In contrast, atoms get smaller as you go across the periods because the nuclear charge effect outweighs the shielding effect, so the attraction between the nucleus and the electron is greater and the atom shrinks in size. Atoms and ions get bigger as you go down the columns because the shielding effect outweighs the effects of the nuclear charge, so the attraction between the nucleus and electrons is weaker and the atom expands in size. These tendencies tell you about atom size. Lastly, the nuclear charge increases as you go across and down the table, while the shielding stays constant across the periods, but increases as you go down the columns. You can use trends in ionization energy and electronegativity to predict whether two elements will form ionic or covalent compounds. The difference in electronegativity between two elements will determine whether they exchange electrons (ionic compounds) or share electrons (covalent compounds). So, fluorine is more likely to take electrons from another element than lithium. Like ionization energy, electronegativity decreases as you go down a column and increases as you go across a period from left to right. So, elements in groups 1 and 2 will tend to form ionic compounds. When you compare elements in groups 1 and 2 (on the left) with those in 16 and 17 (on the right), you'll find that the elements in the first groups have lower ionization energies, won't hold on to their valence electrons as tightly and will tend to form cations. If you look at the periodic table, ionization energy tends to decrease as you move down a column and increase as you move across a period from left to right. Let's see how these factors can help predict what type of chemical reactions any two elements will make. The more energy levels between the valence electrons and the nucleus, the more shielding. Shielding: inner electrons tend to shield the outer electrons from the attractive force of the nucleus.The more protons, the greater the nuclear charge.
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