Chemical bonds form by transferring or sharing electrons

1. Atoms bond in ways that give them a full outer shell, like the outer shell of a noble gas.
2. A metal bonding with a non-metal transfers electrons, forming ionic bonds.
3. Non-metals bonding with each other share electrons, forming covalent bonds.
4. Metal atoms bonding together release their outer electrons into a shared pool, forming metallic bonds.
5. In every case the bond is an electrostatic attraction between opposite charges.

Ionic bonds hold oppositely charged ions together

1. When sodium reacts with chlorine, each sodium atom transfers its one outer electron to a chlorine atom.
   1. Sodium becomes ${Na}^{+}$, with the arrangement $[2.8{]}^{+}$, and chlorine becomes ${Cl}^{-}$, with the arrangement $[\mathrm{2.8.8}{]}^{-}$.
2. The ions attract each other in all directions, so they build a giant structure.
3. A grain of salt contains millions of ions, so the formula NaCl gives the ratio of ions rather than a molecule.
4. In magnesium oxide, magnesium transfers two electrons to oxygen, forming ${Mg}^{2+}$ and $O^{2-}$ ions.

Covalent bonds form molecules, giant structures and polymers

1. A covalent bond forms when two non-metal atoms each put one electron into a shared pair.
   1. The shared pair is attracted to both nuclei, which holds the two atoms together.
2. In simple molecules such as $H_2$, ${Cl}_2$, $H_{2}O$ and ${CH}_4$, a small, fixed number of atoms are joined by covalent bonds.
   1. Separate molecules are held to one another only by weak intermolecular forces, which are much weaker than the covalent bonds inside them.
3. In giant covalent structures such as diamond and silicon dioxide, huge numbers of atoms are joined by covalent bonds in a continuous network.
4. Polymers such as poly(ethene) are very long molecules in which thousands of atoms are joined in chains by covalent bonds.
   1. Neighbouring polymer chains are held together by intermolecular forces, which are stronger than between small molecules because the chains are so long.

Metallic bonding involves a sea of delocalised electrons

1. In a metal, the atoms are packed closely in a regular arrangement.
2. Each atom gives up its outer electrons, leaving positive metal ions.
3. The released electrons spread through the whole metal as a sea of electrons.
4. This strong electrostatic attraction between positive ions and delocalised electrons is called metallic bonding.
5. Ionic, giant covalent and metallic substances are giant structures, while simple molecules and polymers are made of separate molecules.

Dot and cross diagrams show outer electrons

1. Electrons from one atom are drawn as dots and electrons from the other atom as crosses, so it is clear where each came from.
2. For a binary ionic compound, draw the outer shell of each ion inside square brackets, with the charge written outside at the top right.
   1. In $MgO$, magnesium's two outer electrons appear as crosses in the outer shell of oxygen, giving $[Mg{]}^{2+}$ and $[O{]}^{2-}$ with eight outer electrons.
3. For a simple covalent molecule, draw the outer shells overlapping, with each shared pair of one dot and one cross inside the overlap.
   1. In $H_{2}O$, oxygen shares one pair with each hydrogen atom, leaving oxygen with two pairs of outer electrons that are not shared.
4. Finish by counting the outer electrons around each atom to check that every outer shell is full.