Topic 1.8 Valence Electrons and Ionic Compounds: relate the number of valence electrons to an element's group and reactivity, and predict the ions main-group elements form and the formulas of the ionic compounds they make.

What this topic is asking

The College Board (Topic 1.8) wants you to connect an element's valence electrons to its position in the periodic table, its reactivity, and the ions it forms, and then to use those ions to write the formulas of ionic compounds. This topic ties Unit 1's atomic structure to the bonding of Unit 2.

Valence electrons

For a main-group (s- and p-block) element, the number of valence electrons follows the group: group 1 has 1, group 2 has 2, group 13 has 3, up to group 18 with 8 (helium has 2). This is why elements in the same group react similarly: they have the same number of valence electrons.

Valence electrons and reactivity

This explains reactivity patterns. Group 1 metals lose one electron readily, so they are very reactive; group 17 non-metals need just one electron to complete their octet, so they too are very reactive. Group 18 noble gases already have full shells, so they are largely unreactive.

Predicting ion charges

For main-group elements, the charge of the common ion follows from reaching the nearest noble-gas configuration:

• Group 1 forms $1+$, group 2 forms $2+$, group 13 forms $3+$.
• Group 15 forms $3-$, group 16 forms $2-$, group 17 forms $1-$.
• Group 14 elements usually share rather than fully transfer electrons.

Transition metals can form more than one charge (for example iron as $\text{Fe}^{2+}$ or $\text{Fe}^{3+}$), so their charge is given by a Roman numeral in the name.

Writing ionic-compound formulas

An ionic compound must be electrically neutral, so the total positive charge equals the total negative charge. To write a formula, find the smallest whole-number ratio of cations to anions that balances the charges. A quick route is the crossover: use the magnitude of each ion's charge as the other ion's subscript, then simplify.

The chemistry behind the bookkeeping is electron transfer driven by the same Coulombic and effective-nuclear-charge ideas from earlier topics: a low-ionization-energy metal gives up electrons that a high-electronegativity non-metal accepts, and the resulting oppositely charged ions attract in a lattice. So this single topic is where atomic structure (which electrons are loosely held), periodic trends (which elements lose or gain electrons most readily) and bonding (the ionic compounds that result) all meet, which is exactly why the College Board places it at the end of Unit 1 as the hinge into Unit 2.

Try this

Q1. State the number of valence electrons and the common ion charge for a potassium atom (group 1). [2 points]

• Cue. 1 valence electron; it loses it to form $\text{K}^{+}$.

Q2. Predict the formula of the ionic compound formed between calcium and chlorine. [1 point]

• Cue. $\text{Ca}^{2+}$ and $\text{Cl}^{-}$ give $\text{CaCl}_2$.