Oct 15, 2016

Chemical Elements

Now that we have acquired some fundamental ideas about atomic structure, we can more thoroughly discuss the concept of chemical elements. All atoms of a particular element have the same atomic number, Z, and, conversely, all atoms with the same number of protons are atoms of the same element. The elements shown on the inside front cover have atomic numbers from Z = 1 to Z = 112. Each element has a name and a distinctive symbol. Chemical symbols are one- or two-letter abbreviations of the name (usually the English name). The first (but never the second) letter of the symbol is capitalized; for example: carbon, C; oxygen, O; neon, Ne; and silicon, Si. Some elements known since ancient times have symbols based on their Latin names, such as Fe for iron (ferrum) and Pb for lead (plumbum). The element sodium has the symbol Na, based on the Latin natrium for sodium carbonate. Potassium has the symbol K, based on the Latin kalium for potassium carbonate. The symbol for tungsten, W, is based on the German wolfram. Elements beyond uranium (Z = 922) do not occur naturally and must be synthesized in particle accelerators. Elements of the very highest atomic numbers have been produced only on a limited number of occasions, a few atoms at a time. Inevitably, controversies have arisen about which research team discovered a new element and, in fact, whether a discovery was made at all. However, international agreement has been reached on the first 112 elements; each one, except element 112, has an official name and symbol.

To represent the composition of any particular atom, we need to specify its number of protons (p), neutrons (n), and electrons (e). We can do this with the symbolism
This symbolism indicates that the atom is element E and that it has atomic number Z and mass number A. For example, an atom of aluminum represented as Al has 13 protons and 14 neutrons in its nucleus and 13 electrons outside the nucleus. (Recall that an atom has the same number of electrons as protons.)

Contrary to what Dalton thought, we now know that atoms of an element do not necessarily all have the same mass. In 1912, J. J. Thomson measured the mass-to-charge ratios of positive ions formed from neon atoms. From these ratios he deduced that about 91% of the atoms had one mass and that the remaining atoms were about 10% heavier. All neon atoms have 10 protons in their nuclei, and most have 10 neutrons as well. A very few neon atoms, however, have 11 neutrons and some have 12. We can represent these three different types of neon atoms as

Atoms that have the same atomic number (Z) but different mass numbers (A) are called isotopes. Of all Ne atoms on Earth, 90.51% are Ne. The percentages of Ne and Ne are 0.27% and 9.22%, respectively. These percentages 90.51%, 0.27%, 9.22% are the percent natural abundances of the three neon isotopes.

Sometimes the mass numbers of isotopes are incorporated into the names of elements, such as neon-20 (neon twenty). Percent natural abundances are always based on numbers, not masses. Thus, 9051 of every 10,000 neon atoms are neon-20 atoms. Some elements, as they exist in nature, consist of just a single type of atom and therefore do not have naturally occurring isotopes.* Aluminum, for example, consists only of aluminum-27 atoms.

When atoms lose or gain electrons, for example, in the course of a chemical reaction, the species formed are called ions and carry net charges. Because an electron is negatively charged, adding electrons to an electrically neutral atom produces a negatively charged ion. Removing electrons results in a positively charged ion. The number of protons does not change when an atom becomes an ion. For example, 22Ne+ and 22Ne2+ are ions. The first one has 10 protons, 10 neutrons, and 9 electrons. The second one also has 10 protons, but 12 neutrons and 8 electrons. The charge on an ion is equal to the number of protons minus the number of electrons. That is

Another example is the 16O2- ion. In this ion, there are 8 protons (atomic number 8), 8 neutrons (mass number - atomic number), and 10 electrons (18 - 10 = -22).

Isotopic Masses
We cannot determine the mass of an individual atom just by adding up the masses of its fundamental particles. When protons and neutrons combine to form a nucleus, a very small portion of their original mass is converted to energy and released. However, we cannot predict exactly how much this so called nuclear binding energy will be. Determining the masses of individual atoms, then, is something that must be done by experiment, in the following way. By international agreement, one type of atom has been chosen and assigned a specific mass. This standard is an atom of the isotope carbon-12, which is assigned a mass of exactly 12 atomic mass units, that is, 12 u. Next, the masses of other atoms relative to carbon-12 are determined with a mass spectrometer. In this device, a beam of gaseous ions passing through electric and magnetic fields separates into components of differing masses. The separated ions are focused on a measuring instrument, which records their presence and amounts. Figure below illustrates mass spectrometry and a typical mass spectrum.

