Sharing electrons

A molecule is a neutral group of two or more atoms joined together, which can exist on its own and shows the substance's properties: H₂, HCl. Some elements, like helium, exist as single atoms, because they are already stable.

"Why do atoms join at all?" An atom with fewer than 8 electrons in its outer shell (or fewer than 2 in a K shell) is not stable. It becomes stable by sharing electrons or by giving or taking them. Joining lowers the total energy: like restless people settling down. The force that holds the atoms together is a chemical bond.

A covalent bond is a shared pair of electrons. Hydrogen has only 1 electron and can't give it away, so two hydrogen atoms share: each puts in one, and both now feel they have 2. We draw it as H–H, a single bond. Chlorine has 7 valence electrons; two chlorine atoms share one each: Cl–Cl.

Count the dots. Draw each atom's outer electrons as dots, then circle the shared pairs and count round each atom.

• Oxygen (2, 6) needs 2 more: two oxygen atoms share two pairs, a double bond, O=O.
• Nitrogen (2, 5) needs 3 more: two nitrogen atoms share three pairs, a triple bond, N≡N.

One shared pair is drawn as one line; two pairs, two lines; three pairs, three lines.

Compounds work the same way:

• HCl: H and Cl share one pair: H–Cl.
• H₂O: oxygen shares one pair with each of two hydrogens.
• CO₂: carbon shares two pairs with each oxygen: O=C=O. Count: 8 round each oxygen, 8 round the carbon.
• NH₃: nitrogen makes three N–H bonds.

Neon (2, 8) already has a full octet, so it neither shares nor gives away electrons.

The book adds a thought: "Together we are strong." Just as atoms share electrons to form strong bonds, people who share and care build strong friendships, a stronger community, and a strong nation.