Elements in the same group have the same number of electron shells
The non-metallic properties of the elements tend to decrease across each period
Elements in the same period have the same number of valence electrons
The valence electrons of the elements increase progressively across the period
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Here is an explanation:
Across the table means from left to right.since the atomic number increases by one as we move from one element to the other across, the valence electron(s) also increases likewise e.g.period 3 elements;Na[2,8,(1)],Mg[2,8,(2)],Al[2,8,(3)].....Ar[2,8(,8)].these electrons are added onto the same shell making d atoms heavier progressively.
Periods are also known as horizontal rows and anything horizontal move from left to right...so as the elements in the same period move horizontally i.e from left to right, they increase and an atom is added
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