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| Valence Shell and Valence Electron
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| The outermost shell of an atom is called its Valence shell. It is the decisive shell during a chemical reaction. The electrons of only this outermost shell are involved during chemical combinations; electrons are either given out from the outermost shell, or accepted into the outermost shell, or shared with the electrons in the outermost shell of another element.
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| Example:
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| K L M
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| Na - 11 - 2 8 1
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| Valence shell - M
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| Valence electrons - 1
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| Core electrons - 2 + 8 = 10
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| Significance of Valence Electrons
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Valence electrons of an atom are responsible for chemical reactions as they take part in them.
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Elements having same number of valence electrons in their atoms possess similar chemical properties. All alkali metals have one valence electron in their atom. Thus, their chemical properties are similar.
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The number of the valence shell in an atom determines its position in the Periodic Table i.e. the period to which the element belongs.
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Elements having 1, 2 or 3 electrons in the valence shell are metals. Exception is H and He. Elements having 4 to 7 electrons in their valence shell are non-metals.
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| Example: 1
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| The electronic configurations of some of the elements are detailed below. How many valence and core electrons do the following have?
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| (a) Al : 2, 8, 3
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| (b) Mg: 2, 8, 2
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| (c) Cl : 2, 8, 7
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| (d) Ca : 2, 8, 8, 2
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| Solution :
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| (a) Valence electrons = 3; core electrons = 10
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| (b) Valence electrons = 2; core electrons = 10
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| (c) Valence electrons = 7; core electrons = 10
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| (d) Valence electrons = 2; core electrons = 18
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| Example: 2
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| Classify the following elements as metals or non-metals based on valence electrons:
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| Solution:
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| A - 2, 8, 2 - It is a metal as it has 2 valence electrons
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| B - 2, 8, 6 - It is a non-metal as it has 6 valence electrons
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| C - 2, 6 - It is a non-metal as it has 6 valence electrons
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| Example: 3
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| Determine the period to which sodium (Na - 11) belongs.
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| Solution:
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| Na - 11; 2, 8, 1
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| Number of the valence shell - 3
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| Therefore, sodium is placed in period 3 of the periodic table.
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| Example: 4
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| Determine the period to which calcium (Ca - 20) belongs.
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| Solution:
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| Ca - 20; 2, 8, 8, 2
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| Number of the valence shell - 4
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| Therefore, sodium belongs to period 4 of the periodic table.
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| Valency
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| Valency is the combining capacity of an element. It is the number of electrons in an atom that actually take part in bond formation. For example, carbon atom with an atomic number 6 has 4 valence electrons.
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| K L
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| C6 2 4
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| Carbon atom is capable of forming 4 bonds. Hence its valency is four.
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| Calculation of Valency
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The number of valence electrons is the valency of the element.
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The valency of an element can also be calculated by finding the number of electrons required to complete octet (8)
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| i.e., 8 - Number of valence electrons = Valency
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| Example:1
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| Determine the valency of chlorine (Cl - 17).
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| Solution:
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| Cl - 17 : 2, 8, 7
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| Valency = 8 - Number of valence electrons
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| = 8 - 7
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| = 1
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| Example 2:
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| Determine the valency of oxygen (O - 8).
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| Solution:
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| 0 - 8 : 2, 6
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| Valency = 8 - Number of valence electrons
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| = 8 - 6
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| = 2
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| If the outermost shell of an atom is completely filled, its valency = 0. The outermost shells of the noble gases helium, neon, argon, krypton etc. are completely filled. Hence their valency is zero. Such elements are very un-reactive and inert by nature.
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| Let us test our understanding of valency and valence electrons before we proceed ahead.
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| Example 1:
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| Calculate number of valence electrons, the valency and valence shell of an atom whose atomic number is 17.
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| Solution:
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| Electron configuration:
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| Number of valence electrons = 7
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| Valency = 8 - 7 = 1
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| Valence shell = M
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| Example 2:
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| Which of the two elements would be more reactive, element A of atomic number 36 or element B of atomic number 19?
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| Solution:
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| A (36) - Electron configuration: 2, 8, 18, 8
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| B (19) - Electron configuration: 2, 8, 8, 1
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| Since the outermost shell of element A is completely filled it is stable and
unreactive.
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| Element B has one electron in its valence shell thus it is more reactive.
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