The horizontal rows in the periodic table are called periods.Period 1 contains only hydrogen and helium. Period 2 runs from lithium to neon and period 3 from sodium to argon.All the elements in the same period have the same number of electron shells. The period number tells you which shell is the outer shell, so all period 3 elements have their outer electrons in the third shell.Moving across a period, one proton and one electron are added each time, so the number of outer electrons increases. This gives repeating trends across each period in properties such as atomic radius and first ionisation energy, which is called periodicity.