Atomic Structure

Atom

Reason — An atom is the smallest particle of an element (matter) which may or may not have independent existence.
A molecule is the smallest particle of a pure substance (element or compound) which has independent existence. An element is a pure substance that cannot be converted further into anything simpler than itself by any physical or chemical process. 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.

Atomicity

Reason — The number of atoms of an element that join together to form a molecule of that element is known as the atomicity of that molecule.
Valency is the combining capacity of an element or of a radical. A compound is a pure substance formed by the chemical combination of two or more elements in a fixed ratio by mass. The molecular formula of an element is the symbolic representation of its molecule. It indicates the number of atoms present in it.

4

Reason — Phosphorus exists as a molecule of phosphorus, written as P₄. This means one molecule of phosphorus contains four atoms of phosphorus. Since atomicity is the number of atoms present in one molecule of an element, the atomicity of phosphorus is 4.

3

Reason — A water molecule has the chemical formula H₂O. This means it contains 2 hydrogen atoms, and 1 oxygen atom.

two atoms of hydrogen

Reason — The symbol H represents one atom of hydrogen. When a number is written before a symbol, it shows the number of separate atoms. Therefore, 2H means two separate atoms of hydrogen.
It does not represent two molecules of hydrogen because a molecule of hydrogen is written as H₂, not H.

Ca²⁺

Reason — The symbol for calcium ion is Ca²⁺. Ca represents a neutral calcium atom, Ca⁺ is not the usual calcium ion, and Mg²⁺ is the magnesium ion.

oxygen, sulphur

Reason — Oxygen and Sulphur are both present in Group 16 of the periodic table.
Sodium is present in Group 1. Beryllium and magnesium are present in Group 2. Boron is present in Group 13. Carbon is present in Group 14. Nitrogen is present in Group 15.

ammonium ion

Reason — A cation is a positively charged ion, and polyatomic means it is made up of more than one atom. Ammonium ion, NH₄⁺, is made up of nitrogen and hydrogen atoms, both of which are non-metals, and it carries a positive charge. Therefore, it is a non-metallic polyatomic cation.
Nitrate ion and chloride ion are negatively charged anions, while aluminium ion is a metallic monoatomic cation.

3

Reason — The valency of iron in Fe₂O₃ is 3. Two iron atoms (Fe) combine with three oxygen atoms (O) to form the compound.

Carbon

Reason — The valency of Carbon (C) is 4.
The valencies of Aluminium, Oxygen and Phosphorus are 3, 2 and 3, respectively.

CaSO₄

Reason — The formula of Calcium Sulphate is CaSO₄. Sulphate ion (SO₄^2-) has a charge of (-2), while Calcium ion (Ca²⁺) has a charge of (+2). So, Ca²⁺ and SO₄^2- combine in a 1:1 ratio to balance the charges, and form a neutral compound.

Copper

Reason — Variable valency means an element can show more than one valency (i.e., it can lose different numbers of electrons in different compounds). Copper shows two valencies of 1 and 2.

Both A and R are true and R is the correct explanation of A.

Reason — A is true as an atom is the smallest particle of an element that exhibits all the properties of that element.
R is true as atoms of an element have the same atomic number. Since atoms of an element have the same atomic number, they exhibit the properties of that element.

Therefore, Both A and R are true and R is the correct explanation of A is the correct answer.

A is false but R is true.

Reason — A is false as a hydrogen molecule is H₂, which contains two atoms. So it is diatomic, not monoatomic.
R is true as the number of atoms of an element that combine to form a molecule is known as its atomicity.

Therefore, A is false but R is true is the correct option.

A is true but R is false.

Reason — A is true as ions or radicals are formed when atoms lose or gain electrons, giving them a charge.
R is false as cations are formed by losing electrons, not gaining. Gaining electrons forms anions, not cations.

Therefore, A is true but R is false is the correct option.

Both A and R are true and R is the correct explanation of A.

Reason — A is true as periodic table is a tabular arrangement of elements in vertical columns called groups and horizontal rows called periods.
R is true as the periodic table was designed to organize elements so their properties can be studied easily and patterns can be observed.

Therefore, Both A and R are true and R is the correct explanation of A is the correct option.

Both A and R are true but R is not the correct explanation of A.

Reason — A is true as electrons, protons and neutrons are three fundamental particles of an atom.
R is true as the sum of the number of protons and neutrons is called the mass number of an atom.

