Consider the following elements :
Na, Ca, Al, K, Mg, Li
(a) Which of these elements belong to the same period of the periodic table ?
(b) Which of these elements belong to the same group of the periodic table ?
(a) The elements Na, Mg and Al belong to the same period. They belong to the 2nd period of the modern periodic table.
(b) The elements Li, Na and K belong to the same group, they belong to the first group of the modern periodic table.