Chemical Bonding (Basics) - Chemistry

Why atoms bond

Atoms bond to reach a full outer shell of electrons, because that arrangement is stable, the same arrangement the unreactive noble gases already have. Most atoms are aiming for eight electrons in the outer shell (an octet), though the very smallest are happy with two.

There are two ways to get there. Atoms can transfer electrons, giving them away or taking them, which makes charged ions held together by an ionic bond. Or they can share electrons, which makes a covalent bond. Metals with non-metals tend to transfer; non-metals with non-metals tend to share.

Chemical bond: A strong force of attraction that holds atoms together, formed when atoms transfer or share electrons to gain full outer shells.

Ionic bonding

Ionic bonding happens between a metal and a non-metal. The metal atom loses its outer electrons to become a positive ion (a cation), and the non-metal gains them to become a negative ion (an anion). The oppositely charged ions then attract each other strongly, and that electrostatic attraction is the ionic bond.

Take sodium chloride. Sodium (2, 8, 1) has one outer electron it wants to lose; chlorine (2, 8, 7) needs one to fill its shell. Sodium hands its electron to chlorine:

\[ \text{Na} \rightarrow \text{Na}^{+} + e^{-} \qquad \text{Cl} + e^{-} \rightarrow \text{Cl}^{-} \]

Now both ions have full outer shells. The Na⁺ and Cl^- attract and form NaCl.

Dot-and-cross: NaCl and MgO

A dot-and-cross diagram shows where the electrons come from by drawing one atom's outer electrons as dots and the other's as crosses. For an ionic compound we draw the ions in square brackets with their charge outside.

Sodium chloride (NaCl). Picture sodium's single outer electron (a dot) moving across to chlorine. Sodium is left as [Na]⁺ with an empty outer shell showing (its full 2, 8 inner arrangement remains). Chlorine becomes [Cl]^- with eight outer electrons: its own seven crosses plus the one dot it gained from sodium. One electron transferred, so the ratio is 1 to 1, giving the formula NaCl.

Worked example

Describe the dot-and-cross diagram for magnesium oxide (MgO) and give the charges on the ions.

Magnesium is 2, 8, 2, so it has two outer electrons to give away. Oxygen is 2, 6, so it needs two more.

Both of magnesium's outer electrons (two dots) transfer to oxygen. Magnesium becomes [Mg]²⁺ and oxygen becomes [O]^2-.

The oxide ion now shows eight outer electrons: its own six crosses plus the two dots from magnesium. Both ions have full shells.

Mg²⁺ and O^2-, transferring two electrons, so the formula is MgO (1 to 1).

Common slip

In an ionic diagram, electrons are transferred, not shared. The outer shell of the positive ion should end up empty (its full inner shell is what remains), and every electron the negative ion gained should be drawn as the other atom's symbol (dot or cross).

Simple covalent molecules

When two non-metals bond, neither wants to give electrons away, so they share. Each shared pair of electrons is one covalent bond, and it counts towards a full outer shell for both atoms.

Hydrogen (H₂). Each hydrogen atom has one electron and needs two for a full first shell. They share one pair, forming a single bond, so both now count two electrons. Written H-H.
Water (H₂O). Oxygen (2, 6) needs two more electrons, so it forms one bond to each of two hydrogen atoms, two shared pairs in all. Oxygen reaches eight, each hydrogen reaches two.
Methane (CH₄). Carbon (2, 4) needs four more, so it shares a pair with each of four hydrogen atoms, four bonds. Carbon reaches eight, each hydrogen reaches two.

In a dot-and-cross diagram of a molecule, the shared pair sits in the overlap between the two atoms, drawn as one dot and one cross together, showing one electron came from each atom.

Comparing the two

The bonding type explains the everyday properties:

Property	Ionic compounds	Simple covalent molecules
Made from	Metal + non-metal	Non-metal + non-metal
Melting point	High	Low
State at room temperature	Solid	Often liquid or gas
Conducts electricity?	Yes, when molten or dissolved	No

Where this is assessed

Linking a property back to the bonding, such as "ionic compounds conduct when molten because the ions are then free to move", is exactly the reasoning Criterion A rewards. Correct, clearly labelled dot-and-cross diagrams are assessed under Criterion C communication.

Check yourself

1. Why does a magnesium atom lose two electrons when it bonds? +

Magnesium's configuration is 2, 8, 2, so it has two electrons in its outer shell. Losing both leaves the full 2, 8 arrangement underneath as the new outer shell, which is stable. It becomes an Mg²⁺ ion.

2. How many covalent bonds does carbon form in methane, and why? +

Carbon is 2, 4, so it needs four more electrons for a full outer shell. It shares one pair with each of four hydrogen atoms, forming four covalent bonds, giving CH₄.

3. Explain why sodium chloride only conducts electricity when molten or dissolved, not as a solid. +

Conduction needs charged particles that are free to move. In solid NaCl the ions are locked in a fixed lattice, so it does not conduct. When melted or dissolved the ions break free and can move to carry the charge, so it conducts.

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