The nucleus is at the centre which contains protons and neutrons, orbiting the nucleus are electrons. Electrons are arranged in energy levels or shells. Protons have a positive charge, electrons are negatively charged and neutrons have no charge at all.
The total number of protons is the same as the number of electrons. This number is called the atomic number or proton number of the element. Elements are arranged in order of their atomic numbers in the periodic table.
Electrons in atoms are in energy levels that can be represented by shells. Electrons in the lowest or first energy level are in the shell closest to the nucleus. The first energy level can hold two shells. The second and third energy level can hold eight.
The noble gases are unreactive because their atoms have stable arrangements of electrons. Atoms of other elements can achieve stable electronic structures by gaining or losing electrons to form ions, or by sharing electrons to form covalent bonds. When an element in Group 1 reacts with an element in Group 7, an electron is transferred between atoms to form ions with the electronic structure of a noble gas.
Compounds are substances in which elements are chemically combined.
When elements react their atoms achieve stable arrangements of electrons.
Atoms gain or lose electrons to form ions or share electrons to form covalent bonds.
Ions are electrically charged particles formed when atoms lose or gain electrons. This loss or gain leaves a complete highest energy level, so the electronic structure of an ion is the same as that of a noble gas - such as a helium, neon or argon.
Metal atoms and non-metal atoms go in opposite directions when they ionise:
Metal atoms lose the electron, or electrons, in their highest energy level and become positively charged ions.
- Non-metal atoms gain an electron, or electrons, from another atom to become negatively charged ions.
Ionic compounds have high melting and boiling points.
Compounds made of ions have giant structures that are very regular. Strong force of attraction act throughout the lattice to hold the ions together.
A covalent bond forms when two non-metal atoms share a pair of electrons. The electrons involved are in the highest occupied energy levels - or outer shells - of the atoms. An atom that shares one or more of its electrons will complete its highest occupied energy level.
Covalent bonds are strong - a lot of energy is needed to break them. Substances with covalent bonds often form molecules with low melting and boiling points, such as hydrogen and water.
A covalent bond can be represented by a straight line or dot and cross diagram.
A covalent bond is formed when two atoms share a pair of electrons.
The number of covalent bonds an atom forms depends on the number of electrons it needs to achieve a stable electron arrangement.
Many covalently bonded substances consist of small molecules, but some have giant structures.