Atoms And Molecules

Fermi unit is used to measure the atomic radius.

1 fermi = 10^-15m

An atom is the smallest particle of an element that can retain all the chemical properties. It is the indivisible unit of matter that can exist independently. It is building block of matter.

It is not possible to see an atom with the unaided eyes because it is very small, it is smaller than anything that we can imagine or compare with.

Lead phosphate:

By applying criss-cross method:

Chemical formula: Pb₃(PO₄)₂

The combining power or capacity of an element is known as valency. It can be used to find out how the atoms of an element will combine with the atoms of another element to form a chemical compound.

Valency helps in determining the formula of an ionic compound. Valency can be used to find out how the atoms of an element will combine with the atoms of another element to form a compound.

For example: Magnesium chloride

By applying criss-cross method:

Formula: MgCl₂

Thus, the formula for magnesium chloride is MgCl₂.

Molar mass is the mass of one mole of any substance. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecules mass in grams.

Molar mass is represented by “M”

We can write,

Molar mass (M) = mass (m) × Number of moles(n)

Law of conservation of mass: This law states that “atoms are neither created nor destroyed in a chemical reaction”.

This means that the total mass of the products formed in chemical reaction must be equal to the mass of reactants consumed.

Carbon + oxygen → CO₂

Mass of carbon = 12g

Mass of oxygen = 32g

Mass of CO₂ =?

According to the law of conservation of mass,

Sum of masses of reactants = Sum of masses of of products

Hence, 12g + 32g = 44g

Thus, the amount of CO₂ formed is 44g.

Formula unit mass of CuSO₄.5H₂O:

Atomic mass of Cu + Atomic mass of S + (4× atomic mass of O) + 5 (2× atomic mass of H + atomic mass of O)

63.5 u + 32 u + 4× 16u + 5 (2 × 1 u + 16 u)

63.5 u + 32 u + 64 u + 90 u

249.5 u

Thus, the formula unit mass of CuSO₄.5H₂O is 249.5 u

Given:

Mass of He (m) = 52g

Atomic mass of He = 4u

Molar mass of He (M) = 4g/mol

To calculate the number of moles, we will apply the formula given below:

Number of moles = 13

Thus, the number of moles in 52 g of He is 13.

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

Number of moles = 2

Thus, number of moles of He is 2.

Given:

Number of moles = 0.5

Atomic mass of N = 14 u

Atomic mass of N₂ = 2 × Atomic mass of N

= 2 × 14 u

= 28 u

Molar mass of N₂ = 28g/mol

To calculate the mass, apply the formula:

We can write:

Mass (m) = Number of moles (n) × Molar mass (M)

Mass = 0.5 mol × 28g/mol

Mass = 14g

Thus, the mass of 0.5 mole of N₂ gas is 14g

Given:

Number of moles = 0.5

Atomic mass of N = 14 u

Molar mass of N = 14g/mol

To calculate the mass, apply the formula:

We can write:

Mass(m) = Number of moles (n) × Molar mass (M)

Mass = 0.5 mol × 14g/mol

Mass = 7g

Thus, the mass of 0.5 mole of Natoms is 7g.

First we will convert 3.011 × 10²³ number of nitrogen atoms into number of moles:

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

Number of moles = 0.5

Now, to calculate mass, apply the formula:

We can write:

Mass(m) = Number of moles (n) × Molar mass of nitrogen (M)

Mass = 0.5 mol × 14g/mol

Mass = 7g

Thus, the mass of 3.011 × 10²³ atoms of nitrogen is 7g

We know, 1 mole = 6.022 × 10²³

Now, to calculate mass, apply the formula:

We can write:

Mass(m) = Number of moles (n) × Molar mass of N₂ (M)

Mass = 1 mol × 28g/mol

Mass = 28g

Thus, the mass of 6.022 × 10²³ atoms of nitrogen is 28g

Postulates of Dalton’s atomic theory are:

i. All mater is made up of very tiny particles called atoms.

ii. Atoms are indivisible particles.

iii. They cannot be created or destroyed in a chemical reaction.

iv. Atoms of a given element are identical in mass and chemical properties.

v. Atoms of different elements have different masses and chemical properties.

vi. Atoms combine in the ratio of small whole numbers to form compounds.

a) Ammonium sulphate:

By applying criss-cross method:

Chemical formula: (NH₄)₂SO₄

b) Magnesium bicarbonate:

By applying criss-cross method:

Chemical formula: Mg(HCO₃)₂

c) Barium nitrate:

By applying criss-cross method:

Chemical formula: Ba(NO₃)₂

a) Molecular mass: The molecular mass of a substance is the sum of atomic masses of all the atoms in a molecule of the substance.

i. It is used for those substances whose constituent particles are molecules.

ii. The molecular mass remains the same even if millions of molecules are added.

iii. For example: The molecular mass of H₂O = 2 × Atomic mass of hydrogen + Atomic mass of oxygen

= 2 + 16 = 18u

b) Molecular mass of ZnSO₄:

Atomic mass of Zn + Atomic mass of S + 4× atomic mass of O

65 u + 32 u + 4× 16u

65 u + 32 u + 64 u

161 u

Thus, the molecular mass of ZnSO₄ is 161u

Law of conservation of mass: This law of states that “atoms are neither created nor destroyed in chemical reaction”. During a chemical reaction, the sum of the masses of the reactants and products remains unchanged. This is known as the law of conservation of mass.

