A Historic Overview: Mendeleev And The Periodic Table

In order of increasing atomic mass, Mendeleev thought about the elements beryllium, boron, carbon, nitrogen, oxygen,

and fluorine. These elements were all different in their physical and chemical properties, thus seeming to belong to

different families. Mendeleev put their cards in a vertical row, with lithium at the top and fluorine at the bottom.

The known element next most massive after fluorine was sodium. It shared many physical

and chemical properties with lithium. They seemed enough alike to be classified as

belonging to the same family. Thus Mendeleev put sodium’s card as the top of a second

column, just to the right of lithium’s card.

From there things worked amazingly well. Mendeleev was thinking about the similar

properties of the next elements. Magnesium, following sodium, had physical and

chemical properties similar to beryllium, which followed lithium. In the same manner,

Mendeleev placed aluminum next to boron; silicon next to carbon; phosphorus next to

nitrogen; sulfur next to oxygen; and chlorine next to fluorine.

Mendeleev must have felt great pleasure in how this card game was turning out.

Repeating patterns are called periodic. Mendeleev eventually called this arrangement

the periodic table of the elements.

Problems and Predictions

Mendeleev encountered the first problem with his model in the next set of elements.

Potassium headed the third column, since its properties were similar to those of sodium

and lithium. Calcium was next, and it fit well with magnesium and beryllium.

??Ti?

The next known element was titanium. According to Mendeleev’s model, it should

have belonged to the same chemical family as boron and aluminum. But titanium’s

properties were similar to those of silicon.

Mendeleev did not give up. He decided to put

titanium in the row with carbon and silicon. He left a

gap next to boron and aluminum. He predicted that an unknown element would some

day be found with an atomic mass between 40 (for calcium) and 48 (for titanium),

whose properties would be similar to those of boron and aluminum.

In fact, in 1878 the element scandium was discovered. Its atomic mass was almost

45, and it had properties as predicted by Mendeleev.

G E N E S I S

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