Oxygen (O), Sulphur (S) and Selenium (Se) have similar chemical properties because they all have the same number of electrons in their outermost shells.
Atoms with the same number of electrons in their outermost shells can be found in the same group.
A group consists of elements arranged in the same column (from top to bottom) on the periodic table. Elements on the same group have similar chemical properties.
Oxygen (O), Sulphur (S) and Selenium (Se) are in group eight. All three elements are highly electronegative and have 6 electrons in their outermost shell. They also carry ab charge of -2.
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