Although mass numbers are whole numbers, the actual masses of individual atoms (in atomic mass units, u) are never whole numbers, except for carbon-12. However, they are very close in value to the corresponding mass numbers, as we can see for the isotope oxygen-16. From mass spectral data the ratio of the mass of 16O to 12C is found to be 1.33291. Thus, the mass of the oxygen-16 atom is

1.33291 X 12 u = 15.9949 u

Which is very nearly equal to the mass number of 16.

Oct 13, 2016

Reproduction in Diatoms

Despite being unicellular, diatoms are highly organized cells, presenting complex sexual reproduction processes, characterized by a common scheme.

Life Cycle

Diatoms are diplonts with meiosis in the final stage of gametogenesis (figure below). The duration of the haploid phase is relatively short, not exceeding several hours, while the diploid state may last from months to years, depending on the species and the environmental conditions. The life cycle of diatoms is thus composed of two successive phases: a prolonged phase of vegetative multiplication and a very short period of sexual reproduction.

Different Types of Life Cycles Explored by Algal Groups and other Organisms.
Depending upon the precise time and purpose of meiosis and gamete fertilization in the cycle, one can discriminate between haplontic life cycles (also called zygotic meiosis life cycles since meiosis occurs during germination of the zygote, the dominant part of the life cycle is haploid) or diplontic life cycles (also called gametic meiosis life cycles as meiosis is used to produce haploid gametes, the dominant part of the life cycle is diploid). Haplo-diplontic (= diplobiontic) life cycles (also called sporic meiosis life cycles as meiosis is used to produce haploid spores) represent a combination of both, with the gametophyte stage producing gametes and the sporophyte producing spores. Microscopic algae use zygotic (e.g., dinoflagellates), gametic meiosis (diatoms), or sporic meiosis (haptophytes). Sessile macroscopic seaweeds experiment (at least functionally) gametic or sporic meiosis, which ultimately is utilized by all land plants.

Vegetative multiplication is performed through common mitotic division. Under any circumstances, the biggest cells cannot reproduce sexually. However, most diatoms undergo significant size reduction over their life cycle, a phenomenon that puzzles many novice analysts of plankton samples. This happens because of the specific structure of the diatom cell wall also known as the frustule. The silicate frustule is rigid and consists of two parts, the epitheca and the hypotheca, which are joined to each other in a manner similar to the base and lid of a Petri dish. During mitotic division both daughter cells acquire new ‘bases’, i.e., hypothecae. As a result, after each act of vegetative multiplication via mitotic division one daughter cell keeps the parental size, while the other daughter cell becomes a little bit smaller in length (in pennate diatoms) or diameter (in centric diatoms). As cells divide, the mean cell size in a particular population gradually decreases, a phenomenon known as the MacDonald-Pfitzer rule (MacDonald 1869; Pfitzer 1869).The shift from the vegetative to the sexual phase of the life cycle in diatoms is size dependent. Upon reaching a critical size threshold, which is species specific, cells becomes sexually inducible and may enter sexual reproduction. It is not obligatory for cells below a critical size threshold to enter sexual reproduction, they are just allowed by the size factor to do this; many cells in the population continue vegetative multiplication. In some species there is also a second threshold below which cells could again not be induced to become sexual but rather divide mitotically until they die. When favorable conditions combine, such as the proper cell size, availability of the sexual partner (if needed), suitable environmental factors (light conditions, temperature, salinity, etc.), and cells may shift from mitotic to meiotic division. The cells produce gametes, and can therefore be named gametangia. The process results in the reduction of the chromosome number to half that of parent cells (i.e., haploid). Gamete fusion leads to zygotes, restoring the diploid state. Zygotes commonly start to grow without any dormancy period. At this stage, the cells, which are called auxospores, are not surrounded by a siliceous frustule and are thus capable of expansion. The auxospore typically expands very quickly, reaching the maximum (or close to maximal) species specific size in a couple of hours. However, auxospore growth is not simple swelling of the cell and concomitant expansion of the cell wall; a more or less complex structure composed of silicate scales, rings and bands is formed in the growing cell as a skeleton, named in pennate diatoms as the perizonium.

