What is the most important factor in determining an element's place in the periodic table?
There are many factors but some include:
=> The increasing atomic number (periods, or along)
=> Chemical properties (groups, or down)
if we are looking at periods, for example hydrogen, then helium it is based around the increase in atomic number (amount of protons an element has, amount of protons is also equal to the amount of electrons).
But if we look at groups (eg, hydrogen, lithium, sodium etc.) it is based on reactivity, chemical properties and how many electrons each element has in its outer shell (each element in a certain group should have the same). It starts at the top with the less reactive elements and when it goes down, it gets more reactive (for most groups although there are some exceptions).
There are some great wesbites on BBC Bitesize explaining this in further detail: