The Seven Periods

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1. First period

The first period has two elements - hydrogen and helium.

2. Second and third period

The second and third periods are known as short periods. Each consists of eight elements. The second period starts with lithium (Z = 3) and ends with inert gas neon (Z = 10). The third period starts with sodium (Z = 11) and ends with argon (Z = 18).

3. Fourth and fifth period

These two periods are long periods containing eighteen elements each. The fourth period starts with potassium (Z = 19) and ends with krypton (Z = 36). The fifth period starts with rubidium (Z = 37) and ends with xenon (Z = 54).

4. Sixth period

The longest period is the sixth period that contains thirty-two elements starting with cesium (Z = 55) and ending with radon (Z = 86). This period includes 14 lanthanide elements (Z = 58 to 71). The lanthanides are all placed in the same place due to their similar properties.

5. Seventh period

The seventh period is incomplete and contains nineteen elements (Z = 87 to 105). Two elements with atomic number 106 and 107 have been discovered but have not been named. The elements (Z = 87 to 92) are naturally occurring while the elements with atomic numbers 93 to 107 are artificially prepared. The elements after actinium (Z = 89 to 91) are called actinides. Elements after uranium (Z = 92) are called transuranic elements. All the actinides and most of the transuranic elements are radioactive (substances whose nucleus undergoes disintegration).