Vertical columns in Modern periodic table are called groups and horizontal rows are called periods. As we move down the group metallic character increases as the atomic size increases, force of attraction between nucleus and outermost shell decreases so tendency to lose ions increases. When we move left to right in period, the number of electrons in the same shell increases leading to greater electrostatic attraction between nucleus and outermost electron. This increased attraction pulls the outermost electron closer to the nucleus, thereby decreasing the atomic size.
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