Why do atoms bond?
Most atoms are chemically unstable because their outermost shell (valence shell) is not full. To become stable, an atom rearranges its valence electrons so that its electron arrangement copies that of the nearest noble gas. For the smallest atoms this means a full first shell of two electrons (a duplet, like helium); for most other atoms it means eight valence electrons (an octet, like neon or argon).
Three ways to reach stability
- Losing electrons — metals with few valence electrons donate them, forming positive ions (cations), e.g. Na → Na+ + e−.
- Gaining electrons — non-metals with nearly-full shells accept electrons, forming negative ions (anions), e.g. Cl + e− → Cl−.
- Sharing electrons — two non-metals share pairs of electrons so each achieves an octet, e.g. in Cl2.
Electron transfer produces an ionic bond; electron sharing produces a covalent bond. Which route an element takes depends on its position in the periodic table and how many valence electrons it has.
Key idea / 要点
The octet rule: atoms tend to gain, lose or share electrons until they are surrounded by eight valence electrons (or two, for H, Li, Be, B). This stable arrangement lowers the energy of the system.
Worked reasoning
Worked example / 例题
Magnesium has electron arrangement 2.8.2. It has 2 valence electrons. Losing them is easier than gaining six, so Mg forms Mg2+ (2.8, like neon). Oxygen (2.6) needs 2 more electrons, so it forms O2− (2.8). Mg donates 2 electrons to O, giving the compound MgO.
Remember / 记住
- Metals usually lose electrons; non-metals usually gain or share.
- The number of valence electrons decides charge on the ion or number of bonds.
- Noble gases already have full shells, so they rarely form compounds.