Rows and columns
The modern periodic table arranges elements by increasing proton number. Horizontal rows are called periods, and vertical columns are called groups. There are 7 periods and 18 groups.
What the position tells you
An element's position is decided by its electron arrangement:
- The period number equals the number of occupied electron shells.
- The group number (for Groups 1, 2 and 13–18) relates to the number of valence electrons (outermost shell).
Worked example
Chlorine has proton number 17 and electron arrangement 2.8.7. It has 3 occupied shells, so it is in Period 3. It has 7 valence electrons, so it is in Group 17.
Metals, non-metals and blocks
Metals lie on the left and centre, non-metals on the upper right, separated by a zig-zag line of semi-metals (metalloids). Elements in the same group have the same number of valence electrons, so they show similar chemical properties. Going across a period, properties change gradually from metallic to non-metallic.
Remember
- Period number = number of electron shells.
- Group number relates to valence electrons.
- Same group = similar properties (same valence electrons).
- Arranged by increasing proton number.