Chemistry

Structure of the Atom

Covalency of Carbon

The atomic number of carbon is 6, so its electronic configuration is 2, 4. Thus, a carbon atom has 4 valence electrons and it requires 4 more electrons to complete the et'.It gets these electrons by sharing.So,a carbon atom shares its 4 electrons with the four electrons of other atoms to attain the 8-electron inert gas electron arrangement.Since one carbon atom shares its 4 electrons to attain the inert gas electron arrangement,therefore, the covalency (or just valency) of carbon is 4.For example, in the formation of a methane molecule,

CH

₄,the carbon atom shares its 4 electrons with four electrons of four hydrogen atoms, so the valency of carbon in

CH

₄ molecule is 4.The atomic number of silicon is 14 and its valency is also 4. Explain it yourself.
The elements like fluorine, chlorine, bromine, iodine, oxygen, and sulphur, etc., form electrovalent compounds as well as covalent compounds, so they show electrovalency as well as covalency. Please note that the electrovalency and covalency of an element are numerically the same. So, we usually talk of just valency and not electrovalency or covalency. Let us now solve some problems based on valency.
Sample Problem 1. What valency will be shown by an element having atomic number 12 ?
Solution. To find out the valency of the element, we will have to write down its electronic configuration.Now, the atomic number of this element is 12, so its electronic configuration will be

A = \begin{matrix} K \\ 2 \end{matrix}

A = \begin{matrix} L \\ 8 \end{matrix}

A = \begin{matrix} M \\ 2 \end{matrix}

.It has 2 electrons in its outermost shell (M shell). So, one atom of this element can lose 2 electrons to achieve the nearest inert gas electron arrangement of 2,8. Since one atom of the element loses 2 electrons to achieve the inert gas electron configuration, so its valency is 2. It is divalent. (The element having atomic number 12 is actually magnesium).
Sample Problem 2. What valency will be shown by an element having atomic number 15 ?
Solution. The atomic number of this element is 15, so its electronic configuration will be

A = \begin{matrix} K \\ 2 \end{matrix}

A = \begin{matrix} L \\ 8 \end{matrix}

A = \begin{matrix} M \\ 5 \end{matrix}

.It has 5 electrons in its outermost shell, so it needs 3 more electrons to complete the eight electron, inert gas configuration. Since one atom of this element needs 3 electrons to achieve the inert gas electron configuration,therefore, its valency is 3. (The element having atomic number 15 is actually phosphorus).
Sample Problem 3. If Z = 3, what would be the valency of the element ? Also name the element.
Solution. The symbol Z stands for the atomic number of an element. By saying that Z = 3, we mean that the atomic number of this element is 3. The electronic configuration of the element having atomic number 3 is configuration. Since one atom of this element needs 3 electrons to achieve the inert gas electron configuration,therefore, its valency is 3. (The element having atomic number 15 is actually phosphorus). Sample Problem 3. If Z = 3, what would be the valency of the element ? Also name the element.Solution. The symbol Z stands for the atomic number of an element. By saying that Z = 3, we mean that the atomic number of this element is 3. The electronic configuration of the element having atomic number 3 is

A = \begin{matrix} K \\ 2 \end{matrix}

A = \begin{matrix} L \\ 1 \end{matrix}

.It has 1 electron in its outermost shell (L shell). So, one atom of this element can lose 1 electron to achieve the nearest inert gas electron arrangement of

A = \begin{matrix} K \\ 2 \end{matrix}

(which is the same as that of helium gas).Since one atom of this element loses 1 electron to achieve the inert gas electron configuration, therefore,its valency is 1. The element having atomic number 3 is lithium.
Sample Problem 4. The number of valence electrons in a chloride ion,

Cl

^-,are :
(a) 16 (b) 8 (c) 17 (d) 18
Choose the correct answer.
Solution. A chlorine atom (Cl) has 7 valence electrons. A chloride ion is formed by the addition of 1 more electron to a chlorine atom. So, the number of valence electrons in a chloride ion (

Cl

^-) will be 7 + 1 = 8.

Notes

Structure of the Atom - Notes

1. Drawback of Rutherford’s Model of the Atom