Chapter Six Atomic structure
CHECK YOUR EXISTING KNOWLEDGE
Atomic theory
It is important to realize at the outset that the descriptions of atoms and atomic particles that you will encounter here and in other texts are merely models designed to help us understand how atoms behave and interact. As with any model, there is a degree of simplification – in the case of atomic modelling, this simplification is often so gross that the model bears no relationship to reality. Atoms are, pretty much, incomprehensible things: you can’t see them, even with the most powerful microscopes (you will learn why in Chapter 8); you can’t taste them, feel them, hear them; you can’t touch them, merely the fields that surround them.
The Rutherford atom
For some time, it had been known that α-particles could be slightly deflected when fired at thin sheets of metal foil. This was entirely consistent with the Thomson model of the electron with weak electric forces being exerted on the passing α-particles by the uniformly distributed charges of the ‘plum pudding’ atoms.
He realized that the only model that could account for the α-particles’ extraordinary behaviour was if the positive charge was all concentrated into a small, tightly bound, dense nucleus with the electrons more diffusely dispersed at a distance (Fig. 6.2). This alone could account for the findings: it would mean that much of apparently solid objects was empty space, which was why most of the α-particles passed through the foil without deviation. The light, widely separated electrons had little influence on the passage of the incoming particles but if they came within the electric field of the large, positive charge of the nucleus, they could be deflected or even repelled (Fig. 6.3). In fact, it transpires that 99.999999% of matter is empty space; if the nucleus of a hydrogen atom is represented by a thumb-tack stuck in the middle of a football field, then the electron would be lurking somewhere in the furthest, uppermost row of seats in the surrounding stadium. In Rutherford’s model, the electric field intensity of the gold atomic nucleus is 100 000 000 times that of the Thomson model.
Isotopes
Perhaps the isotope that has the greatest public awareness is carbon-14. Carbon, which has six protons (Z = 6), normally has an atomic mass number of 12; however, it also exists in a form where A = 14. Conveniently, this form is radioactive and decays slowly back into carbon-12. By finding the ratio of carbon-12 to carbon-14, scientists are able to calculate the approximate age of organic materials, all of which contain large numbers of carbon atoms.
It is also the carbon atom that forms the basis for the definition of the mole, which, as we learned in Chapter 1, is one of the seven fundamental SI units (Table 1.1). You may recall that the mole was defined as the amount of a given substance that contains as many elementary entities as there are atoms in 0.012 kilogram of carbon-12.
The three isotopes of carbon we discussed become (six protons and six neutrons),
(six protons and seven neutrons) and
(six protons and eight neutrons). Examples of some of the elements that we have discussed, and others that you are likely to commonly encounter, are given in Table 6.1.
Table 6.1 Some common elements, their isomer nomenclature and comments
Element (Z) | Nomenclature of common (>1%), natural isotopes | Comment |
---|---|---|
Hydrogen (1) | ![]() ![]() | Ubiquitous in animal tissue, the element that facilitates magnetic resonance imaging |
Helium (2) | ![]() | Nucleus is an α-particle ![]() |
Carbon (6) | ![]() | Carbon dating (see above). The backbone of organic molecules |
Nitrogen (7) | ![]() | Main component of air. Component of all proteins and DNA |
Oxygen (8) | ![]() | Essential for cellular respiration |
Sodium (11) | ![]() | Na+ ion is essential in cell membrane transport |
Magnesium (12) | ![]() | Catalyst for enzyme reactions in carbohydrate metabolism |
Aluminium (13) | ![]() | Used for X-ray filters and step-wedges |
Chlorine (17) | ![]() | Cl− ion is essential in cell membrane transport |
Potassium (19) | ![]() | Man-made isotopes used in medicine |
Calcium (20) | ![]() | Major component of bones and teeth; Ca2+ triggers muscle contraction |
Iron (26) | ![]() | Critical component of haemoglobin |
Cobalt (27) | ![]() | Man-made isotopes used in medicine |
Copper (29) | ![]() | Key enzymatic component. Used for X-ray filters and anodes/targets |
Zinc (30) | ![]() | Key enzymatic component |
Tin (50) | ![]() | Used for X-ray filters |
Iodine (53) | ![]() | Component of thyroid hormone. Radioactive, synthesized ![]() |
Barium (56) | ![]() | Radio-opaque medium used in radiographic examination of the gastrointestinal tract |
Gadolinium (64) | ![]() | Commonest contrast agent for musculoskeletal MRI |
Tungsten (74) | ![]() | X-ray machine filament |
Gold (79) | ![]() | Used in treatment of rheumatoid arthritis. Was the original foil used by Geiger and Marsden |
Mercury (80) | ![]() | Used in traditional thermometers |
Lead (82) | ![]() | Used for X-ray and other radiation shielding |
Radon (86) | ![]() | Synthetic isotopes used in radiotherapy |
Radium (88) | ![]() | Synthetic isotopes used in radiotherapy |
* Not occurring at > 1% but listed because of inclusion in discussion above.