How chemists arranged elements in a useful table
Early chemists tried to organise elements by their properties. Döbereiner grouped elements in triads, where the middle element’s atomic mass was roughly the average of the other two, as in lithium, sodium and potassium. Newlands noticed that every eighth element had similar properties, his law of octaves, but it worked only up to calcium. Mendeleev arranged elements by increasing atomic mass and left gaps for undiscovered elements, correctly predicting the properties of gallium and germanium.
The modern periodic table arranges elements by increasing atomic number, which removed problems such as the position of isotopes. It has 18 vertical groups and 7 horizontal periods. Elements in a group have the same number of valence electrons and the same valency, while across a period the number of valence electrons increases by one. Moving across a period, atomic size decreases and metallic character decreases, while down a group atomic size and metallic character increase. Metals are on the left, non-metals on the right and metalloids lie along a zig-zag line.
Why it matters
The periodic table is the most important map in chemistry. It lets you predict an element’s valency, the formula of its compounds and whether it is a metal or non-metal just from its position. Mendeleev’s successful predictions showed the power of scientific patterns. This topic leads to detailed periodic trends in Class 11.
Key terms
- Group
- A vertical column of elements in the periodic table with the same number of valence electrons.
- Period
- A horizontal row of elements with the same number of electron shells.
- Modern periodic law
- Properties of elements are a periodic function of their atomic numbers.
- Metalloid
- An element with properties between those of metals and non-metals, such as silicon.