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Q3.32(a):
Predict the formulas of the stable binary compounds that would be formed by the combination of the following pairs of elements. (a) Lithium and oxygen

Solution :

Step 1: Identification of Electronic Configuration

To predict the chemical formula, we first determine the valence electron configurations of the constituent elements using the Aufbau principle and the periodic table [Per Bohr-Bury scheme].

  • Lithium ($Li$): Atomic number $Z = 3$. The electronic configuration is $1s^2 2s^1$. Lithium has $1$ valence electron in the $2s$ orbital.
  • Oxygen ($O$): Atomic number $Z = 8$. The electronic configuration is $1s^2 2s^2 2p^4$. Oxygen has $6$ valence electrons and requires $2$ more electrons to complete its octet [Per Octet Rule].

Step 2: Determination of Valency and Ion Formation

The stability of binary compounds is governed by the tendency of atoms to attain a stable noble gas configuration.

  • Lithium: Being an alkali metal, $Li$ tends to lose $1$ electron to achieve the $He$ configuration ($1s^2$). Thus, it forms a unipositive ion:
    $Li \rightarrow Li^+ + e^-$
  • Oxygen: Being a chalcogen, $O$ tends to gain $2$ electrons to achieve the $Ne$ configuration ($1s^2 2s^2 2p^6$). Thus, it forms a dinegative ion (oxide ion):
    $O + 2e^- \rightarrow O^{2-}$

Step 3: Charge Balance and Stoichiometric Calculation

For the compound to be electrically neutral, the total positive charge from the cations must balance the total negative charge from the anions [Per Law of Electroneutrality].

Let the formula be $Li_xO_y$.

Equating the total charges: $x(+1) + y(-2) = 0$ $x = 2y$

Substituting the smallest integer ratio for $y=1$, we find $x=2$.

Visual Representation of Ionic Bonding

Li Li O e- transfer

Step 4: Final Formula Synthesis

By applying the criss-cross method of valence mapping:

Element Valency
$Li$ $+1$
$O$ $-2$

The subscripts are determined by the cross-multiplication of the numerical values of the valencies, resulting in $Li_2O$. This compound is known as Lithium Oxide.

Final Solution: The stable binary compound formed by Lithium and Oxygen is $Li_2O$.


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