Select all that apply.

What is common about the elements of a period?

They have the same number of electron shells.

They have similar chemical characteristics.

They have similar physical characteristics.

They transition from a metal to noble gas.

They have the same number of electron shells.

Explanation:

All elements in the same period have the same number of electron shells. The period to which any element belong automatically tell the number of shells the element is expected to possess. For instance, period 2 has Lithium, Beryllium, Boron, Carbon, etc as members and each possess only two shells.

The answer is

They have the same number of electron shells

They transition from a metal to noble gas. Electron shells are also referred to as atomic orbitals. The number of electrons increase from left to right of the periodic table (until the orbital shell is filled and the atom becomes stable/noble) while the number of atomic orbitals increases from top to bottom of the periodic table.