The deposition of perizonial bands causes the rupture of the primary zygote wall (the membrane surrounding the zygote) and the remnants of such a membrane can be seen in many cases on both tips of the elongating auxospore in the form of caps. As soon as the auxospore has reached the maximum size, the diploid nucleus undergoes an acytokinetic division that precedes the deposition of the two initial valves (epitheca first, followed by hypotheca) inside the fully-grown auxospore. As a result, a new cell of the biggest size (an initial cell) arises. The initial cells may differ more or less from the vegetative cells in shape and structure of silica frustules. After being released from the auxospore envelope, the initial cell resumes vegetative multiplication and thereby creates a new clonal lineage of cells, which are renovated genetically and have the biggest sizes.

The close relationship between sexual reproduction and cell size restoration is an intriguing feature of the diatom life cycle. In a few species vegetative enlargement has also been observed (Gallagher 1983; Nagai 1995; Chepurnov et al. 2004). Usually, auxosporulation occurs in conditions that are favorable for vegetative growth. Certain centric and pennate diatoms can form resting stages in response to environmental stress (for details see Round et al. 1990). Nevertheless, in most cases the transition to dormant stages such as resting spores/cells or winter forms is not a typical characteristic of the diatom life cycle and is obligatory only in a few species (e.g., French and Hargraves 1985).

Diagrammatic Representation of the Life Cycle in a Centric Diatom Melosira sp. (a) an initial cell formed in a mature auxospore, (b-f), because of specifi c construction of the frustule the cell size decreases while cells pass through mitotic cycles, (g) male and (h) female gametogenesis; n and 2n, haplontic and diplontic phases (cell diameter range ca. 20–80 µm).
Generically, if compared with other groups of algae, ‘lower plants’, higher plants, and vertebrates (Mable and Otto 1998), the diatoms have a life cycle similar to those of the evolutionarily most advanced organisms (figure above). In contrast to many other algae, where closely related taxa can exhibit widely variant life cycles with respect to the duration of haploid and diploid phases, the life cycle strategy in diatoms appears to be more permanent and uniform.

Patterns of sexual reproduction
A great diversity of copulation processes has been revealed in diatoms. Centric diatoms (Figure below) were shown to be oogamous (von Stosch and Drebes 1964; Schultz and Trainor 1968); they produce large non-motile female gametes (eggs) which are fertilized by small motile flagellated male gametes (sperms). Male cells usually undergo a series of successive differentiating mitoses giving rise to a variable number (2, 4, 8, 16, and 32) of small diploid spermatogonia, which complete their further development by sperm formation (spermatogenesis). Two main types of spermatogenesis are known, depending on the formation of residual bodies, the merogenous and the hologenous types. The flagellum of the sperm cell is directed forwards during swimming and does not conform to the usual eukaryotic 9+2 arrangement, but shows a 9 + 0 pattern, i.e., central microtubules are lacking (Manton and von Stosch 1966; Heath and Darley 1972). In contrast to the spermatogonia formed by de-pauperizing mitoses, the oogonia usually develop directly from vegetative cells. Three types of egg formation are recognized: (1) oogonia containing two eggs, (2) oogonia containing a single egg and a polar body, (3) oogonia containing a single egg. Independent from the meiotic nuclear stage of the oocyte, fertilization may happen as soon as the egg surface is mechanically (partly or totally) exposed, but in all cases nuclear fusion (karyogamy) does not follow before the female nucleus has reached the mature haploid stage (Drebes 1977). As a rule during fertilization the flagellum is discarded. Unlike centric, gametes released by pennate, diatoms are more or less equal in size (isogamy), however, they may differ morphologically and behaviorally depending on the species (see image below). Taken into account these and many other details, Geitler elaborated a comprehensive system of sexual reproduction patterns (Geitler 1973; Mann 1993). What is important, a flagellate stage has never been described in pennate diatoms (see also discussion in Subba Rao et al. 1991, 1992; Rosowski et al. 1992; Davidovich and Bates 1998). Automixis in the form of paedogamy or autogamy is also possible; and some diatoms are known to be apomictic (Geitler 1973; Vanormelingen et al. 2008).

