Periodic Classification of the Elements

The electrons occupying the outermost shell of an atom are valence electrons, while electrons occupying the inner principal energy levels are core electrons.

A sodium atom with a [Ne]3s¹ electron configuration has three principal energy levels. The complete inner principal energy levels with [He]2s²2p⁶ indicate that there are ten core electrons followed by the third outermost level containing the remaining one electron. Therefore sodium has one valence electron. Similarly, chlorine has ten core electrons and seven valence electrons.

Valence electrons are the farthest from the nucleus and are held most loosely. Hence, they are the easiest to lose or share and play an important role in chemical bonding.

Elements that have the same number of valence electrons exhibit similar chemical properties, as can be seen from the arrangement in the modern periodic table.

For main-group elements, the lettered group number equals the number of valence electrons and the row number equals the highest principal quantum number of that element.

Each element in a group has the same number of electrons available for bonding. Down the group, the principal quantum number increases by one, whereas the number of valence electrons remains the same.

The two far-left columns of the periodic table constitute the s-block. For these elements, the last electron enters an s-orbital. Group one elements except for hydrogen are called the alkali metals and are extremely reactive as they have only one valence electron. Group two elements are the alkaline earth metals with two electrons in the valence shell.

The six far-right columns form the p-block. The valence shell of these elements has completely occupied s-orbitals and the last electron enters the p-orbital. From group three A to group eight A, the number of electrons in p orbitals increases by one. The noble gases have eight valence electrons except for helium, which belongs to the s-block, and only has two electrons.

The d-block consists of the ten columns placed between the s and p-block. These elements are referred to as the transition metals. The last electron enters the d-orbital of the principal shell number one less than the row number. In the fourth row, three d-orbitals fill, in the fifth row, four d-orbitals fill, and so on.

Inner transition elements constitute the f-block and have the last electron entering an f-orbital. The principal quantum number of the f-orbitals that fill across each row is two less than the row number. In the sixth row, the four f-orbitals fill, and in the seventh row, the five f-orbitals fill. Inner transition elements are arranged in the lanthanide series and the actinide series.

The number of columns in each block indicates how many electrons can be filled in the sublevel of the block. Two columns in the s-block correlate to an s-orbital with two electrons, six columns in the p-block represent three p-orbitals with six electrons, while ten columns in the d-block correspond to five d-orbitals with two electrons each.

Lastly, the f-block comprises fourteen columns indicating the maximum capacity of seven f-orbitals.