Which of these describes the periodic trend of the first ionization energy for metals? Question 4 options:
1. From left to right across a period of elements, the first ionization energy increases. If the valence shell of an atom is less than half full, it requires less energy to lose an electron than to gain one. Conversely, if the valence shell is more than half full, it is easier to pull an electron into the valence shell than to donate one. From top to bottom down a group, the first ionization energy decreases. This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius. Important exceptions of the above rules include the noble gases, lanthanides, and actinides. As for the transition metals there is little variance among them across the period and up and down a group.
2. First ionization energy decreases from left to right within a period. This is caused by the increase in the number of protons and electrons across a period. One proton has a greater effect than one electron; thus, electrons are pulled towards the nucleus, resulting in a smaller radius. First ionization energy increases from top to bottom within a group. This is caused by electron shielding.
3. First ionization energies decrease from left to right across a period. This is caused by the decrease in radius (caused by Zeff) of the atom that allows the outer electrons to ionize more readily. First ionization energies increase down a group. Electron shielding causes the atomic radius to increase thus the outer electrons ionizes more readily than electrons in smaller atoms.
4. The first ionization energy of the elements within a period generally increases from left to right. This is due to valence shell stability. The first ionization energy of the elements within a group generally decreases from top to bottom. This is due to electron shielding. The noble gases possess very high first ionization energies because of their full valence shells as indicated in the graph. Note that helium has the highest ionization energy of all the elements.