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Periodic Trends: Chemical Reactivity

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Periodic Trends: Chemical Reactivity - Lesson Summary

The recurrence of similar outermost electronic configuration after certain regular intervals causes periodicity in the physical properties of elements.

Affect of periodicity on the chemical properties of elements:

Valence or oxidation state is one of the properties which show periodicity.

According to IUPAC, an oxidation state can be defined as: "A measure of the degree of oxidation of an atom in a substance." or

Number of electrons that have been removed from an element called as positive oxidation state and the number of electrons that have been added to the element called as negative oxidation state.

The oxidation state of any element in the uncombined form is zero.

To calculate the valence or oxidation state of elements they can be deduced from the outer electronic configurations of elements.

For representative elements, it is usually equal to:

  • The number of electrons in the outermost shell and or
  • 8 - Number of outermost electrons


For representative elements belonging to groups 1, 2, 13 and 14, the oxidation state is equal to the number of valence electrons.

For groups 15 through 18, the oxidation state is given by number of electrons in the outermost shell and/or 8 - number of valence electrons.

Non-representative elements, including transition metals and actinides tend to show multiple oxidations states.

The number of atoms of each element required for compound formation depends on the oxidations states of the elements.

Ex: Magnesium belongs to Group 2 shows an oxidation state of +2, whereas oxygen belonging to group 16 has an oxidation state of -2.

One atom of each of these two elements required in forming compound, the formula of magnesium oxide will be MgO.

Sum of the oxidation numbers of all the atoms or ions in a neutral compound like magnesium oxide is zero.

The oxidation state exhibited by an element in a covalent bond is equivalent to the charge acquired by its atom based on its electronegativity with respect to the other atoms in the compound.

The more electronegative element in a substance is gets negative oxidation state whereas the less electronegative element gets positive oxidation state.

Ex: in chlorine oxide, oxygen has a greater electronegativity as compared to chlorine. So oxygen atom shows an oxidation state -2, and two chlorine atoms show an oxidation state of +1 each.

As moving from left to right in the periodic table, the oxidation number increases first and then decreases.

The oxidation number in a group remains constant and depends on its generic electronic configuration.

All the elements in a group of periodic table show similar chemical properties.

The elements in the second period do not share many periodic trends with their other group members.

These elements include Lithium in group 1, Beryllium in group 2 and Boron to Fluorine in groups 13 to 17.

Lithium, in group 1, forms covalent compounds unlike other alkali metals that primarily form ionic compounds.

The behavior of Lithium is more similar to the second element of group 2, Magnesium.

Similarly, beryllium behaves more like Aluminium, which is placed in the third period and group 13.

This similarity in properties found amongst the elements that exist diagonally to each other in the periodic table is called diagonal relationship.

There are three reasons for this anomalous behavior of second period

  • small size
  • large charge to radius ratio
  • high electronegativity of elements


The first member of each group has only four valence orbitals (one 2s and three 2p orbitals available for bonding)

The second member of the groups have nine valence orbitals (one 3s, three 3p and five 3d orbitals).

So the maximum covalence or the number of sharable electron pairs of the first member of each group is 4. But other members of the groups can expand their valence shell to accommodate more than four pairs of electrons.

Ex: Boron can only form single charge ion with fluorine whereas Aluminum can form higher charged ions with fluorine.

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