Periodic Trends
Certain properties and characteristics of isolated elemental atoms occur in general trends across the periodic table. These trends include metal activity, metallic character, atomic radius, ionic radius, ionization energy, electron affinity, and electronegativity.
Metal activity
In general, the activity of a metal increases down a group and decreases across a period. Francium is the most active element. For the nonmetals of group 17, activity decreases down the group.
Metallic character
In general, the metallic character of an element, specifically a metal, increases down a group and decreases across a period.
Atomic radius
The radius of an atom increases down a group. Electrons added further from the nucleus feel less attraction to it. (The shielding effect makes radius larger.) Radius size decreases across a period because electrons feel a greater pull from the nucleus as the number of protons increases. In this instance, shielding does not increase; it remains the same.
Ionic radius
Radius of ions increases down a group and decreases across a period.
Cations (positively-charged ions)
Cations are smaller than their parent ions because the same number of protons in the nucleus pulls on less electrons. Metals commonly become cations.
Anions (negative ions)
Anions are larger than their parent ions because the same number of protons in the nucleus pulls on more electrons. Nonmetals commonly become anions.
Ionization energy
Ionization energy (IE) is the energy required to remove an electron from an atom (creating a cation). IE decreases down a group because, as the size of the atom increases, shielding increases. IE increases across a period. More protons in the nucleus create greater nuclear pull where shielding remains the same.
The energy needed to remove a second electron from an atom is always greater than that to remove the first. After the first is removed, the protons have greater pull on each of the remaining electrons. Also, there is a always jump in the IE when removing an inner shell electron--the greatest nuclear pull exists in the inner shell.
Electron affinity
The energy change that occurs when an atom gains an electron is its electron affinity. An atom is more stable when more energy is released (an exothermic reaction). Affinity decreases down a group and increases across a period. Exceptions to this rule are groups 2 and 18. Group 18, the noble gases, have essentially no electron affinity.
Electronegativity
Electronegativity (EN) is the attraction of an atom in a compound for an electron. The scale for this characteristic was developed by Linus Pauling. The scale shows that fluorine is the most electronegative element at 4.0. The scale can be used to determine the type of bond that will form between two atoms. EN decreases down a group and increases across a period. Group 18 is again an exception.
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