Lesson 6: Periodic Table Trends II
Let's continue our examination of the trends in atomic properties on the periodic table.
Tendency to Lose Electrons | Tendency to Gain Electrons | Electron Affinity | Ionic Size
Tendency to Lose Electrons
Ionization energy measures how difficult it is for atoms to lose electrons but quite often we will want to talk about how easy it is for atoms to lose electrons. A low ionization energy means that it is easy for an atom to lose electrons. A high ionization energy means that it is hard for an atom to lose electrons.
Using this terminology, it gets harder for atoms to lose electrons as you go across the periodic table and it gets easier for atoms to lose electrons as you go down the periodic table. |
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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. |
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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.
Electron Affinity
The electron affinity is a very specific measure of the tendency for atoms to gain electrons. It is the amount of energy released by an electron when the electron joins or attaches to an isolated atom. This measurement is very sensitive to the difference between the types of orbitals and to the number of electrons in those orbitals. Consequently, the zigs and zags in a graph of electron affinity are even more pronounced than in the graph of ionization energy. Therefore we won't deal with the electron affinity trends - they get kind of lost in the fluctuations. You should remember what electron affinity is but don't worry about how it varies as you go across and up and down the periodic table.
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. 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. Cations are smaller than their parent atoms because they have lost electrons (sometimes the entire outermost energy level) and the electrons that remain behind simply don't take up as much room. |
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There is a chart on the wall of the lab that shows not only the sizes of atoms, but also the sizes of ions. You might want to take a look at that. It is on the south wall of the lab. Also note that when comparing cations and anions, the anions are larger.
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.