Why classify the elements

Too many elements to remember

In 1800 chemists knew about 31 elements. By 1865 they knew 63, and today we know 118. Learning the chemistry of each one separately is impossible. Chemists needed a way to group similar elements, so that knowing one member of a family tells you about the rest. This search for order led, step by step, to the periodic table.

Dobereiner's triads

In 1829 Johann Dobereiner noticed groups of three elements with similar properties. In each group, the atomic mass of the middle element was close to the mean of the other two. He called these groups triads.

• Lithium, sodium, potassium: $6.94$ u, $22.99$ u and $39.10$ u. The mean of the outer two is $23.02$ u.
• Chlorine, bromine, iodine: $35.45$ u, $79.90$ u and $126.90$ u. The mean of the outer two is $81.18$ u.

The rule worked for only a few families, so it was not taken further. But it was the first sign that atomic mass and properties are linked.

Set the masses of chlorine, bromine and iodine, then try numbers of your own. How far from the mean can the middle element be before the rule stops looking useful?

Newlands' law of octaves

In 1865 John Newlands listed the known elements in order of increasing atomic mass. He saw that every eighth element had properties like the first, just as the eighth note of a musical scale repeats the first. He called it the law of octaves.

Starting from lithium, the eighth element is sodium, and the eighth after sodium is potassium: three alkali metals. The pattern held only as far as calcium. After that, elements with very different properties fell into the same column, and the idea was laughed at. In 1887 the Royal Society gave Newlands its Davy Medal for it.

Lothar Meyer's curves

Lothar Meyer plotted physical properties, such as the atomic volume, the melting point and the boiling point, against the atomic mass. The graphs rose and fell in repeating waves. Similar elements sat at similar points on each wave. By 1868 he had drawn a table very like Mendeleev's, but Mendeleev published first.

Mendeleev's periodic law

Dmitri Mendeleev arranged the elements in order of atomic mass, in rows and columns, so that elements with similar properties fell in the same column. He stated:

The properties of the elements are a periodic function of their atomic masses.

Mendeleev did two bold things. He did not follow the order of atomic mass blindly. Where the properties disagreed, he trusted the properties. And he left gaps for elements not yet found, and predicted what they would be like.

He named two missing elements eka-aluminium and eka-silicon: one step below aluminium and below silicon. When gallium (1875) and germanium (1886) were found, they matched his predictions closely.

Predictions like these turned his table from a list into a tool. He also corrected the atomic masses of some elements, such as beryllium and indium, to fit their places.

Practise with new numbers

Try it in Python

The program checks three triads. It finds the mean of the outer masses and how far the middle element is from it.

Common mistakes

• Thinking Mendeleev's table used the atomic number. Atomic numbers were not known in 1869. He used atomic masses and properties.
• Saying Mendeleev never broke the mass order. He did, where the properties demanded it. Tellurium ($127.6$ u) comes before iodine ($126.9$ u).
• Mixing up the early ideas. Triads are about three elements and the mean mass. Octaves are about every eighth element.
• Calling the gaps mistakes. The gaps were places kept for elements still to be found.

Remember these

• Dobereiner: triads, the middle mass near the mean of the other two.
• Newlands: the law of octaves, every eighth element alike, true only up to calcium.
• Lothar Meyer: physical properties rise and fall in waves with the atomic mass.
• Mendeleev: properties are a periodic function of atomic mass. He left gaps and predicted gallium and germanium.

Check yourself

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

1. Why could Dobereiner's idea not organise all the elements?

2. Starting from fluorine in Newlands' list, which element is the eighth one, and does it behave like fluorine?

3. What did Mendeleev do when the order of atomic mass disagreed with the properties?

4. Why did the discovery of gallium matter so much?

5. What did Lothar Meyer plot, and what did he see?