An ion is an atom that has lost or gained electrons. Figure 1 shows that cations are smaller than their original neutral atoms while anions are larger than their neutral atoms. This can be explained through the shielding effect.
When you remove an electron from an atom there is one less electron involved repelling the other electrons, therefore the shielding effect is less in the cation than in the original atom. Consequently the effective core charge will be greater in the cation, thus pulling the electrons in tighter making the cation smaller the original atom. As shown in Figure 1 the more electrons you remove the smaller the cation becomes. For this reason as you go across a periods section that contains metals the ionic size will decrease because you are taking away more valence electrons to reach a stable configuration.
Also when you add an electron to an atom, there is one extra electron present to repel other electrons, thus increasing the shielding effect in the anion and consequently reducing the effective core charge. Since the the electrons are less tightly held in the size of the anion will be greater than the size of the neutral atom. As shown in Figure 1 the more electrons you add the larger the anion becomes. For this reason as you go across a periods section for non-metals (left to right), the ionic sizes decrease because you are adding less electrons.
When you remove an electron from an atom there is one less electron involved repelling the other electrons, therefore the shielding effect is less in the cation than in the original atom. Consequently the effective core charge will be greater in the cation, thus pulling the electrons in tighter making the cation smaller the original atom. As shown in Figure 1 the more electrons you remove the smaller the cation becomes. For this reason as you go across a periods section that contains metals the ionic size will decrease because you are taking away more valence electrons to reach a stable configuration.
Also when you add an electron to an atom, there is one extra electron present to repel other electrons, thus increasing the shielding effect in the anion and consequently reducing the effective core charge. Since the the electrons are less tightly held in the size of the anion will be greater than the size of the neutral atom. As shown in Figure 1 the more electrons you add the larger the anion becomes. For this reason as you go across a periods section for non-metals (left to right), the ionic sizes decrease because you are adding less electrons.
The trend for ionic radius is similar to that of atomic radius in that as you go down a group the ionic radius increases due to the shielding effect. However, as you go across a period the similarly charged ions decrease in size, it is not a uniform decrease as you go across. This is because when you get to group 15 elements, they accept 3 electrons to make a stable configuration thus making the ionic sizes much larger than then the previous groups who gave away electrons. Therefore going across a period, the cations decrease in size and then the anions are bigger in size but also decrease in size as shown in figure 2.