An atom is made up of a nucleus, itself made up of protons and neutrons held together by the strong nuclear force, and electrons, which form a 'cloud' and the nucleus. The following points must be made:
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The number of electrons is equal to the number of protons, since the atom as a whole must be neutral.
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An element is determined by it's atomic number, which is the number of protons it contains. The number of neutrons may be different for each atom without changing the element. Different elements may have the same number of neutrons and atoms of the same element may have different numbers of neutron. Atoms of the same element with different numbers of neutrons are called isotopes.
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The chemical properties of an atom are a result of interaction between the electrons around the nucleus with other electrons around other nuclei. The number of neutrons in the nucleus does not affect the electrons in any way, so does not change any chemical properties of the atom.
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Different isotopes have different physical properties and this can be used to separate them. For example, centrifuges are used to separate isotopes of Uranium for use in nuclear weapons.
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Protons and neutrons have very nearly the same mass, while electrons weigh very little. The mass number of an atom is equal to the number of protons plus the number of neutrons Different isotopes of the same element have different masses therefore. The mass of an element need not be an integer even though the mass number of an atom always is. This is because the mass of an element as given in the period table is the mass in grams of a certain number of atoms (
called Avagadro's number). Any sample of of an element will be made up of several different isotopes. The mass of chlorine from the period table is 35.5 grams. Chlorine has two main isotopes, Cl – 35 and Cl – 37, which occur in the ratio 3:1. A weighted average in this proportion gives an average mass of 35.5 grams. -
An isotope may be written in the form
where
is the mass number and
is the atomic number. Thus
has238 protons and neutrons altogether. Of these 92 are protons so there are 238-92=146 neutrons. The number of electrons is equal to the number of protons, so the are 92 electrons. -
The notation
is often used to refer also to the nucleus alone. This is a source of confusion.