VALENCE ELECTRONS

VALENCE ELECTRONS

A valence electron is a negatively charged particle, located in the outermost shell of an  atom that can be transferred to or shared with another atom.

valence(or valency) is an atom or group of atoms’ ability to chemically unite with other atoms or groups.Electrons  are negatively charged subatomic particles surrounding the nucleus of an atom in shells. Valence electrons, then, are electrons in the outermost shell of the atom that determine an atom’s or group’s ability to bond with others.

In chemistry, a group refers to two or more atoms that are specifically arranged. Groups are also called radicals. Free radicals are unstable, highly reactive atoms or molecules that have unpaired valence electrons.

Source: This topic is taken from NCERT TEXTBOOK

CONCEPT OF VALENCY

CONCEPT OF VALENCY

The number of electrons gained, lost or shared so as to make the octet of electrons in the outermost shell, gives us directly the combining capacity of the element, that is, the valency discussed in the previous chapter. For example, hydrogen/ lithium/sodium atoms contain one electron each in their outermost shell, therefore each one of them can lose one electron. So, they are said to have valency of one. Can you tell, what is valency of magnesium and aluminium? It is two and three, respectively, because magnesium has two electrons in its outermost shell and aluminium has three electrons in its outermost shell.

If the number of electrons in the outermost shell of an atom is close to its full capacity, then valency is determined in a different way. For example, the fluorine atom has 7 electrons in the outermost shell, and its valency could be 7. But it is easier for fluorine to gain one electron instead of losing seven electrons. Hence, its valency is determined by subtracting seven electrons from the octet and this gives you a valency of one for fluorine. Valency can be calculated in a similar manner for oxygen. What is the valency of oxygen that you get from this calculation?

Therefore, an atom of each element has a definite combining capacity, called its valency. Valency of the first eighteen elements is given in the last column of Table 4.1.

Table 4.1: Composition of Atoms of the First Eighteen Elements with Electron Distribution in Various Shells

Questions

How will you find the valency of chlorine, sulphur and magnesium?

Source: This topic is taken from NCERT TEXTBOOK

BOHR-BURY SCHEME

BOHR-BURY SCHEME

From the Bohr-Bury scheme, we also know that the outermost shell of an atom can accommodate a maximum of 8 electrons. It was observed that the atoms of elements, completely filled with 8 electrons in the outermost shell show little chemical activity. In other words, their combining capacity or valency is zero. Of these inert elements, theaccommodate a maximum of 8 electrons. It was observed that the atoms of elements, completely filled with 8 electrons in the outermost shell show little chemical activity. In other words, their combining capacity or valency is zero. Of these inert elements, the helium atom has two electrons in its outermost shell and all other elements have atoms with eight electrons in the outermost shell.

The combining capacity of the atoms of elements, that is, their tendency to react and form molecules with atoms of the same or different elements, was thus explained as an attempt to attain a fully-filled outermost shell. An outermost-shell, which had eight electrons was said to possess an octet. Atoms would thus react, so as to achieve an octet in the outermost shell. This was done by sharing, gaining or losing electrons.

Source: This topic is taken from NCERT TEXTBOOK