Atomic Structure

(a) Atom — An atom is the smallest particle of an element that exhibits all the properties of that element. It may or may not exist independently but takes part in every chemical reactions.

Example : O — Oxygen atom, H — Hydrogen atom

(b) Molecule — A molecule is the smallest particle of an element or compound, made up of two or more atoms of an element or compound which has independent existence exhibiting all of their properties.

Example : O₂ — Oxygen molecule, H₂O — Water molecule

(c) Radical — A radical is an atom of an element or a group of atoms of different elements that behaves as a single unit with a positive or negative charge on it.

Example : NH₄⁺ — Ammonium radical

Below labelled diagram shows the basic structure of an atom:

Atomic number [z] = no. of protons [p] = no. of electrons [e] = 7

Therefore, Atomic number of nitrogen is 7.

Contradiction of Dalton’s atomic theory by the Modern Atomic Theory:

(a) O₂ — Oxygen molecule and N₂ — Nitrogen molecule

(b) H₂O — Water molecule and NaCl — Sodium chloride molecule

Atomicity — It is the number of atoms present in a molecule of an element.

(a) Helium (He) is considered monoatomic because it contains only one atom of helium while hydrogen contains two atom of hydrogen (H₂) therefore it is considered a diatomic molecule.

(b) Ozone is considered triatomic because it is composed of three atoms of Oxygen (O₃) while phosphorous is composed of four atoms (P₄), therefore, it is considered a tetratomic molecule.

(a) Monoatomic

(b) Diatomic

(c) Diatomic

(d) Monoatomic

(e) Octatomic or Polyatomic

(a) Ammonium [NH₄]: 1 nitrogen atom and 4 hydrogen atoms

(b) Bisulphate [HSO₄]: 1 hydrogen atom, 1 sulphur atom and four oxygen atoms

(c) Carbonate [CO₃] : 1 carbon atom and 3 oxygen atoms

In calcium sulphate [CaSO₄], the cation is [Ca²⁺] and anion is [SO₄^2-]

Valency of an element or radical is the number of hydrogen atoms which can combine with or displace one atom of the element or radical forming a compound.

(a) One atom of hydrogen combines with one atom of chlorine to form HCl, hence the valency of chlorine in hydrogen chloride is 1^-.

(b) Two atoms of hydrogen combine with one radical of sulphate to form sulphuric acid (H₂SO₄), hence the valency of sulphate in sulphuric acid is 2^-.

(c) Three atoms of hydrogen combine with one atom of nitrogen to form ammonia (NH₃) hence the valency of nitrogen in ammonia is 3^-.

(d) We know that valency of oxygen is 2^-. Hence, in magnesium oxide [MgO], valency of magnesium is 2⁺.

Three elements exhibiting variable valency are:

1. Iron (Fe) can show a valency of 2 and 3.
2. Copper (Cu) can show a valency of 1 and 2.
3. Lead (Pb) can show a valency of 2 and 4.

Chemical formula is the representation of a substance by means of symbols. It denotes in a compound, the number of atoms of each element present.

(a) Sodium hydroxide

$$\begin{aligned}\text{Na}^{1+} \phantom{\nearrow} \text{OH}^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{OH}} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Na}\bold{OH}$

(b) Sodium nitrate

$$\begin{aligned}\text{Na}^{1+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{NO}_3} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Na}\bold{NO}_3$

(c) Sodium bicarbonate

$$\begin{aligned}\text{Na}^{1+} \phantom{\nearrow} \text{HCO}_3^{1-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{HCO}_3} \Rightarrow \underset{\phantom{}1{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{HCO}_3} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Na}\bold{HCO}_3$

(d) Sodium sulphite

$$\begin{aligned}\text{Na}^{1+} \phantom{\nearrow} \text{SO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_3} \Rightarrow \underset{\phantom{}2{}}{\text{Na}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_3} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Na}_2\bold{SO}_3$

(e) Magnesium nitrate

$$\begin{aligned}\text{Mg}^{2+} \phantom{\nearrow} \text{NO}_3^{1-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \overset{1}{\text{NO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{Mg}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{NO}_3} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Mg}\bold{(NO}_3)_2$

(f) Ammonium sulphate

$$\begin{aligned}\text{NH}_4^{1+} \phantom{\nearrow} \text{SO}_4^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{NH}_4} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{SO}_4} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{(NH}_4)_2\bold{SO}_4$

(g) Carbonic acid

$$\begin{aligned}\text{H}^{1+} \phantom{\nearrow} \text{CO}_3^{2-} \\[0.5em] \overset{\phantom{1}{1}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \overset{2}{\text{CO}_3} \Rightarrow \underset{\phantom{2}{2}}{\text{H}} \space {\searrow}\mathllap{\swarrow} \space \underset{1}{\text{CO}_3} \\[0.5em]\end{aligned}$$ So, we get the formula as $\bold{H}_2\bold{CO}_3$

(h) Calcium phosphate

$$\begin{aligned}\text{Ca}^{2+} \phantom{\nearrow} \text{PO}_4^{3-} \\[0.5em] \overset{\phantom{1}{2}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\searrow}\mathllap{\swarrow} \space \underset{2}{\text{PO}_4} \\[0.5em]\end{aligned}$$

So, we get the formula as $\bold{Ca}_\bold{3}\bold{(PO_4)_2}$

Word equation:

Sodium hydroxide + hydrochloric acid ⟶ sodium chloride + water

Molecular equation:

NaOH + HCl ⟶ NaCl + H₂O

In the above equation,

As, the number of atoms of each element of reactants = the number of atoms of each element of products. Hence, the equation is balanced.

Group number signifies the number of electrons in the outer shell of an atom of the element.

As the number of electrons in the outer shell of all elements in a group is the same. Hence all elements in the same group of the periodic table have same valency and similar chemical properties.

1. A molecule of an element is chlorine.

2. A radical with one atom of sulphur and three atoms of oxygen is sulphite.

3. An element with variable valency is copper.

4. The formula of aluminum sulphate is Al₂(SO₄)₃.

5. The element lithium in the Modern Periodic Table with valency 1⁺ belongs to the group 1.

1. C: Atom

2. A: Molecule

3. E: Radical

4. B: Orbits

5. D: Valency

1. Ca(OH)₂

2. NaHSO₃

3. (NH₄)₂SO₄

4. AlPO₄

5. Cu(NO₃)₂

1. Nitrogen dioxide [NO₂]

2. Ammonium [NH₄]

3. Iron (II) oxide [FeO]

4. Group 2

5. Phosphorous (P₄)