This means that the total mass of the products formed in chemical reaction must be equal to the mass of reactants consumed.

For example:

CaCO₃ → CaO + CO₂

100g 56g 44g

_{Sum of masses of reactants = Sum of masses of products}

a) Given:

Mass of sulphur (m) = 16g

Atomic mass of sulphur (S₈) = 8 × 32u = 256 u

Molar mass of S₈ (M) = 256g/mol

To calculate the number of moles, we will apply the formula given below:

Number of moles = 0.0625

Thus, the number of moles in 16 g of solid sulphur is 0.0625.

b) Given:

Number of moles = 10

Molecular mass of sodium sulphite (Na₂SO₃) = 2 × Atomic mass of Na + atomic mass of sulphur + 3 × Atomic mass of O

= 2 × 23 u + 32 u + 48 u

= 126 u

Molar mass of Na₂SO₃ = 126g/mol

To calculate the mass, apply the formula:

We can write:

Mass = Number of moles × Molar mass

Mass = 10 mol × 126g/mol

Mass = 1260g

Thus, the mass of sodium sulphite is 1260g.

c) Given:

Mass of Na (m) = 11.5g

Atomic mass of sulphur = 23u

Molar mass of sulphur (M) = 23g/mol

To calculate the number of moles, we will apply the formula given below:

Number of moles = 0.5

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

We can write, number of atoms = Number of moles × N_a

Number of atoms = 0.5 × 6.022 × 10²³

Number of atoms = 3.022 × 10²³

Thus, the number of atoms in 11.5 g of Na is 3.022 × 10²³

(a) First we will convert 1.0505 × 10²³ molecules of carbon dioxide (CO₂) into number of moles:

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

Number of moles = 0.17

Now, to calculate mass, apply the formula:

We can write:

Mass(m) = Number of moles (n) × Molar mass of CO₂ (M)

Mass = 0.17 mol × 44g/mol

Mass = 7.48g

Thus, the mass of 1.0505 × 10²³ molecule of CO₂ is 7.48g.

(b) Given: Number of moles = 0.25

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

We can write:

Number of molecules = Number of moles (n) × N_a

Number of molecules = 0.25mol × 6.022 × 10²³

Number of molecules = 1.5 × 10²³

Thus, the number of molecules of 0.25 moles of NH₃ is 1.5 × 10²³

(c) Formula unit mass of Na₂SO₃ :

2 × Atomic mass of Na + Atomic mass of sulphur + 3 × Atomic mass of O

= 2 × 23 u + 32 u + 3 × 16 u

= 126 u

Thus, the formula unit mass of Na₂SO₃ is 126 u

Mole: One mole of any species (Atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecules mass in grams.

(a) We will convert 1.0505 × 10²³ atoms of C into number of moles:

We know, 1 mole = 6.022 × 10²³(N_a = Avogadro’s number)

Number of moles = 0.5

Thus, the number of moles in 3.011 × 10²³ atoms of C is 0.5.

(b) Given mass: 32g

Molar mass of oxygen gas (O₂) = 2 × atomic mass of oxygen = 2 × 16 u = 32g /mol

To calculate the number of moles, we will apply the formula given below:

Number of mole = 1

Thus, the number of mole in 32 g of oxygen gas is 1.

(a) Given mass: 112g

Molar mass of iron (Fe) = 56 g/mol

To calculate the number of moles, we will apply the formula given below:

Number of moles = 2

Thus, the number of moles in 112 g of iron is 2

(b) Given: Number of moles = 0.5

Molar mass of C₁₂H₂₂O₁₁ = 12 × atomic mass of C + 22 × atomic mass of H + 11 × atomic mass of O

= 12 × 12 + 22 × 1 u + 11 × 16 u = 342 u or 342 g/mol

Now, to calculate mass, apply the formula:

We can write:

Mass(m) = Number of moles (n) × Molar mass of C₁₂H₂₂O₁₁ (M)

Mass = 0.5 mol × 342g/mol

Mass = 171g

Thus, the mass of 0.5 mole of sugar (C₁₂H₂₂O₁₁) is 171g.

(a) Atom:

i. The building blocks of all matter are atoms. Atoms are very small.

ii. They are smaller than anything we can imagine or compare with.

(b) Molecule:

i. A molecule is general a group of two or more atoms that are chemically bonded together. They are tightly held together by attractive forces.

ii. A molecule can be defined as the smallest particles of an element or a compound that is capable of an independent existence.

iii. A molecule shows all the properties of a particular substance.

iv. Atoms of the same element or different element join together to form molecules.

(c) Avogadro’s number:

i. Avogadro’s number is the number of particles (atoms, ions or ions) present in one mole of any substance is fixed with a value of 6.022 × 10²³.

ii. This is an experimentally obtained value.

(d) Valency:

i. The combining power or capacity of an element is known as valency.

ii. Valency can be used to find out how the atoms of an element will combine with the atoms of another element to form a chemical compound.

(e) Molar mass:

Molar mass is the mass of one mole of any substance. One mole of any species (atoms, molecules, ions or particles) is that quantity in number having a mass equal to its atomic or molecules mass in grams.

Molar mass is represented by “M”

We can write,

Molar mass (M) = mass (m) × Number of moles(n)