Diagrammatic Representation of the Life Cycle in the Pennate Diatom Haslea karadagensis. (a) an initial cell, (b-e) vegetative cells passing through mitotic cycles, (f) pairing of gametangia, (g) gametogenesis, (h) zygotes, (i-j) auxospore formation; n and 2n, haplontic and diplontic phases (cell length range: ca. 20–95 µm).
Delivery of the Gametes to the Place of Syngamy
The whole content of the gametangial cell transforms into only one or two gametes (more in centric males). Therefore, the cost of sex is very high in diatoms (Lewis 1984). Several mechanisms that promote the encounter of gametes and thus their fusion have evolved. Oogamous fertilization in centric diatoms is effective because of motile male gametes; they swim by being dragged and not pushed by the anterior flagellum. Very often, the gametes of the raphe-bearing pennate diatoms are brought together by a prior pairing of their mother cells (gametangia) of the complementary sexes. For this reason the term ‘gametangiogamy’ is also in use (Wiese 1969).

To secure fertilization, in several benthic pennates a mucilage capsule is secreted around the copulating partners. Once gametangia are physically close, another type of active movement, amoeboid movement may be employed. This movement allows gamete transfer to the place of syngamy and is sufficient for short distance translocation of the gamete from closely positioned gametangia. Vegetative cells of a very diverse group of araphid pennates, however, are sessile and only a few species are known for their slow motility (Kooistra et al. 2003; Sato and Medlin 2006). An unusual mechanism of male gamete motility has recently been described in araphid pennates, involving formation of thread-like cytoplasmic projections on the gamete cell surface (Sato et al. 2011; Davidovich et al. 2012).

Factors triggering sexual reproduction
If mating is heterothallic, interaction between sexual partners is required to initiate meiosis and gametogenesis. There is some evidence that the process of sexualisation is triggered by a sophisticated multi-stage exchange of several pheromones (Sato et al. 2011; Gillard et al. 2012). For now, it is unclear how species-specific these pheromones are. If pheromones or their combination are unique for each species, a minor change in their structure may lead to certain problems in the control of cell pairing that predetermine a very short and efficient way for speciation in diatoms, even in sympatric populations.

Comprehensive observations gained by previous investigators, mainly by Geitler (Geitler 1932), allowed Drebes to write in his review (Drebes 1977, p. 271): “In contrast to other protists, induction of sexualisation in diatoms depends not only on the genotype and specific environmental conditions but also on a suitable cell size as an internal non-genetic factor”. The concept of ‘cardinal points’ in the life cycle of diatoms developed by Geitler (Geitler 1932) declares that cells cannot be induced to become sexual until they have declined in size below the critical size. The upper threshold for sexual induction can range from 30% to 75% of the maximal size of the initial cells. However, for more than half of the species examined, the threshold was at 45% to 55% (Davidovich 2001).

Apart from suitable cell sizes, one of the necessary conditions for successful sexual reproduction is an excellent physiological state. Laboratory practice has shown that the best results can be achieved in exponentially growing cultures. Stressed cultures never undergo sexual reproduction (Chepurnov et al. 2004). Beside internal cues, a number of external factors have been shown to influence sexual reproduction in diatoms (Drebes 1977). Appropriate light conditions are highly important, sometimes being a key factor in triggering sexual reproduction (e.g., Hiltz et al. 2000; Mouget et al. 2009).

