C4.2 How do chemists explain the patterns in the properties of elements?
At the centre of every atom is a nucleus containing protons and neutrons and electrons arranged in shells around the nucleus. All atoms of the same elements have the same number of protons
- An atom has the same number of protons (positive) as electrons (negative), so the atom as a whole is neutral (i.e. it has no electrical charge)
- A proton has the same mass as a neutron
- The mass of an electron is negligible (nearly nothing) when compared to a proton or neutron
- A substance that contains only one sort of atom is called an element
- The elements are arranged in the periodic table in order of increasing (proton) number
When some elements are heated, they emit distinctive coloured flames – the colour given off by each element is different and can be used to identify them.
Lithium, sodium and potassium compounds can be recognised by the distinctive colours they produce in a flame test:
Lithium, Li – Red
Sodium, Na – Yellow
Potassium, K – Lilac
The light emitted from the flame of an element produces a characteristic line spectrum. The study of spectra has helped chemists to discover new elements – in fact the discovery of some elements, such as rubidium and caesium, was not possible until the development of spectroscopy.
Electrons are arranged in shells at different distances around the nucleus. Electron configuration tells us how the electrons are arranged around the nucleus of an atom in shells (energy levels).
- The first level (or shell) can only contain a maximum of two electrons
- The second shell can hold a maximum of eight electrons
- The third shell can hold up to 18 electrons
- The electron configuration is written as a series of numbers, e.g., oxygen is 2.6; aluminium is 2.8.3 and potassium is 2.8.8.1
The chemical properties of an element are determined by its electron arrangement
C4.3 How do chemists explain the properties of compounds of Group 1 and Group 7 elements
Chemists use their observations to develop theories to explain the properties of different compounds.
For example, experiments show that molten compounds of metals and non-metals such as lithium chloride, conduct electricity. It can therefore conclude that there must be charged particles in molten compounds – these particles are known as ions.
An ions is an atom (or group of atoms) that has gained or lost electrons and so has an overall charge because the proton and electron number are no longer equal.
If the ion has been formed by an atom losing electrons, it will have an overall positive (+) charge because it now has more protons than electrons – it is called a cation.
If the ion has been formed by an atom gaining electrons, it will have an overall negative (-) charge because it now has more electrons than protons – it is called an anion.
An iconic bond occurs between a metal and a non-metal and involves the transfer of electrons from an atom to another to form electrically charged ions. Each electrically charged ion has a complete outer energy level or shell – compounds of group 1 and group 7 elements are ionic compounds.
E.g. Sodium and chlorine bond ionically to form sodium chloride, NaCl. The sodium has one electron on its outer shell which is transferred to the chlorine atom – so they both have eight electrons in their outer shell.
The positive ion and negative are then electrostatically attracted to each other to each other to form a giant crystal lattice.
Sodium chloride has a high melting point and dissolves in water – it conducts electricity when it is a solution or is molten, but not when it is solid.
There is a strong force of attraction between the ions, which take a lot of energy to break, so ionic compounds have high melting and boiling points. When the compound is molten or dissolved the forces of attraction are weakened so much that the charged ions are free to move –meaning they can conduct electricity.