Atomic and Ionic Radius

Objectives Compare period and group trends of several properties Relate period and group trends in atomic radii to electron configuration

Atomic Radius The electron cloud surrounding the nucleus is based on probability and does not have a defined edge Atomic size is determined by how closely an atom lies to a neighboring atom.

Trends within periods In general, there is a decrease in atomic radii as you move left to right across a period This is caused by the increasing positive charge in the nucleus and the fact that the principal energy level stays the same. Because the electrons are in the same energy level, there is no “sheild” from the increased pull of the nucleus. The result is that the increased nuclear charge pulls the outermost electrons closer to the nucleus.

Look at the trend within the periods.

Trends within groups Atomic radii generally increase as you move down a group. Moving down a group the outermost orbital increases in size along with the increasing principal energy level. The nuclear charge increases, but the electrons are in a larger energy level with more other levels “shielding” in between.

Look at the trend within groups.

Summarize this trend with arrows and descriptive words.

Practice Problems For each of the following pairs, predict which atom is larger? Mg, Sr Sr, Sn Ge, Sn Ge, Br Cr, W

Ionic Radius When an atom gains or loses electrons, it becomes an ion. Because an electron is negatively charged, atoms that lose or gain acquire a net charge If an atom loses electrons it becomes _________ If an atom gains electrons it becomes _________

Ionic Radius Continued When an atom loses electrons, they always become smaller. When atoms gain electrons, they always become larger.

Trends within periods In general as you move left to right across a period, the size of the positive ions gradually decreases. Then, beginning in groups 5A or 6A, the size of the much larger negative ions also gradually decreases.

Look at the trends for ionic size as you move across a period.

Trends within groups As you move down a group, an ion’s outer electrons are in higher principal energy levels, resulting in increased size.

Look at the trends in ionic size as you move down a group

Practice Problems For each of the following pairs, predict which atom or ion is larger Mg, Mg2+ S, S2- Ca2+, Ba2+ Cl-1, I-1 Na+1, Al3+