Answer:
The elements on the left-side of the periodic table are relatively electron deficient. So due to their comparatively low effective nuclear charges (the net positive charge of the protons minus the shielding core electrons below the valence level), their electrostatic hold on these electrons are weak.
Elements further right on the period table though, have higher effective nuclear charges and stabilize electrons more effectively. Which leads to localized covalent bonding and the formation of molecules.
The right side contains non metals while the left side contains metals.
Metals lose electrons (negative electrons). They now have more protons, therefore making the ion positive.
Non metals gain electrons (positive electrons). So the ion has more electrons than protons which makes the ion negative.