Using the table to predict

Read an element from its place

The strength of the table is this. If we know the position of an element, we can say a lot about it, even if we have never seen it.

Example. An element has atomic number $4$.

1. The arrangement is $2, 2$.
2. Two shells: period 2. Two outer electrons: group 2.
3. The valency is $2$, and it forms $\text{X}^{2+}$.
4. It is a metal, an alkaline earth metal, like magnesium. Its oxide is basic: $\text{XO}$.
5. Its chloride is $\text{XCl}_2$.

It is beryllium.

Example. An element has the arrangement $2, 7$.

1. Group $17$, period $2$: a halogen, fluorine.
2. The valency is $1$. It forms $\text{X}^-$, and it makes $\text{F}_2$.
3. It is a non-metal.
4. It is the most reactive halogen. It would displace all the others from their salts.

Estimate from the neighbours

A property often changes steadily down a group, so we can estimate the middle one as the average of the neighbours. This is just Dobereiner's idea, used on any property.

The estimates are near. But they are not exact, since the change is not perfectly even.

You should find that each estimate falls a little short of the real radius, because the steps down the group get smaller.

The transition metals

Between groups $2$ and $13$, from period $4$ onwards, there are ten columns: groups $3$ to $12$. These are the transition metals, such as iron, copper, zinc, nickel and gold. They are hard, strong metals that conduct well. In their compounds, they often have more than one valency: iron makes $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$. Many of their compounds are coloured, and many of them act as catalysts. Iron is used for ammonia, and nickel for the hydrogenation of oils, which we saw in Chapter 8.

What the table cannot do

• Hydrogen is hard to place. With one outer electron, it looks like group $1$, but it is a gas, and it makes $\text{H}_2$ like the halogens. In our table, it is placed at the top of group $1$ and kept apart.
• The estimates by the average give only rough values.
• The rows for the lanthanoids and actinoids, which are placed in two rows below the table, are outside this chapter.

Practise with new numbers

Try it in Python

This program estimates a property from the two neighbours, and it compares it with the real value.

Remember these

• The position tells the valency, the kind of ion, the formula of the oxide and chloride, and the reactivity.
• A property in the middle of a group can be estimated by the average of its neighbours.
• Transition metals have more than one valency. Hydrogen has no perfect place.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. Element X is 2, 8, 1. Predict its valency and the formula of its oxide.

2. Why does an estimate from two neighbours not give the exact value?

3. Name two transition metals, and say what is unusual about their valency.

4. Where are the transition metals in the table?