- In the middle of an atom is a small nucleus. This contains protons and neutrons. Orbiting the nucleus are the electrons. Any atom has the same number of electrons orbiting its nucleus as it has protons in its nucleus.
- Protons are positively charged, electrons are negatively charged and neutrons have no charge.
- The number of protons in the nucleus of an atom is its atomic number or proton number.
- The atomic mass of an atom is just the total of the protons and neutrons.
The arrangement of electrons in atoms
- Electrons are arranged around the nucleus in shells. Each shell represents a different energy level.
- An energy level can only hold a certain number of electrons. The first holds 2 electrons. The second holds 8 electrons, as does the third. The fourth usually holds two electrons.
- The atoms in each group of the periodic table have the same number of electrons in their outer shell as the name of their group - e.g. atoms in group one have one electron in their outer shell, while atoms in group 7 have 7 electrons in their outer shell.
- Ionic bonds are usually formed between a metal and a non-metal. As the name suggests, these bonds are formed between ions, e.g. Negative sodium and positive chloride ions.
- Usually one of the atoms, usually the metal, loses electron(s) to the other atom, creating ions. The ions are then attracted to each other and form an ionic compound.
- If an atom needs to gain or lose more electrons than the other has to lose or gain, two or more atoms of each element may react.
- Covalent bonding usually occurs between two non-metals. It is when two electrons share one or two pairs of electrons to gain a full outer shell. These bonds are very strong and hard to break.
- Sometimes one atom will need several electrons, while the other only needs one more for a stable arrangement. In this case, more atoms become involves.
- We can represent covalent bonds by drawing dot-cross diagrams.
- Metallic bonding usually occurs in a lattice of metal atoms.
- The outer electrons in each atom can easily move from one atom to the next, forming a 'sea of delocalised' (free) electrons.
- Strong electrostatic attraction between the negatively charged electrons and positively charged ions bond the metal ions together.