Figure 3.3 John Dalton (1766–1844), the father of the modern atomic theory
42 moduLE 1: the chemical Earth
❉ DalTon’S aToMiC TheoRy
Early in the nineteenth century, an English school teacher, John dalton, revived the concept of atoms and proposed an atomic theory based on the experimental evidence available at that time. in this theory, dalton proposed a different kind of atom for each element.
the main ideas or postulates of dalton’s theory can be stated as follows.
1 Elements consist of extremely small, indivisible particles called atoms. in chemical reactions, atoms are conserved; that is, they are not created, destroyed or changed into different kinds of atoms. 2 All atoms of the same element are identical and therefore have the
same properties, such as mass and size.
3 Atoms of different elements have different properties.
4 compounds are formed when atoms of more than one element combine.
5 in a given compound, the relative numbers of atoms of each kind are definite and constant. in general, these relative numbers are simple integers.
6 Atoms of two or more elements may combine in different ratios to form more than one compound.
Although dalton’s theory is still largely accepted, his first two postulates have since been modified. dalton proposed that atoms were small, indivisible particles. it is now known that atoms can be split if they are bombarded by high-energy particles, and some atoms spontaneously disintegrate by radioactive decay. However, under normal conditions most atoms are essentially indivisible.
dalton’s second postulate, that all atoms of the same element are identical, has also been modified. it has been found that different atoms of the same element are not necessarily all the same. An element may exist in different atomic forms with slightly different masses. these different forms are called isotopes and are discussed in unit 3.1.
Figure 3.2 aerial view of the Fermi national accelerator laboratory at Batavia, illinois. Particles are accelerated to very high energies using strong electric and magnetic fields. (Fermilab Photo Department)
Electrons
Nucleus consisting of protons and neutrons
Figure 3.4 a model for the structure of an atom
cHAPtEr 3: Atoms combine to form compounds 43 quantity of charge, the proton and electron are usually described as having charges
of +1 and –1 respectively. Neutrons are uncharged (neutral).
Since as early as the 1950s, physicists have known of the existence of subatomic particles other than protons, electrons and neutrons. They have discovered that protons are made of even smaller particles called quarks. The number and types of particles continue to increase with improvements in the quality of linear and cyclic accelerators, which cause high-energy beams of charged particles to collide. Knowledge of these other subatomic particles is beyond the scope of this course.
A model of the atom
The atom can be visualised in terms of two main regions. These regions are the nucleus and the surrounding space occupied by the electrons.
The structure of the nucleus is as follows:
• It is the central part of the atom, and contains the protons and neutrons. • It has a positive charge equal to the number of protons.
• It is exceedingly small compared with an atom. The diameter of an average
nucleus is 10–15 m, compared with an average atomic diameter of 10–10 m. Thus,
the atomic diameter is about 100 000 times the diameter of the nucleus. (This is the size of a golf ball compared with the Sydney Football Stadium.)
• It contains over 99.9% of the mass of an atom. This is due to the relatively large masses of the proton and neutron compared with the mass of the electron.
• It is exceptionally dense. This is due to its large mass and small volume. (A matchbox full of nuclear material would have a mass of the order of billions of tonnes.)
The arrangement of the electrons around the nucleus is as follows:
• The electrons move through a relatively large space outside the nucleus.
• They are kept moving around the nucleus by various forces.
• In an uncharged atom, the number of electrons equals the number of protons in the nucleus. A diagrammatic representation of the structure of an atom is shown in Figure 3.4.
How atoms differ
The composition of a particular atom is
conveniently described in terms of two numbers: the atomic number and the mass number.
The atomic number, Z, is the number of protons in the nucleus of an atom. This has a fixed value for atoms of any one element. Carbon, for example, has an atomic number of six. Every carbon atom contains six protons in the nucleus. In an electrically neutral atom, the numbers of electrons and protons are equal.
Z A
X
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Therefore, if an atom has no net charge, the atomic number determines the number of electrons in the atom as well as the number of protons in the nucleus.
The mass number, A, is the sum of the numbers of protons and neutrons in the nucleus of an atom. The mass number, the atomic number and the number of neutrons in a nucleus are related in the following way:
A = Z + number of neutrons
If the mass number and atomic number are known, it is possible to work out the number of neutrons in a nucleus. For example, fluorine has an atomic number of nine and a mass number of nineteen, so a fluorine nucleus must contain nine protons and ten neutrons.
It is sometimes convenient to identify the structure of an atom using the following convention:
X is the element symbol
where Z is the atomic number
A is the mass number.
From this shorthand representation it is possible to identify the element, the numbers of protons and neutrons in the nucleus, and the number of electrons in an electrically neutral atom. For example, a neutral phosphorus atom contains 15 protons and 16 neutrons in the nucleus, and 15 electrons. The elemental symbol for phosphorus is P, Z = the number of protons = 15, and A = the number of protons and neutrons = 31. Therefore the atom is represented as
15 31P.
Isotopes
All atoms of the same element have the same number of protons in the nucleus. However, all atoms of the same element do not necessarily have the same mass. Atoms of the same element with different masses have different numbers of neutrons in the nucleus. For example, oxygen atoms can exhibit one of three different masses. Over 99% of oxygen atoms have a mass number of 16, while 0.20% have a mass number of 18, and 0.04% have a mass number of 17. All
oxygen atoms have eight protons in the nucleus, so the three types of atoms can be represented as 168
8 18
O, Oand O.178
The difference between these three types of atoms is that 168O atoms have eight
neutrons in the nucleus, 178O have nine and 8
18O have ten. These different types of
oxygen atoms are called isotopes and are often written as oxygen-16, oxygen-17 and oxygen-18.
Isotopes are defined as different atoms of an element that have the same number of protons but different numbers of neutrons in the nucleus. Table 3.2 gives a list of some well-known isotopes. The three isotopes of hydrogen are given special names because of their particular importance. Although the isotopes of an element have different masses, they have the same chemical properties.