Patterns Across Periods and Down Groups: Quick Revision

A brief note on Patterns Across Periods and Down Groups. Read it before a test. Try each question at the end before you open its answer.

Valency

Valency is the same down a group and runs $1, 2, 3, 4, 3, 2, 1, 0$ across a period.

Trends

Oxides

Metal oxides are basic, non-metal oxides acidic, and aluminium oxide amphoteric. Across period 3 they turn from basic to acidic.

A worked example

Silicon has the atomic number $14$. Find its valency and the formula of its oxide.

1. Arrangement. $14 = 2 + 8 + 4$: 2, 8, 4.
2. Valency. Four outer electrons: valency $4$.
3. Oxide. Oxygen has valency $2$. Crossing gives $\text{Si}_2\text{O}_4$, which simplifies to $\text{SiO}_2$.

Practice with new numbers

A program to try

The program finds the valency and the oxide from the outer electrons.

Mistakes to avoid

• Taking the valency as the outer electrons for every element. From five on, use $8$ minus them.
• Saying atoms grow across a period. The extra electrons go into the same shell and are pulled in.
• Calling every metal oxide basic. Aluminium oxide is amphoteric.
• Forgetting to simplify a formula. $\text{Mg}_2\text{O}_2$ is MgO.

Check yourself

1. What is the valency of an element with seven outer electrons?

2. Which is bigger, a lithium atom or a fluorine atom?

3. Where in the table are the most non-metallic elements?

4. What kind of oxide does phosphorus form?