Ions

• When an atom loses an electron, or gains one, it becomes a charged ion. Because electrons are negative, when an atom loses an electron, the atom becomes positively charged. This is called a cation. An atom that gains an electron becomes negatively charged. This is an anion.
  
  

Ionization energy

• Ionization energy is the amount of energy required to remove the outermost electron from an atom. A high ionization energy means it takes a lot of energy to remove an electron. Conversely, a low ionization energy means it takes little energy to remove an electron. Looking at the periodic table, remember that ionization energy increases as you move up the table and to the right. This means that the alkali metals and the alkali earth metals on the left have low ionization energies, while the halogens and noble gases on the right have high ionization energies.
  
  

Electron Affinity

• Electron affinity is a measure of the change in energy when an atom gains an electron. When an atom gains an electron, it becomes more negative, so its electron affinity becomes more negative. When an atom loses an electron, it becomes more positive, so its electron affinity becomes more positive. Electron affinity becomes more positive going down and to the left on the periodic table. This means that the alkali metals and the alkali earth metals on the left have more positive electron affinities; the halogens and noble gases on the right have more negative electron affinities.
  
  

Electronegativity

• Electronegativity is a measure of the ability of an atom in a bond to attract electrons to it. Elements with a high electronegativity tend to attract electrons; elements with a low electronegativity tend to release electrons. Electronegativity increases going up and to the right on the periodic table. Thus, the alkali metals and alkali earth metals on the left have low electronegativities, while the halogens and noble gases on the right have high electronegativities.
  
  

General Trend

• As ionization energy decreases, electronegativity decreases and electron affinity becomes more positive, the tendency to lose electrons increases. Thus, the tendency to lose electrons increases as you move down and to the left on the periodic table. The elements that most readily give up electrons are sodium, potassium, lithium, rubidium, cesium, francium, beryllium, magnesium, calcium, strontium, barium, and radium. Other cations commonly encountered are chromium, manganese, iron, copper, nickel, lead and silver. Conversely, as ionization energy increases, electronegativity increases and electron affinity becomes more negative, the tendency to accept electrons increases. Thus, the tendency to accept electrons increases as you move up and to the right on the periodic table. The elements that most readily accept electrons are nitrogen, chlorine, fluorine, sulfur, iodine and bromine.