Double Award - Science for GCSE

Chapter 17

Atomic structure

1.	Metals, non-metals and metalloids One of the most useful ways of classifying elements is as metals and non-metals. The most convenient way to decide whether an element is a metal or a non-metal is to check its electrical conductivity. Some elements, like graphite, are difficult to classify as metals or nonmetals because they have some properties like metals and other properties like non-metals. These elements are called metalloids.
2.	Patterns of elements – the Periodic Table During the 19th century, chemists identified families of similar elements (e.g. chlorine, bromine and iodine). This led chemists to search for a pattern in the properties of all elements. In 1869, Mendeleev found a pattern in the properties of elements and their relative atomic masses, which he summarised in a table. In time, Mendeleev’s table of elements led to the modern Periodic Table. The modern Periodic Table shows that the properties of elements are closely related to their atomic number. Elements with similar properties fall in the same vertical column in the Periodic Table. – These vertical columns of similar elements are called groups. – The horizontal rows are called periods.
3.	Atomic structure All atoms are built up from protons, neutrons and electrons. The proton and the neutron have a relative mass of one unit. The electron has a mass of of a unit. The proton has a charge of +1, the neutron has no charge, and the electron has a charge of -1. Protons and neutrons occupy the nucleus at the centre of the atom. Electrons occupy layers or shells at different distances from the nucleus. Electrons determine the chemical properties of atoms. Protons, neutrons and electrons are the building blocks for all atoms. E.g. helium atoms have 2 protons, 2 or 3 neutrons and 2 electrons; carbon atoms have 6 protons, 6, 7 or 8 neutrons and 6 electrons; uranium atoms have 92 protons, 143 or 146 neutrons and 92 electrons. Uncharged atoms always have the same number of protons as electrons.
4.	Atomic number and mass number The atomic number of an atom = the number of protons = the order of the element in the Periodic Table. E.g. fluorine is the ninth element in the Periodic Table with 9 protons, i.e. its atomic number = 9. The term atomic number is sometimes called proton number. The mass number of an atom = number of protons+ number of neutrons. Protons and neutrons which occupy the nucleus are sometimes called nucleons. So the term mass number is sometimes called the nucleon number of an atom.
5.	Isotopes Isotopes are atoms of the same element with the same atomic number, but different mass numbers.