However, the reason only defines mass number. It does not explain why an atom consists of electrons, protons and neutrons. Hence, Both A and R are true but R is not the correct explanation of A is the correct option.

(a) indivisible

(b) Cation

(c) Anion

(d) molecules

(e) Ozone

(f) variable

(g) Sodium hydroxide (NaOH)

(a) True

(b) False
Correct Statement — The molecules of compounds are formed when atoms of different elements combine chemically.

(c) False
Correct Statement — A basic radical is positively charged.

(d) False
Correct Statement — The valency of oxygen is two.

(e) True

(a) Atom

(b) Atom

(c) Proton

(d) Nucleus

(e) Neutron

(f) Atomicity

(g) Molecular formula

(h) Radical

(i) Valency

(j) Periodic Table

(a) Sulphate ion

(b) Bicarbonate ion

(c) Hydroxide ion

(d) Dichromate ion

(a) A represents orbits or shells and B is nucleus.

(b) Electrons are present in A.
Protons and neutrons are present in B.

(c) The particles are charged as follows:

(i) Electrons are negatively charged.
(ii) Protons are positively charged.
(iii) Neutrons are neutral.

(a) Ammonium sulphate

(b) Calcium nitrate

(c) Iron (II) sulphide

(d) Sodium phosphate

(e) Ammonium hydroxide

(f) Copper (II) carbonate

(g) Mercuric oxide

(h) Zinc chloride

(i) Zinc sulphide

(j) Hydrogen sulphide

(a) 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 reaction.

(b) A molecule is the smallest particle of an element or compound which has independent existence and exhibits all the properties of that element or compound.

(c) 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.

(d) Valency is the combining capacity of an element or of a radical.

(e) Periodic table is a tabular arrangement of elements in vertical columns and horizontal rows indicating the regular trends in the properties of elements.

(a) IIA

(b) IVA

(c) VIA

(d) Zero

(a) 2 molecules of carbon dioxide

(b) 2 molecules of hydrogen sulphide

(c) 5 molecules of sulphuric acid

(d) 6 molecules of sodium nitrate

(a) Oxide (O^2-) valency is 2

(b) Hydroxide (OH^-) valency is 1

(c) Carbonate (CO₃^2-) valency is 2

(d) Chloride (Cl^-) valency is 1

(a) Na₂S

(b) MgO

(c) Ca(OH)₂

(d) HCl

(e) H₂SO₄

(f) FeS

(g) Fe₂(SO₄)₃

(h) HNO₃

(i) Ca₃(PO₄)₂

(j) Al₂(SO₄)₃

(k) Mg₃N₂

Phosphorus (P₄) and Sulphur (S₈)

The name and symbol of an ion each with the valency of +1, +2 and +3 are:

The name and symbol of an ion each with the valency of -1, -2 and -3 are:

The particles present in the nucleus of an atom are:

• Protons – positively charged particles
• Neutrons – neutral particles (no charge)

A molecule of an element is made up of atoms of the same element, whereas a molecule of a compound is made up of atoms of different elements chemically combined together.
For example, O₂ is a molecule of an element because it contains only oxygen atoms, while H₂O is a molecule of a compound because it contains hydrogen and oxygen atoms.

A basic radical (cation) is formed when an atom loses one or more electrons. When electrons are lost, the atom becomes positively charged because the number of protons (positively charged) exceeds the number of electrons (negatively charged). For example:

Na → Na⁺ + e^-

Five characteristics of an atom are given below:

1. An atom is the smallest particle of matter which cannot be divided further into smaller particles.
2. Atoms of the same element are all identical but they differ from the atoms of other elements.
3. An atom of an element exhibits all the properties of that element. Different elements differ in their properties.
4. Atoms can neither be created nor be destroyed.
5. Atoms may or may not have independent existence but they can take part in chemical reactions.

Five characteristics of a molecule are given below:

1. A molecule is formed by the chemical combination of two or more atoms of the same element or different elements.
2. A molecule exhibits all the properties of that element or compound.
3. In a molecule of a compound, atoms of different elements combine in a fixed whole number ratio.
4. In a molecule, the atoms are chemically bonded together by attractive forces.
5. A molecule can be represented by a chemical formula.

