Mendeleev's periodic table

A card game

The Russian chemist Dmitri Mendeleev wrote the properties of each element on a card, and he played with the cards, like a game of patience, until the similar elements were together. In $1869$, he published a table.

Mendeleev's periodic law

Mendeleev's rule, the periodic law, was:

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

"Periodic" means that they repeat after a regular step.

How the table was built

• He put the elements in order of increasing atomic mass.
• A horizontal row is a period. A vertical column is a group. There were $8$ groups and $6$ periods in his first table.
• Elements in the same group have similar properties.
• He tested each place with the formulae of the hydrides and oxides of the element. A group has the same formulae, such as $\text{Na}_2\text{O}$ and $\text{K}_2\text{O}$ for group I.

What made it a success

1. He left gaps. If an element did not fit a place, he did not force it. He left a gap, and he said that an element would be found. He gave it a name, eka (the Sanskrit word for "one") with the name of the element above it: eka-aluminium is the element one place below aluminium.

2. He predicted properties. He wrote the mass and other properties of the missing elements. Later, the elements were found, and the match was very close.

3. The noble gases fitted in. Helium, neon and argon were found at the end of the $1800\text{s}$. They went into a new group, with no change to the rest of the table.

4. He corrected masses. Where the atomic mass of an element did not fit, he suggested that it was measured wrongly. He was often right.

You should find two places where the mass falls although the atomic number rises: argon to potassium, and cobalt to nickel.

The limits of the table

• Hydrogen had no fixed place. It is like group I, since it has one electron. It is also like the halogens, since it makes the molecule $\text{H}_2$.
• Isotopes of the same element have different masses. In a table of atomic mass, each would need its own place. But they are the same element.
• The order of mass was not always right. Argon ($39.9\ \text{u}$) has a higher mass than potassium ($39.1\ \text{u}$), but it fits before potassium. Tellurium ($127.6\ \text{u}$) and iodine ($126.9\ \text{u}$) are the same.
• There was no way to say how many elements were still to be found.

All of these were solved when the table was arranged by a different number. We see it in the next lesson.

Practise with new numbers

Try it in Python

This program finds the error of Mendeleev's predictions.

Remember these

• Periodic law (Mendeleev): the properties of elements repeat in order of atomic mass.
• Rows are periods, and columns are groups. Elements of a group are alike.
• He left gaps and predicted eka-aluminium (gallium), eka-silicon (germanium) and eka-boron (scandium).
• Limits: the place of hydrogen, isotopes, and pairs such as argon and potassium.

Check yourself

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

1. What does the prefix eka mean in eka-aluminium?

2. Tellurium, $127.6\ \text{u}$, is heavier than iodine, $126.9\ \text{u}$. Why did Mendeleev still put tellurium first?

3. Why could Mendeleev's table not say how many elements were still missing?

4. Scandium was predicted at $44\ \text{u}$ and found at $45.0\ \text{u}$. What was the error, as a percentage of the true value?