As you know, each horizontal row on the periodic table is called a period. And, each vertical column on the periodic table is called a group. In any group on the periodic table, all of the elements have the same number of electrons in their outermost shell. Electrons that are located in an atoms outermost shell are called valence electrons. These electrons are most responsible for an atoms properties and chemical behavior.

Rule number 5:

With progression from left to right across a period, valence electrons are more tightly bound to the atom. With progression down a group, valence electrons are less tightly bound to the atom.

Five important Periodic Trends

1. As we move from left to right across a period or up a group, ionization energy increases because the atoms positively charged nucleus tends to hold the electrons in the atom. It takes energy to remove an electron. The amount of energy necessary to take one electron out of an atom is called the ionization energy.
   
2. As we move from left to right across a period, electron negativity increases. As we go down a group, electron negativity decreases because valence electrons are less and less tightly bound. Electron negativity refers to the degree of pull that an atom exerts on another atom’s electrons when it is involved in a bond.
   
3. As we move from left to right across a period, atomic radius decreases. As we go down a group the increased atomic radius increases. Every atom has a radius. And the radius is called atomic radius.
   
4. As we move from left to right across a period, electron affinity increases. As we move from the top to the bottom of a group, the electron affinity decreases or becomes more positive. Electron affinity of an element is the amount of energy either released or absorbed when an electron is added to a neutral atom. A negative electron affinity value means that energy is released when an electron is added. A positive electron affinity value means that energy is absorbed in order to take an extra electron.
   
5. As we move from left to right across a period, metallic character decreases. As we move down a group, metallic character increases. Metallic character requires a high degree of mobility of valence electrons.

Rule number 6:

Try to remember these periodic trends. As you move from left to right across a period, atomic radius decreases, ionization energy increases, electron negativity increases, electron affinity increases, metallic character decreases. As you move down a column, atomic radius increases, ionization energy decreases, electron negativity decreases, electron affinity decreases, metallic character increases.

Rule number 7:

An atom’s chemical behavior is largely determined by the number of electrons in its highest energy shells. The more valence electrons or electrons in the outer shell, the more stable an atom is. When we see an element symbol with dots around it we are looking at a Lewis Dot Structure.