Two differences between molecules and radicals are given below:

(a) An atom is made up of three fundamental or subatomic particles. They are:

• Electrons : Electrons are the negatively charged particles in an atom with one unit negative charge and negligible mass.
• Protons : Protons are the positively charged particles present in an atom with one unit positive charge and one unit mass.
• Neutrons : Neutrons are the particles with no electrical charge, so they are neutral but have one unit mass.

(b) The two main parts of an atom are:

• Nucleus : Contains protons and neutrons.
• Orbits or Shells : The region around the nucleus where electrons revolve in circular paths.

(c) The three fundamental particles in an atom are located as given below:

• Electrons are located in orbits around the nucleus.
• Protons and Neutrons are present in the centrally located part called the nucleus.

Certain elements exhibit more than one valency that means these elements show variable valency.

Examples: Iron and copper.

(a) Periodic table is a tabular arrangement of elements in vertical columns and horizontal rows indicating the regular trends in the properties of elements.

(b) The horizontal rows of the periodic table are called periods. The vertical columns are called groups.

(c) The completed periodic table is given below:

The names and symbols of the missing elements, along with their type are:

The molecular formulae of the oxides, sulphides and chlorides of the given metal ions (cations) are provided below:

(a) Calcium phosphate

Formula of the compound formed between calcium ion and phosphate ion is :

$$\begin{aligned}\text{Ca}^{2+} \space \text{PO}_4^{3-} \\[1em] \overset{\phantom{22}{2}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \space \overset{3}{\text{PO}_4} \Rightarrow \underset{\phantom{2}{3}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \underset{{2}}{\text{PO}_4} \\[1em] \text{Ca}_3(\text{PO}_4)_2\end{aligned}$$

∴ Chemical formula of calcium phosphate is Ca₃(PO₄)₂.

(b) Aluminium sulphate

Formula of the compound formed between aluminium ion and sulphate ion is :

$$\begin{aligned}\text{Al}^{3+} \space \text{SO}_4^{2-} \\[1em] \overset{\phantom{22}{3}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{2}{\text{SO}_4} \Rightarrow \underset{\phantom{2}{2}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \underset{{3}}{\text{SO}_4} \\[1em] \text{Al}_2(\text{SO}_4)_3\end{aligned}$$

∴ Chemical formula of aluminium sulphate is Al₂(SO₄)₃.

(c) Iron (II) hydroxide

Formula of the compound formed between iron (II) ion and hydroxide ion is :

$$\begin{aligned}\text{Fe}^{2+} \space \text{OH}^{1-} \\[1em] \overset{\phantom{22}{2}}{\text{Fe}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{2}{1}}{\text{Fe}} \space {\swarrow}\mathllap{\searrow} \underset{{2}}{\text{OH}} \\[1em] \text{Fe}(\text{OH})_2\end{aligned}$$

∴ Chemical formula of iron (II) hydroxide is Fe(OH)₂.

(d) Calcium bicarbonate

Formula of the compound formed between calcium ion and bicarbonate ion is :

$$\begin{aligned}\text{Ca}^{2+} \space \text{HCO}_3^{1-} \\[1em] \overset{\phantom{22}{2}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{HCO}_3} \Rightarrow \underset{\phantom{2}{1}}{\text{Ca}} \space {\swarrow}\mathllap{\searrow} \underset{{2}}{\text{HCO}_3} \\[1em] \text{Ca}(\text{HCO}_3)_2\end{aligned}$$

∴ Chemical formula of calcium bicarbonate is Ca(HCO₃)₂.

(e) Aluminium hydroxide

Formula of the compound formed between aluminium ion and hydroxide ion is :

$$\begin{aligned}\text{Al}^{3+} \space \text{OH}^{1-} \\[1em] \overset{\phantom{22}{3}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \space \overset{1}{\text{OH}} \Rightarrow \underset{\phantom{2}{1}}{\text{Al}} \space {\swarrow}\mathllap{\searrow} \underset{{3}}{\text{OH}} \\[1em] \text{Al}(\text{OH})_3\end{aligned}$$

∴ Chemical formula of aluminium hydroxide is Al(OH)₃.

(i) The scientist who proposed these characteristics was John Dalton.

(ii) Statement (b) is true. Statement (a) was accepted earlier as a characteristic proposed by Dalton, but it is not considered true now because an atom is made up of still smaller subatomic particles — electrons, protons and neutrons.

(iii) When two or more atoms combine chemically, they form molecules.