# Summary of session 3

1. The chemical formulae of many substances can be understood by arguing that their atoms attain noble gas structures by chemical combination.
2. In ionic compounds, this is achieved by the transfer of electrons from one atom to another; in molecular substances, it happens through the sharing of electron pairs in covalent bonds. But in both cases, bonds between atoms consist of shared pairs of electrons. In covalent compounds the sharing is fairly equitable; in ionic compounds it is much less so.
3. In metals, the sharing takes a different form. An 'electron gas' is created by removing electrons from the atoms of the metallic elements. The result is an array of ions steeped in a pool of free electrons. The negatively charged electron gas occupies the space between the ions and pulls them together.

OpenLearn - Discovering chemistry
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