Valency: Bohr's Model, Noble Gas and Sample Questions

Valency can be defined as the combining capacity of an atom. It describes the number of bonds that one atom can form with another compatible atom. This concept arises from Niel Bohr’s model of the atom. The combining capacity, or affinity of an atom of a given element is determined by the number of hydrogen atoms that it combines with. In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of 2; and in hydrogen chloride, chlorine has a valence of 1. Chlorine, as it has a valence of one, can be substituted for hydrogen.

Keyterms: Valency, atom, bonds, hydrogen chloride, chlorine, hydrogen, nitrogen, ammonia

Read Also: How to find Atomic Mass?

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Bohr’s Model of the Atom

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According to Bohr’s model, which was the first model to incorporate quantum theory, atoms have a fixed nucleus around which electrons orbit. These discrete orbits, also known as energy levels, have fixed radii and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. There are a fixed number of electrons in each orbit, and the maximum number allowed in each shell is determined by the formula 2n², where n is the position of the orbit from the nucleus (1,2,3…) and are represented by the letter K, L, M, N and so on. This is known as the electronic configuration of an atom. The outermost orbit can hold a maximum of 8 electrons, in order to be stable, irrespective of the number of the energy level. Each shell must be fully filled before the electrons are distributed to the next one. Based on this model of the atom, the atom’s ability to combine with other atoms was theorised. 

Bohr’s Model

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Valency

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It was initially believed that the valency was an indication of the number of atoms of hydrogen with which an atom of the element can merge or that it can replace in a compound. It was the American chemist, G.N.Lewis, who discovered that the bonds between the electrons of a compound were held jointly by the different atoms. The tendency of atoms to react and form molecules was then defined by the tendency of atoms to seek an octet formation, that is, have 8 electrons in their outermost shell. 

The electrons in the outermost shell of an atom are known as the valence electrons. They determine the combining capacity of the atom. When the outermost shell is completely filled, like in noble gases (argon, neon, xenon), the atoms are not pressured to combine with others, since they are already stable. However, if the outermost shell is not completely filled, the atom is compelled to combine with other atoms in order to become stable. They do this by gaining, losing or sharing electrons, thereby completing their final orbit and forming an octet. The capacity of an atom is determined by the total number of electrons lost, gained or shared, and this defines its valency. 

Check out Important Question on: Valency to Copper and Iron

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Finding the Valency of Different Elements

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Elements like sodium and hydrogen have only one electron in their last orbit, and can hence lose that one electron to become stable, resulting in a valency of 1. In the same way, elements like magnesium and chlorine have a valency of 2 and elements like aluminium have a valency of 3. This is for elements whose outermost shell has less than 4 electrons.

When the outermost shell of an atom has more than 4 electrons and is close to its full capacity, the valency is the number of electrons that it is capable of gaining. For example, fluorine has 7 electrons in its valence shell (the outermost shell of the atom). The valency of the electron could be 7, but it is easier to gain one electron and become stable than to lose 7 electrons. The valence of the atom would then be determined by the number of electrons it can gain, rather than lose, and for this, we subtract the number of electrons in the outermost shell from 8. . The valence of fluorine would then be 1. 

Any element, then, has a fixed combining capacity and can combine only with atoms that are compatible with their valency. In the periodic table, elements in the same group have the same valency. The valence of an atom does not denote the charge of its ion, and hence valency has no charge. 

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Noble Gases

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When we speak of valency, noble gases are an important topic to cover, because they are naturally stable and do not combine with any other element. Examples of such gases are helium, neon, argon, xenon and many others. The noble gases do not lose or gain electrons and exist as they are. Noble gases in their natural form have octets and are hence chemically the least reactive elements. The valency of noble gases is 0 since they don't need to lose, gain or share electrons. 

Noble Gases

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Things To Remember

• Valency can be defined as the combining capacity of an atom. It describes the number of bonds that one atom can form with another compatible atom.
• There are a fixed number of electrons in each orbit, and the maximum number allowed in each shell is determined by the formula 2n²,where n is the position of the orbit from the nucleus (1,2,3…) and are represented by the letter K, L, M, N and so on.
• The electrons in the outermost shell of an atom are known as the valence electrons. They determine the combining capacity of the atom
• The valence of an atom does not denote the charge of its ion, and hence valency has no charge.
• The valency of noble gases is 0 since they don't need to lose, gain or share electrons. 

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Previous Year Question

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