Valency

Valency of an element is a measure of an atom's ability to combine with other atoms to create molecules or chemical compounds. The characteristics of an element that indicate how many more atoms can join one of its atoms in a covalent bond are known as valence, or valency, in chemistry. The valence electrons determine what valences are and what their meaning is in chemistry.

• The different orbitals (shells) in which electrons are grouped in atoms are denoted by the letters K, L, M, N, and so on.
• Valence electrons are those found in the outermost orbit or shell of an atom.
• Because the outermost orbit often contains more energy than the electrons found in other orbits, the valence electrons participate in every chemical reaction.
• The Bohr-bury theory predicts that an atom's outermost orbit will contain up to 8 electrons

Finding Valency of Elements

The periodic table shows that all members of the same group of an element have the same valency. If we look at group 8 in the periodic table, all of the elements have achieved octet organization and have entirely occupied their outermost orbit. Therefore, group 8's elements have no valencies.

Octet Rule

• The octet rule is applied if valency cannot be determined using the periodic table.
• According to this principle, atoms of an element or chemical have the propensity to gain or lose electrons in the compound it is present in, giving rise to an increase or decrease in the number of electrons in their outermost orbit. If outermost electrons are:
• 1, 2, 3 → Valency = number of electrons (they lose electrons)
• 4 → Valency = 4
• 5, 6, 7 → Valency = 8 – number of electrons (they gain electrons)
• 8 → Valency = 0
• Example: Sodium (2,8,1) → 1 electron in outer shell → Valency = 1
• Oxygen (2,6) → 6 electrons → 8 – 6 = 2 → Valency = 2

Periodic Table

• With this approach, the periodic table graphic is used to calculate valency.
• For instance, all of the metals in column 1 have a valency of +1, including hydrogen, lithium, sodium, and so on.
• Similar to this, all of the elements in column 17—including fluorine, chlorine, and other elements have valency 1.

Chemical Formulae

• Use the known valency of one element.
• Apply the criss-cross method to determine unknown valency.
• Example:

In NaCl:

• Sodium valency = 1
• Chlorine valency = 1

In MgCl₂:

• Magnesium valency = 2
• Chlorine valency = 1

Examples

Sodium (Na)

The Atomic number of Na is 11 (K = 2, L = 8, M = 1). They have 1 electron in their outermost shell.

Carbon (C)

The Atomic number of C is 6 (K = 2, L = 4). They have 4 electrons in their outermost shell.

Oxygen (O)

The Atomic number of O is 8 (K = 2, L = 6, M= 0), They have 6 electrons in their outermost shell. So, the valency of Oxygen is: 8 - 6 = 2

Nitrogen (N) 

The Atomic number of N is 7 (K = 2, L = 5, M = 0). They have 5 electrons in their outermost shell. So, the valency of Nitrogen is: 8 - 5 = 3

Valency Chart of Ions

Relation Between Valency and Valence Electrons

Valency explains the bond formation of atoms. Whereas, valence electrons are related to elemental characters. Valency is only a concept or idea and doesn't involve the transmission of electrons. Whereas, valence electron involves the transmission of electrons in the formation of bonds. Both valency and valence electrons are applied to any chemical element.

• Since only the valence electrons (outermost electrons) participate in chemical bonding, an element's valency is determined by the number of valence electrons (outermost electrons) in its atom.
• The amount of valence electrons in an element's atom or the number of electrons necessary to complete eight electrons in the valence shell determines the element's valency.
• Sodium, for example, contains one valence electron and thus a valency of one.
• As a result, the valency of sodium is equal to the number of valence electrons in its atom.
• A metal element's valency is equal to the number of valence electrons in its atom in general.

Valency of a Metal = Number of Valence Electrons in its Atom

• A valence electron, like an electron in an inner shell, may receive or release energy in the form of a photon.
• Atomic excitation occurs when an electron gains enough energy to migrate (jump) to an outer shell.
• Ionization occurs when an electron breaks away from its associated atom's valence shell, resulting in the formation of a positive ion.
• When an electron loses energy (and so emits a photon), it might migrate to an inner shell that isn't completely occupied.

Valency of a Non-Metal = 8 - Number of Valence Electrons in its Atom

Related Articles:

• Difference between Valency and Electrovalency
• Valence Bond Theory
• Chemical Formula?