Early attempts: Dobereiner's triads and Newlands' octaves

Why do we need a table?

Think of a library. A million books would be of little use if they were thrown in a heap. We put them on shelves, by subject, so that we can find them.

The elements were in the same position. By the early $1800\text{s}$ chemists knew about thirty elements. By $1869$, they knew $63$. Each one had its own properties, and nobody could remember them all. So chemists looked for a pattern, a way to sort the elements into families.

Dobereiner's triads

In $1817$, the German chemist Johann Dobereiner noticed that some elements are like each other, and that they come in threes. He called such a group a triad.

When he put the three elements of a triad in order of atomic mass, the mass of the middle element was close to the average of the other two.

The problem. Dobereiner could find only three triads in all of the known elements. For most elements, there was no triad. So his idea was interesting, but it could not sort all of the elements.

You should find that the triad of lithium, sodium and potassium hits the average almost exactly, while most other threes miss by far.

Newlands' law of octaves

In $1866$, the English chemist John Newlands arranged the elements in order of increasing atomic mass. He found that every eighth element had properties like the first, as in music, where the eighth note of a scale repeats the first. He named his idea the law of octaves.

For example, starting from lithium: lithium, beryllium, boron, carbon, nitrogen, oxygen, fluorine, and sodium. Sodium is the eighth. It is like lithium.

The problems.

• The pattern held only up to calcium. After that, the elements did not fit their columns.
• Newlands thought that only $56$ elements existed. When new elements were found, there was no room for them.
• To make the pattern work, he put two elements in one place, and he put some very unlike elements in the same column. For example, iron is not like oxygen and sulphur, but it was placed under them.

So his law was not accepted at first. But the idea that the properties repeat was right. It is the idea that the periodic table is built on.

Practise with new numbers

Try it in Python

This program tests whether a group of three elements is a triad.

Remember these

• Dobereiner: a triad has three similar elements, and the middle mass is near the average of the other two. Only three triads were known.
• Newlands: in order of atomic mass, every eighth element is like the first. It worked only up to calcium.
• Both showed that properties repeat, but neither could sort all of the elements.

Check yourself

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

1. Phosphorus is $31.0\ \text{u}$ and antimony $121.8\ \text{u}$. What mass does Dobereiner's rule give arsenic, between them?

2. Why is Newlands' idea named after music?

3. Why did Newlands' law fail after calcium?

4. How many triads could Dobereiner find among all the known elements?