Periodic Trends in Practice

Let's continue our examination of the trends in atomic properties on the periodic table. This time, we will move from the basic properties of ionization energy and atomic size to see how periodic variation plays out when elements undergo chemical reactions.

Tendency to Lose Electrons

Tendency to Gain Electrons

Next let's consider the opposite of losing electrons, which is, of course, the gaining of electrons. Atoms can attract additional electrons if there is room for them in the valence energy level. When an extra electron moves into the valence shell, it can feel the attraction exerted by the effective nuclear charge. Because the effective nuclear charge is largest for the elements on the right side of the periodic table, those atoms provide the greatest attraction for electrons and have the greatest tendency to gain electrons.

Thus the tendency of atoms to gain electrons increases as we go from left to right across the periodic table. At least it increases until we get to the inert gases. There it drops off to zero because there is no room for additional electrons in the valence energy level. A new electron would have to start a new energy level, but there would not be an additional proton in the nucleus to provide any effective nuclear charge.

As we look at elements going down the periodic table, the effective nuclear charge remains the same, so the increase in the number of energy levels is the important factor. The tendency of atoms to gain electrons decreases as we go down the periodic table. The reason for this is simply that with the larger atoms the added electron is not as close to the nucleus and therefore the attractive force exerted by the effective nuclear charge is not as powerful as it is in the smaller atoms.

Practice Comparing Tendencies to Gain Electrons

For each of the following sets of atoms, decide which has the least and which has the greatest tendency to gain electrons and why. (These are also shown in exercise 11 in your workbook.) Check your answers below and then continue with the lesson.

a. Li, C, N

b. C, O, Ne

c. Si, P, O

d. K, Mg, P

e. S, F, He

Answers for Comparing Tendencies to Gain Electrons

a. Li, C, N: Li has the least tendency to gain electrons because it has the lowest effective nuclear charge (and all use the same number of energy levels). N has the greatest tendency to gain electrons because it has the highest effective nuclear charge (and all use the same number of energy levels).

b. C, O, Ne: Ne has the lowest tendency to gain electrons because its outer energy level is full and there is no room for an additional electron. O has the greatest tendency to gain electrons because it has a higher effective nuclear charge than C (and both use the same number of energy levels).

c. Si, P, O: O has the greatest tendency to gain electrons because it has the highest effective nuclear charge and uses the smallest number of energy levels. Si has the lowest tendency to gain electrons because it has the lowest effective nuclear charge and is tied (with P) for using the most energy levels.

d. K, Mg, P: P has the greatest tendency to gain electrons because it has the highest effective nuclear charge and is tied (with Mg) for using the smallest number of energy levels. Neither Mg nor K have much attraction for electrons, but K has the lowest tendency to gain electrons because it has the lowest effective nuclear charge and uses the most energy levels.

e. S, F, He: He has the lowest tendency to gain electrons because its outer energy level is full and there is no room for an additional electron. F has the greatest tendency to gain electrons because it has a higher effective nuclear charge and uses fewer energy levels than S.

Ionic Size

When atoms gain or lose electrons, the atom becomes an ion. When an atom gains an electron, it becomes a negatively charged ion that we call an anion, as you have learned. Anions are larger in size than their parent atoms because they have one or more additional electrons, but without an additional proton in the nucleus to help moderate the size.

When an atom loses an electron, it becomes a positively charged ion called a cation (again, these terms should be familiar from Lesson 5). Cations are smaller than their parent atoms because they have lost electrons (often the entire outermost energy level) and the electrons that remain behind simply don't take up as much room.

Practice Comparing Ionic Sizes

Try your hand at the making the following comparisons (also shown in exercise 12 in your workbook), based on your understanding of ionic size comparisons and without reference to the wall chart, except to check your answers. Answers also follow on this page.

For each of the following sets of atoms and ions, decide which is the smallest and which is the largest.

a. Na, Na⁺

b. Cl, Cl^-

c. Na⁺, Cl^-

d. H⁺, H, H^-

e. Fe²⁺, Fe³⁺

f. F^-, Ne, Na⁺

Answers for Comparing Ionic Sizes

a. Na is largest, Na⁺ is smallest.

b. Cl is smallest, Cl^- is largest.

c. Na⁺ is smallest, Cl^- is largest.

d. H⁺ is smallest, H, H^- is largest.

e. Fe²⁺ is largest, Fe³⁺ is smallest.

f. F^- is largest, Ne, Na⁺ is smallest.