Atoms And Molecules

● Isotopes are the atoms of same chemical element having different

atomic mass. This means they differ in neutron number. For ex- ₁₇Cl³⁵ and ₁₇Cl³⁷. Here both atoms are chlorine atoms and have atomic number 17. So, each atom has 17 electrons and 17 protons. But the former one has atomic mass 35 and the latter one has atomic mass 37. This implies that former one has 18 neutrons (35-17) and latter one has 20 neutrons (37-17). Hence, they are isotopes.

● Isobars are the atoms of different elements having same atomic mass.

This implies that the sum of protons and neutrons in each atom is same. For ex- ₁₈Ar⁴⁰ and ₂₀Ca⁴⁰. Here, the former one is argon atom and latter one is calcium. Their atomic numbers are different. But their atomic mass is same i.e. 40. This implies that the number of nucleons in both atoms is same, even though the proton, electron and neutron numbers are different.

Atomicity is the number of atoms in the molecule of an element. Molecular mass is calculated by adding up the masses of respective atoms present in the molecule. Nitrogen molecule contains only nitrogen atoms. Given -

Mass of molecule of nitrogen = 28

Mass of atom of nitrogen = 14

Let number of atoms in the molecule = x

So,

Hence, the atomicity of nitrogen is 2. 2 atoms of nitrogen make one

molecule of nitrogen.

Given - Gram molecular mass of Oxygen = 32 g

Density of Oxygen = 1.429 g/litre

Gram molar volume of Oxygen = ?

Formula used is -

Note –

Atoms combine to form molecules.

If the molecular mass of a given substance is expressed in grams, it is known as gram molecular mass of that substance. Molecular mass is the sum of the atomic masses of all the atoms present in one molecule of the substance.

One molecule of water is H₂O, which contains 2 hydrogen and one oxygen atoms. So, molecular mass is sum of atomic masses of hydrogen and oxygen atoms.

Gram molecular mass of water is 18g.

Number of particles in one mole of any substance is 6.023 x 10²³

Note:

1. If a compound or molecular substance is taken, like CO₂, N₂ etc, then 1

mole of the substance contains Avogadro number of molecules. 1 mole contains the mass equal to molecular weight of substance.

2. If a element is taken like C, S, Fe, then 1 mole contains Avogadro

number of atoms. 1 mole contains the mass equal to atomic weight of substance.

isotones

These are the atoms of different elements with same number of neutrons. Example : ₆C¹³ and ₇N¹⁴

Number of neutrons is obtained by subtracting atomic number from atomic mass number.

Number of neutrons in ₆C¹³ is (13-6) 7.

Number of neutrons in ₇N¹⁴ is (14-7) 7.

● Isobars are the atoms of different elements having same mass number but different atomic numbers.

● Isotopes are the atoms of same element with same atomic number but

different mass number.

● Mass numbers is the sum of protons and neutrons.

● chlorine is diatomic because its molecule contains 2 atoms of chlorine. Cl₂

● neon is monoatomic because its molecule contains 1 atom of neon. Ne

● Phosphorous is polyatomic because its molecule contains 4 atoms of

phosphorous. If a molecule contains more than 3 atoms in its molecule its polyatomic. P₄

● ozone is triatomic because its molecule contains 3 atoms of oxygen. O₃

(i) The molar volume of gas at STP is 22.4 litres (not cm³.)

(ii) 2 x V.D = R.M.M

(iii) An atom may or may not exist independently. Atoms like Fe, Na

can exist independently.

(iv) The ratio of atoms in a molecule may be fixed and integral but

may not be simple.

(v) H₂O is a hetero atomic molecule. It contains more than one type

of atoms i.e hydrogen and oxygen. So, it’s a hetero atomic molecule , not homo atomic molecule.

(i) 1 mole of a compound or molecular substance / Avogadro number of molecules.

(ii) Molar volume

(iii) One atomic mass unit

(iv) Vapour density

(v) atomicity

● An atom is the smallest particle which takes part in chemical reaction.

● An atom is considered to be a divisible particle.

● The atoms of the same element may not be similar in all respects.

ex: Isotopes (₁₇Cl³⁵, ₁₇Cl³⁷ )

● The atoms of different elements may be similar in some respects.

ex. Isobars ( ₁₈Ar⁴⁰ , ₂₀Ca⁴⁰ )

● The ratio of atoms in a molecule may be fixed and integral but may not be simple. Ex.C₁₂H₂₂O₁₁ is not a simple ratio. (Sucrose)

● The atoms of one element can be changed into the atoms of another element by transmutation.

● The mass of an atom can be converted into energy. This is in accordance with Einstein’s equation E = mc² where E = energy, m = mass and c = velocity of light

Relative Molecular Mass: It is defined as the ratio of the mass of 1 molecule of the gas or vapour to the mass of 1 atom of hydrogen.

Vapour Density (V.D): It is defined as the ratio of the mass of a certain volume of the gas or vapour to the mass of the same volume of hydrogen at the same temperature and pressure.

Applying Avogadro’s Law,

Since hydrogen is diatomic,

2 x V.D = relative molecular mass of a gas or vapour

2 x Vapour density = Relative molecular mass

we know that to find the number of moles, the following formulae are used:

(i) Using formula 3,

So, 2 moles of copper contain 12.046 x 10²³ atoms of copper.

(ii) Using formula 1,

So, 0.5 moles consist of 27.95 grams of iron.

(iii) Using formula 4,

So, 0.25 moles of CO₂ consists of 1.51 x 10²³ molecules.

