Write down the electronic configuration of the 3Li, 14Si, 2He, 11Na, 15P from the given atomic numbers. Which of these elements belongs to the period 3? Answer with explanation.

The electronic configuration of these elements are as given below:

Electronic configuration of Li is 1s²2s¹

That of Si is 1s²2s²2p⁶3s²3p²

Electronic configuration for He = 1s²

Electronic Configuration of Na is 1s²2s²2p⁶3s¹

That of P is 1s²2s²2p⁶3s²3p³

From this it is seen that elements Na, Si and P have the same value as the principle quantum number of the outermost shell.