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# Key concepts in bonding

## Valence electrons

The valence electrons of an atom are the electrons found in its outermost electron shell. These are the electrons that can be used to form chemical bonds.

The first electron shell holds up to two electrons while the second shell holds a maximum of eight electrons.

Atoms usually form bonds to obtain a full valence electron shell (or a noble gas electron configuration).

Helium (He), neon (Ne) and argon (Ar) are noble gases. They are elements with complete electron valence shells (i.e. the outermost shells).

A complete valence electron shell holds eight electrons, unless the first electron shell is the valence shell. In this case only two electrons are required for a complete shell.

Helium ($\ce{He}$) is a noble gas that only has one electron shell. Its valence shell contains two electrons. Neon ($\ce{Ne}$) has two electron shells and its valence shell contains eight electrons.

## Octet rule

The octet rule states that atoms tend to combine with other atoms to obtain the electron configuration of a noble gas.

The octet rule is valid for atoms with atomic numbers under 20 (i.e. up to calcium).

Atoms tend to achieve the noble gas electron configuration as it is very stable.

Noble gases are usually found as atoms and do not react easily, which reflects the stability of their electron configuration.

There are lots of exceptions to the octet rule, but it is useful for understanding the way that many simple ions and covalent compounds form. Atoms usually donate, receive, or share electrons in bonds in order to satisfy the octet rule.

Halogens (chlorine, bromine and iodine are shown) have seven valence electrons and are highly reactive because only one more electron is needed to complete their valence electron shells.