Mating system
While centric diatoms are believed to be genetically monoecious and hence are capable of homothallic reproduction (Drebes 1977; however see comments in Chepurnov et al. 2004), pennate diatoms are for the most part heterothallic, or combine homo- and heterothallic modes of sexual reproduction (Roshchin and Chepurnov 1999; Chepurnov et al. 2004; Davidovich et al. 2009, 2010; Amato 2010). In the case of strict heterothally (genetic dioecy) clones are divided into two mating types, which are usually indistinguishable at the stage of vegetative growth, but in certain species may clearly differ by morphology and/or behavior of their gametes (e.g., Stickle 1986; Davidovich et al. 2009). These diatoms (cis-anisogamous sensu Mann 1982) are most suitable as model species for studies of sex determination and sex inheritance. If differences are visible and correspond to two mating categories, these can be designated as ‘male’ and ‘female’. Otherwise, mating types are conventionally termed plus and minus, ‘+’ and ‘–’.

Sex determination
Mechanism of sex determination in diatoms is poorly understood. Sex chromosomes are unknown, at the same time obligate heterothallic reproduction and existence of two mating types in some pennates suggest genetic dioecy. In such a case sex factors must be located on different chromosomes. It has been shown that sibling clones, derived from the two initial cells formed during sexual reproduction of a single pair of gametangia (in those species where each gametangium produces two gametes) were of opposite sexes (Mann et al. 2003). This and other experiments (e.g., Chepurnov and Mann 2004) indicate that sex determination in heterothallic pennate species is not developmental or phenotypic.

Sex differentiation is impossible in the case of automictic reproduction, but automixis is not common among diatoms. A genetic base for sex determination can be reasonably elucidated in those species which are able to reproduce both intra- and interclonally. The mode of sex inheritance revealed in a homothallic progeny may give an answer to the question of how sex factors are distributed (Davidovich 2002; Chepurnov et al. 2004; Davidovich et al. 2006). Data acquired suggests coupling of chromosomes bearing male (M) and female (F) genetic factors in combinations MF and FF for male and female sexes accordingly. Male sex is thus heterogametic.

However, it cannot be ruled out that this simple model will prove to be more challenging as more data become available. For example, in several pennate diatoms some clones behaved as males when mated with female clones, but as females when mated with male clones (Chepurnov et al. 2004); this suggests mating system to be more complex than the bipolar and obscures the sex determination mechanism.
An example of phenotypic sex determination can be found in centric diatoms (Drebes 1977), where sex appearance generally depends on the stage of the life cycle. In one and the same clone, relatively big cells recently entered into the sexual size region produce eggs, while getting smaller they shift to spermatozoid production. At the same time, in some centric diatoms there were particular clones which acted as ‘pure’ males, never producing female gametes (Chepurnov et al. 2004).

Asexual auxosporulation
Auxospore formation is usually regarded as a process intrinsically connected with the phenomenon of sexual reproduction. However, sometimes auxospores may occur in the absence of any sexual process (e.g., Nagai et al. 1995; Sabbe et al. 2004; Chepurnov et al. 2004). Apomixis treated as diploid parthenogenesis is associated with asexual auxosporulation, and was reported both in centric and pennate diatoms. In other cases, unfused gametes can transform into auxospores (haploid parthenogenesis). The last is facultative in some allogamous pennate diatoms. During vegetative cell enlargement noted in some diatoms (e.g., Gallagher 1983; Nagai et al. 1995) the cells escape from the ‘trap’ of critical size diminution, but cells developed directly from vegetative cells do not produce typical perizonium and their size is approximately half the size of the normal auxospores produced by the same clone.

The Nuclear Atom

In 1909, Rutherford, with his assistant Hans Geiger, began a line of research using α particles as probes to study the inner structure of atoms. Based on Thomson's plum-pudding model, Rutherford expected that most particles in a beam of α particles would pass through thin sections of matter largely un-deflected, but that some α particles would be slightly scattered or deflected as they encountered electrons. By studying these scattering patterns, he hoped to deduce something about the distribution of electrons in atoms.

The apparatus used for these studies is pictured in figure below. Alpha particles were detected by the flashes of light they produced when they struck a zinc sulfide screen mounted on the end of a telescope. When Geiger and Ernst Marsden, a student, bombarded very thin foils of gold with a particles, they observed the following:
• The majority of α particles penetrated the foil un-deflected.
• Some α particles experienced slight deflections.
• A few (about 1 in every 20,000) suffered rather serious deflections as they penetrated the foil.
• A similar number did not pass through the foil at all, but bounced back in the direction from which they had come.

