The Periodic Table · Grade 9

What did Mendeleev actually notice when he arranged the elements?

He arranged the known elements by atomic mass and saw their properties repeat at regular intervals — a similar element turning up again after a fixed number of steps. That is what makes a property periodic. He trusted the pattern enough to leave gaps in the table for elements nobody had found, and described their properties in advance.

Learning objectives

Element, compound, mixture

An element is a substance in which every atom is of the same kind, such as copper or oxygen. It cannot be broken down chemically into anything simpler.

A compound is a substance in which atoms of different kinds are bonded in a fixed ratio, such as water. Its properties are nothing like those of the elements it is built from — hydrogen and oxygen are gases and water is a liquid.

A mixture is things mixed with no chemical bond, such as salt water or air. It can be separated physically and the ratio of its parts is not fixed. The distinction matters here because the periodic table is a table of elements only.

What the arrangement revealed

By the middle of the nineteenth century about sixty elements were known and there was no system among them. Mendeleev wrote each one on a card with its atomic mass and its properties, and laid them out in order of increasing mass.

Then the pattern appeared: elements with similar properties recurred at regular intervals. Lithium, sodium and potassium — all soft metals that react violently with water — fell one below another once the rows were broken in the right place.

That is exactly what a periodic property is: a property that repeats at regular intervals along the arrangement. Not coincidence, and not a quantity that simply grows — a repeat.

The gaps

To keep the pattern intact Mendeleev had to leave boxes empty. He could have packed the known elements in and filled everything; instead he said the pattern was real and that elements were missing which nobody had yet found.

He even described their properties in advance from their neighbours in the table — density, mass, the colour of their compounds. When germanium was discovered a few years later, its properties matched the description with surprising accuracy.

That is the scientific lesson of this page. An idea that explains what is already known is useful; an idea that correctly predicts something nobody has seen is much harder to fake, and therefore much more convincing.

Worked examples

  1. Classify these: gold, water, air, carbon dioxide.

    1. Gold is made of atoms of one kind
    2. Water and carbon dioxide are atoms bonded in a fixed ratio
    3. Air is gases mixed with no chemical bond and in a varying ratio

    Answer: Gold an element; water and carbon dioxide compounds; air a mixture

  2. The density of the elements rises steadily along the arrangement. Is that a periodic property?

    1. A periodic property repeats at intervals
    2. A steady rise does not repeat, it grows

    Answer: No. That is a trend, not periodicity

  3. Mendeleev predicted a missing element below silicon. How could anyone check whether he was right?

    1. The description included mass, density and the properties of its compounds
    2. Those could be measured on the element once found
    3. Germanium was found and its properties measured

    Answer: By comparing measurement against a prediction written before it. That is how a pattern is tested

Common mistakes

Calling a mixture a compound
A compound has a chemical bond and a fixed ratio; a mixture has neither. Salt water is a mixture; the salt itself is a compound.
Thinking periodicity means the property grows continuously
Periodicity is repetition, not a trend. What returns at regular intervals is periodic; what merely increases is not.
Assuming Mendeleev knew about protons
The inside of the atom was not worked out until decades after him. He ordered by mass and saw a pattern whose explanation arrived later.

What to remember

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