Note:

1. If a compound or molecular substance is taken, like CO₂, N₂ etc, then 1

a mole of the substance contains Avogadro number of molecules. 1 mole contains the mass equal to the molecular weight of the substance.

2. If an element is taken like C, S, Fe, then 1 mole contains Avogadro

number of atoms. 1 mole contains the mass equal to the atomic weight of the substance.

Molecular mass is the sum of the atomic masses of all the atoms present in one molecule of the substance.

1. H₂O = (1gX2) + 16g = 18g

2. CO₂ = 12g + (2 x16g) = 12 + 32 = 44g

3. NaOH = 23g + 16g + 1g = 40g

4. NO₂ = 14 + (2 x 16) = 46g

5. H₂SO₄ = (1gx2) + 32g + (4x16g) = 98g

Formula used is

So, the complete table is

According to mole concept we know that gram molecular weight of any compound consists of Avogadro number of molecules.

This implies that 18 grams of water consists 6.023 x 10²³ molecules of water.

18 grams 6.023 x 10²³ molecules

0.18 grams ?

i) 1 mole of Ca ( 40 g) and 1 mole of an Oxygen atom ( 16 g) combine to form 1 mole of CaO ( 56 g).

ii) 1 mole of Ca ( 40 g) and 1 mole of C ( 12 g) and 3 moles of an Oxygen atom ( 3x16 = 48 g) combine to form 1 mole of CaCO₃ ( 100 g)

Note:

1. If a compound or molecular substance is taken, like CO₂, N₂ etc, then 1

a mole of the substance contains Avogadro number of molecules. 1 mole contains the mass equal to the molecular weight of the substance.

2. If an element is taken like C, S, Fe, then 1 mole contains Avogadro

number of atoms. 1 mole contains the mass equal to the atomic weight of the substance.

So, 1 mole of Ca is 40 grams, 1 mole of O atom is 16 grams and 1 mole of C is 12 grams of C. 1 mole of CaO is 56 grams (40+16) of CaO and 1 mole of CaCO₃ is 100 grams ( 40+12+48).

we know that in the case of compounds –

Molecular mass of water (H₂O) = (2 + 16 ) = 18 grams

Molecular mass of ammonia (NH₃) = ( 14+3) = 17 grams

Molecular mass of glucose ( C₆H₁₂O₆) = ( 72 + 12+ 96) = 180 grams

Applying the above formula to all 3 cases-

(i) Mass = 5 x 18 = 90 grams

(ii) Mass = 2 x 17 = 34 grams

(iii) Mass = 2 x 180 = 360 grams

(i) 3 moles

(ii) 53.5 grams (14 + 4 + 35.5)

(iii) Ammonia ( 2 moles will be remaining )

(iv) NH₃ + HCl NH₄Cl

Explanation -1 mole of any gas at STP occupies 22.4 litres of volume. So, 1 mole of ammonia occupies 22.4 litres of volume. 67.2 litres (22.4 x 3) of volume is 3 moles of ammonia. 22.4 litres of HCl is 1 mole of HCl. According to reaction ,1 mole of ammonia reacts with 1 mole of HCl to form 1 mole of NH₄Cl, which comes out as white fumes.

NH₃ + HCl NH₄Cl

Even though 3 moles of ammonia is present in bulb, only one 1 mole undergoes reaction with 1 mole of HCl. The other 2 moles of ammonia remain unreacted.

1. 4 moles of nitroglycerine ; CO₂ (12) + N₂(6) + O₂(1) = 19 moles of

gas molecules are formed.

2. 4 moles of nitroglycerine gives 19 moles of gas molecules. So, 1 mole

of nitroglycerine gives (19/4) 4.75 moles of gas molecules.

3. Gram molecular mass of nitroglycerine is 1 mole of nitroglycerine.

So, mass of 1 mole of nitroglycerine is 227 grams.

[(3x12)C + (5x1)H + (3x14)N + (9x16)O]

(i) Two (coefficient of sodium bi carbonate)

(ii) 1 mole of sodium bi carbonate contains 84 grams. Two moles of

sodium bi carbonate contains 168 grams. So, 168 grams of sodium bi carbonate is used in the equation.

(iii) 1 mole (coefficient of CO₂ )

(i) 16 grams of O

(ii) 1 mole of oxygen atoms

(iii) 1 mole

(iv) 714.38 grams.

56 grams of CaO gives 40 grams of Ca. 1000 grams of CaO gives (1000x40)/56 = 714.28 grams of calcium

Explanation –

CaO Ca + O

56 40 16

1 mole of any compound (or gas in molecular form) contains the mass equal to its gram molecular mass. 1 mole of any element contains the mass equal to its gram atomic mass. So,

1 mole of CaO = 40 + 16 = 56 grams

1 mole of Ca = 40 grams

1 mole of oxygen atoms = 16 grams

1 mole of calcium oxide dissociates to form 1 mole of Ca and 1 mole of O atoms.

we know that when gases are present in the molecular form, 1 mole of any gas is equal to 6.023 x 10²³ molecules of that gas, which is equal to the gram molecular weight of gas in the combined state.

(i) 1 mole of chlorine molecules contains 71 grams of Cl₂. (2×35.5)

(ii) 2 moles of sulphur molecules contain 512 grams of S₈. (2×32×8)

(iii) 4 moles of ozone molecules contain 192 grams of O₃. (4× 16× 3)

(iv) 2 moles of nitrogen molecules contain 56 grams of N₂. (2× 14× 2)

We know the formula that

Applying the above formula for all cases.

(i)

(ii)

(iii)

(iv)

(v)