[You may click to read Atoms (1) ]
In 1805, the English chemist and physicist, John Dalton, stated that all matter consists of small particles which he called ‘atoms’. Dalton’s theory, which helped to explain many different observations that he and other scientists had made, has been supported  and changed by scientists since his time, but it is basic to an understanding of chemistry and biology today.

The word ‘atom’ comes from a Greek word which means ‘indivisible’. However, scientists in our century have found that atoms are not indivisible. All atoms are made up of different combinations of  three smaller particles: electrons, neutrons and protons. Electrons are the main units of electricity and they carry a negative electrical charge (-). Protons carry a positive electrical charge (+). Neutrons, as their name suggests, are neutral. They carry no electrical charge.

Atoms – Electrons – Nucleus – Neutrons – Electrons


How are these particles arranged inside the atom? The protons and  neutrons together form the nucleus of the atom. The nucleus is in the centre and occupies a very small amount of the total space of the atom. All the rest of the empty space in the atom is used by the rapidly moving electrons. As a result of these rapid movements of electrons, an ‘electron cloud’ is formed around the nucleus. They seem to be everywhere at once.

The number of electrons outside the nucleus equals the number of protons inside the nucleus. Thus, an atom is electricallly balanced, or neutral. All chemical reactions involve only electrons, which travel around the nucleus in different orbits. These electrons can interact with the electrons of another atom to form  compounds. The nucleus of an atom is not changed in a chemical reaction. It is changed only in nuclear reactions, which occur, for example, in radioactive minerals and in atomic reactors.

Atoms of different elements differ from one another in the number of protons, neutrons and electrons. On the other hand, atoms of the same element always have the same number of protons and electrons, although they may differ in the number of neutrons. These are called isotopes.



For example, over 99% of all the oxygen atoms in nature are made up of 8 protons, 8 neutrons and 8 electrons. This is known as the Oxygen-16 isotope. 16 is the sum of the number of protons and  neutrons. (The number of electrons is not included in this number because the number of electrons is the same as that of protons.) However, there is also a small amount of the Oxygen-18 isotope, whose atoms contain 8 protons and 10 neutrons. Isotopes are important in biology because they can be used in  following many processes in living cells.

Radioactive isotopes are the most useful ones for this purpose. The nuclei of radioactive isotopes are not stable. They give off radiation and finally come apart. The radiation can be detected with a Geiger counter. For example, the radioactive isotope of carbon, Carbon-14, has helped biologists to  follow the path of carbon in many complicated reactions inside living cells.

Structure within the atom


Many of the chemical elements which occur in nature are made up of mixtures of non-radioactive isotopes with stable nuclei. Others are composed of radioactive isotopes. In addition, radioactive isotopes of all the chemical elements can be produced artificially. These are called radioisotopes. The most important source of radioisotopes is the atomic reactor, which yields large quantities of some isotopes from the fission of uranium. Other radioisotopes may be produced by the bombardment  of suitable elements by neutrons in the reactor and some others by nuclear reactions.


A. What do the following refer to?
1. ‘if (line 5):
2. ‘these particles’ (line 14):
3. ‘If (line 26):
4. ‘they’ (line 31):
5. ‘Others’ (line 48): Other
B. Mark the best choice.

1. Compounds are formed .

a) as a result of the rapid movements of the electrons

b) because electrons travel in different orbits around the nucleus

c) when the nucleus is changed in nuclear reactions

d) when the electrons of an atom interact with those of another atom


2. A chemical reaction .

a) helps to form an ‘electron cloud’ around the nucleus

b) causes no change in the nucleus and it involves only electrons

c) causes the electrons to move in different orbits

d) causes a change in the arrangement of all the particles of an atom


3. Which of the following statements is true?

a) The nuclei of radioactive isotopes give off radiation and they are stable.

b) There are several ways of producing radioisotopes artificially.

c) The number of protons, neutrons and electrons are always the same in the atoms of the same element

 d) Both (a) and (b).

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