Rutherford's initial expectation and his explanation of the α-particle experiments are described in the figure below.

Discovery of Protons and Neutrons
Rutherford's nuclear atom suggested the existence of positively charged fundamental particles of matter in the nuclei of atoms. Rutherford himself discovered these particles, called protons, in 1919 in studies involving the scattering of a particles by nitrogen atoms in air. The protons were freed as a result of collisions between a particles and the nuclei of nitrogen atoms. At about this same time, Rutherford predicted the existence in the nucleus of electrically neutral fundamental particles. In 1932, James Chadwick showed that a newly discovered penetrating radiation consisted of beams of neutral particles. These particles, called neutrons, originated from the nuclei of atoms. Thus, it has been only for about the past 100 years that we have had the atomic model suggested by the figure below.

Properties of Protons, Neutrons, and Electrons

The number of protons in a given atom is called the atomic number, or the proton number, Z. The number of electrons in the atom is also equal to Z because the atom is electrically neutral. The total number of protons and neutrons in an atom is called the mass number, A. The number of neutrons, the neutron number, is A - Z. An electron carries an atomic unit of negative charge, a proton carries an atomic unit of positive charge, and a neutron is electrically neutral. Table above presents the charges and masses of protons, neutrons, and electrons in two ways.

The atomic mass unit is defined as exactly 1/12 of the mass of the atom known as carbon-12 (read as carbon twelve). An atomic mass unit is abbreviated as ‘amu’ and denoted by the symbol u. As we see from the table above, the proton and neutron masses are just slightly greater than 1 u. By comparison, the mass of an electron is only about 1/2000th the mass of the proton or neutron.

Oct 12, 2016

Electrons and Other Discoveries in Atomic Physics

Fortunately, we can acquire a qualitative understanding of atomic structure without having to retrace all the discoveries that preceded atomic physics. We do, however, need a few key ideas about the interrelated phenomena of electricity and magnetism, which we will briefly discuss here. Electricity and magnetism were used in the experiments that led to the current theory of atomic structure.

Certain objects display a property called electric charge, which can be either positive (+) or negative (-). Positive and negative charges attract each other, while two positive or two negative charges repel each other. As we learn in this section, all objects of matter are made up of charged particles. An object having equal numbers of positively and negatively charged particles carries no net charge and is electrically neutral. If the number of positive charges exceeds the number of negative charges, the object has a net positive charge. If negative charges exceed positive charges, the object has a net negative charge.

Sometimes when one substance is rubbed against another, as in combing hair, net electric charges build up on the objects, implying that rubbing separates some positive and negative charges (as shown in the figure below). Moreover, when a stationary (static) positive charge builds up in one place, a negative charge of equal size appears somewhere else; charge is balanced.

The second figure below shows how charged particles behave when they move through the field of a magnet. They are deflected from their straight-line path into a curved path in a plane perpendicular to the field. Think of the field or region of influence of the magnet as represented by a series of invisible "lines of force" running from the North Pole to the south pole of the magnet.

The Discovery of Electrons
CRT, the abbreviation for cathode-ray tube, was once a familiar acronym before liquid crystal display (LCD) was available, the CRT was the heart of computer monitors and TV sets. The first cathode-ray tube was made by Michael Faraday (1791- 1867) about 150 years ago. When he passed electricity through glass tubes from which most of the air had been evacuated, Faraday discovered cathode rays, a type of radiation emitted by the negative terminal, or cathode. The radiation crossed the evacuated tube to the positive terminal, or anode. Later scientists found that cathode rays travel in straight lines and have properties that are independent of the cathode material (that is, whether it is iron, platinum, and so on). The construction of a CRT is shown in figure below.

The cathode rays produced in the CRT are invisible, and they can be detected only by the light emitted by materials that they strike. These materials, called phosphors, are painted on the end of the CRT so that the path of the cathode rays can be revealed. (Fluorescence is the term used to describe the emission of light by a phosphor when it is struck by energetic radiation.) Another significant observation about cathode rays is that they are deflected by electric and magnetic fields in the manner expected for negatively charged particles (see figure below).

