Ions and ionic bonding.

Step 1 of 6

A gap and a spare

In the last Unit the noble gases, with full outer shells, hardly reacted at all. Most other atoms reach a full shell when they meet: one gives electrons away and the other takes them.

The Scene shows a chlorine atom, 2,8,7: it is one electron short of a full shell, and the free place in its outer shell is marked. Sodium, 2,8,1, carries one spare electron in a third shell. Predict what happens when they meet, then move Bring in sodium to Meets sodium.

Step 2 of 6

Counting the ions

An atom that has given or taken electrons is no longer neutral: it is an ion. Sodium is left with $11$⁠ protons and $10$⁠ electrons, so its charge is $+1$⁠: a sodium ion, Na⁺, arranged 2,8, like neon. Chlorine took an electron, so its charge is $-1$⁠: a chloride ion, Cl⁻, arranged 2,8,8, like argon.

Notice the sizes too: the sodium ion lost a whole shell and is much smaller, and the chloride ion, with an extra electron in the same shell, has grown. The ions, positive and negative, attract each other with a strong electric force. That attraction is the ionic bond.

Step 3 of 6

A full shell says no

Now the Scene shows argon, 2,8,8. Predict what happens when sodium comes close.

Step 4 of 6

The charge from the group

Each atom gives or takes the number of electrons that most quickly brings it to a full shell. Metals, with few outer electrons, give them away and become positive ions. Non-metals, close to a full shell, take electrons and become negative ions.

Group	1	2	13	15	16	17
Outer electrons	$1$⁠	$2$⁠	$3$⁠	$5$⁠	$6$⁠	$7$⁠
Charge of the ion	$+1$⁠	$+2$⁠	$+3$⁠	$-3$⁠	$-2$⁠	$-1$⁠
Example	Na⁺	Mg²⁺	Al³⁺	N³⁻	S²⁻	F⁻

The Scene shows fluorine, which took an electron and became F⁻. Oxygen sits next to it in the table, in group 16.

Step 5 of 6

Balancing the charges

An ionic compound is neutral overall: the positive and negative charges cancel. So the ions combine in the ratio that balances them. Na⁺ and Cl⁻ balance one to one: NaCl. A calcium ion, Ca²⁺, and an oxide ion, O²⁻, are also one to one: CaO. But aluminium and oxygen need two Al³⁺, a charge of $+6$⁠, against three O²⁻, a charge of $-6$⁠: Al₂O₃.

The formula of an ionic compound does not describe a molecule but the ratio of the ions. The lattice they sit in comes in the next Lesson.

Step 6 of 6

Small ions hold tighter

When ions pull together, energy is released, shown here as a flash. The smaller the ions, the closer their centres, and the stronger the pull. The Scene shows bromine and sodium: sodium bromide, NaBr. Change Period (row) and compare the flashes and the energy in the note beside the ions.