Oxygen has an atomic number 8, because it has 8 protons and 8 electrons.
The first shell of an atom can hold up to 2 electrons but oxygen has 8 electrons, in that eight electrons 2 are in the first shell, so it has 6 more electrons left. The second shell can hold up to 8 electrons, oxygen has only 6 more electrons after the first shell is full, so it will have 6 electrons in the second shell
From this we know that oxygen has 2 shells so it is in period 2, and by counting from left to right, the sixth box in period 2 lies on group 16
Therefore Oxygen lies on group 16 and period 2
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