In 1897, by the method outlined in Figure (c), J. J. Thomson (1856 1940) established the ratio of mass (m) to electric charge (e) for cathode rays, that is, m/e. Also, Thomson concluded that cathode rays are negatively charged fundamental particles of matter found in all atoms. (The properties of cathode rays are independent of the composition of the cathode.) Cathode rays subsequently became known as electrons, a term first proposed by George Stoney in 1874.

Robert Millikan (1868 1953) determined the electronic charge e through a series of oil-drop experiments (1906 1914), described in Figure below. The currently accepted value of the electronic charge e, expressed in coulombs to five significant figures, is (-1.6022 x 10 - 19 C).  By combining this value with an accurate value of the mass-to-charge ratio for an electron, we find that the mass of an electron is 9.1094 x 10 -28 g.

Once the electron was seen to be a fundamental particle of matter found in all atoms, atomic physicists began to speculate on how these particles were incorporated into atoms. The commonly accepted model was that proposed by J. J. Thomson. Thomson thought that the positive charge necessary to counterbalance the negative charges of electrons in a neutral atom was in the form of a nebulous cloud. Electrons, he suggested, floated in a diffuse cloud of positive charge (rather like a lump of gelatin with electron fruit embedded in it).

This model became known as the plum-pudding model because of its similarity to a popular English dessert. The plum-pudding model is illustrated in figure below for a neutral atom and for atomic species, called ions, which carry a net charge.

X-Rays and Radioactivity
Cathode-ray research had many important spin-offs. In particular, two natural phenomena of immense theoretical and practical significance were discovered in the course of other investigations.

In 1895, Wilhelm Roentgen (1845 1923) noticed that when cathode-ray tubes were operating, certain materials outside the tubes glowed or fluoresced. He showed that this fluorescence was caused by radiation emitted by the cathode-ray tubes. Because of the unknown nature of this radiation, Roentgen coined the term X-ray. We now recognize the X-ray as a form of high-energy electromagnetic radiation, which is discussed in other blog posts (use the search bar on the top).

Antoine Henri Becquerel (1852 1908) associated X-rays with fluorescence and wondered if naturally fluorescent materials produce X-rays. To test this idea, he wrapped a photographic plate with black paper, placed a coin on the paper, covered the coin with a uranium-containing fluorescent material, and exposed the entire assembly to sunlight. When he developed the film, a clear image of the coin could be seen. The fluorescent material had emitted radiation (presumably X-rays) that penetrated the paper and exposed the film. On one occasion, because the sky was overcast, Becquerel placed the experimental assembly inside a desk drawer for a few days while waiting for the weather to clear. On resuming the experiment, Becquerel decided to replace the original photographic film, expecting that it may have become slightly exposed. He developed the original film and found that instead of the expected feeble image, there was a very sharp one. The film had become strongly exposed because the uranium-containing material had emitted radiation continuously, even when it was not fluorescing. Becquerel had discovered radioactivity.

Ernest Rutherford (1871 1937) identified two types of radiation from radioactive materials, alpha and beta Alpha particles (α) carry two fundamental units of positive charge and have essentially the same mass as helium atoms. In fact, alpha particles are identical to He2+ ions. Beta particles (β) are negatively charged particles produced by changes occurring within the nuclei of radioactive atoms and have the same properties as electrons. A third form of radiation, which is not affected by electric or magnetic fields, was discovered in 1900 by Paul Villard. This radiation, called gamma rays (γ), is not made up of particles; it is electromagnetic radiation of extremely high penetrating power. These three forms of radioactivity are illustrated in figure below.

By the early 1900s, additional radioactive elements were discovered, principally by Marie and Pierre Curie. Rutherford and Frederick Soddy made another profound finding: The chemical properties of a radioactive element change as it undergoes radioactive decay. This observation suggests that radioactivity involves fundamental changes at the subatomic level in radioactive decay, one element is changed into another, a process known as transmutation.