What does the period tell us on the periodic table?

The period tells us 3 things:
# As you go from left to right across each period, the number of protons in the nucleus increases # As you go from left to right across each period, the reactivity of each element decreases (E.g. Sodium is very reactive but argon is very stable as it is a noble gas) ∴ tells us about the reactivity # It tells us the number of shells there are for each element (E.g. Magnesium has 3 shells [2.8.2], ∴it is in the 3rd period)