Atoms And Molecules

One atomic mass unit is a mass unit equal to the exactly 1/12^th the mass of one atom of carbon-12.

Carbon-112 was chosen as the standard reference for measuring

atomic masses.

The chemical formula of aluminium sulphate is Al₂(SO₄)₃

It contains 2 atoms of aluminum, 3 atoms of sulphate and 12 atoms of oxygen. Hence, it contains total 17 atoms.

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

The postulateof Dalton’s atomic theory: Atoms are indivisible particles which cannot be created or destroyed in a chemical reaction.

Atoms are the building blocks of all matter. They are very small; they are smaller than anything we can imagine or compare with.

Law of definite proportions: This law states that “in a chemical substance, the elements are always present in definite proportions”.

Postulateof Dalton’s atomic theory: Atoms of given elements are identical in mass and chemical properties.

For sulphur:

Molar mass of S = 32g/mol

Sulphur is made up of 8 sulphur atoms.

Hence, molar mass of sulphur (S₈) is:

8 × 32mol/g = 256mol/g

For phosphorus:

Molar mass of P = 31g/mol

Phosphorus molecule is made up of 4 phosphorus atoms.

Hence, molar mass of sulphur (P₄) is:

4 × 31mol/g = 124 mol/g

It is incorrect to say that atoms are indivisible particles because later on it was discovered that atoms can be further divided. It contains some fundamental sub-atomic particles which are electrons, protons, and neutrons. Even neutrons and protons are sub divided into different types.

The following diagram shows the division of particles in matter:

Atoms of most elements are not able to exist independently. Atoms form molecules and ions. These molecules or ions aggregate in large numbers to form the matter that we can see, feel or touch.

Molecules of many elements, such as argon (Ar), helium (He), neon (Ne), etc., are made-up of only one atom of the element because:

i. These elements are noble gases which are unreactive in nature.

ii. They have a completely filled outermost shells.

iii. As a result, they are unable to combine with any other element.

iv. Hence, they exist independently in the form of a single atom.

The atoms of elements such as helium, neon, argon, radon, etc., do not form ions easily because:

i. These elements are noble gases which are unreactive in nature.

ii. They have a completely filled outermost shells.

iii. As a result, they do not loose or gain any electron.

iv. Hence, they exist independently in the form of a single atom and do not form ions easily.

Importance of valency:

i. The combining power of an element is known as its 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 compound.

Difference between ion and atom:

Difference between cation and anion:

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

For example: The molecular mass of CO₂ = 1× atomic mass of carbon + 2× atomic mass of oxygen

⇒ 12 + 2 × 16

⇒ 12 + 32 = 44u

(b) Chemical formula:

The chemical formula of a compound is a symbolic representation of its composition. The chemical formula for different compounds can be written easily. For writing a chemical formula, we need to learn the symbols and combining the capacity of elements.

For example Chemical formula of carbon tetrachloride:

Valency of carbon is 4

Valency of chlorine is 1

By applying the criss-cross method:

Formula: CCl₄

Thus, the formula for carbon tetrachloride is CCl₄.

(c) One mole of any species (atoms, ions or molecules) having that quantity in number having a mass equal to its atomic or molecular mass in grams.

Postulates of Dalton’s atomic theory are:

i. Elements are made up of very small indivisible particles called atoms.

ii. All atoms of a particular element are identical in shape.

iii. The properties of atoms of a particular element are different from the atoms of other elements.

iv. An atom can neither be created nor destroyed.

v. Atoms of one element cannot be converted into those of another element.

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 a chemical reaction must be equal to the mass of reactants consumed.

For example:

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

Law of constant proportions:

This law states that “in a chemical substance, the elements are always present in definite proportions by mass”.

For example: In ammonia (NH₃), nitrogen and hydrogen are always present in the ratio of 14:3 by mass, whatever the method or source from which it is obtained.

In water, hydrogen and oxygen are always present in the ratio of 1:8 by mass, whatever the method or source from which water is obtained.

(i) Magnesium bicarbonate:

Valency of magnesium = 2

Valency of carbonate = 2

By applying criss-cross method:

Formula: MgCO₃

(ii) Cupric oxide:

Valency of cupric = 2

Valency of oxide = 2

By applying criss-cross method:

Formula: CuO

(iii) Ferric oxide:

Valency of ferric = 3

Valency of oxide = 2

By applying criss-cross method:

Formula: Fe₂O₃

(iv) Ammonium hydroxide

Valency of ammonium = 1

Valency of hydroxide = 1

By applying criss-cross method:

Formula: NH₄OH

(v) Calcium carbonate

Valency of calcium = 2

Valency of carbonate = 2

By applying criss-cross method:

Formula: CaCO₃

(vi) Potassium carbonate

Valency of potassium = 1

Valency of carbonate = 2

By applying criss-cross method:

Formula: K₂CO₃

The rules which are to be kept in mind while writing a chemical formula is:

i. The valencies or charges on the ion must balance.

ii. When a compound consists of a metal and a non-metal, the name or symbol of the metal is written first.

iii. For ex: calcium oxide (CaO), sodium chloride (NaCl) where oxygen and chlorine are written on the right whereas calcium and sodium are written on the left.

iv. If the valencies of two elements are same, we will simplify the formula.

v. In compounds formed with polyatomic ions, the ion is enclosed in a bracket before writing the number to indicate the ratio.

To calculate the mass of 1 atom of oxygen:

Atomic mass of oxygen = 16

1 mole of atom = 6.022 × 10²³ (Avogadro’s no.)

Mass of 1 oxygen atom = 2.65 × 10²³g

To calculate the mass of 1 atom of sodium:

Atomic mass of sodium = 23

1 mole of atom = 6.022 × 10²³ (Avogadro’s no.)

Mass of 1 oxygen atom = 3.81 × 10²³g

100 grams of magnesium:

1 mole of Mg atom = 6.022 × 10²³ atoms

This means 24g of Mg has 6.022 × 10²³ atoms

Then 100 g contains:

Number of Mg atoms = 2.5 × 10²⁴ atoms

100 grams of aluminium:

1 mole of Al atom = 6.022 × 10²³ atoms

This means 27g of Al has 6.022 × 10²³ atoms

Then 100 g contains:

Number of Al atoms = 2.2 × 10²⁴ atoms

Thus, 100g of magnesium has more atoms.

Given: At. Mass of S = 32 u

Atomic mass of S₈ = 8 × 32u = 256u

1 mole of Al atom = 6.022 × 10²³ atoms

This means 265g of S₈ has 6.022 × 10²³ molecules

Then 64g g contains:

⇒ Number of S₈ molecules = 1.5 × 10²³ molecules

Thus,the number of molecules of sulphur (S₈) is1.5 × 10²³ molecules.

Given: At. Mass of AL = 27 u

Molar mass of aluminium oxide (Al₂O₃) = 2 × 27 + 3× 16 = 54 + 48 = 102g

This means 102g of Al₂O₃ has 2 × 6.022 × 10²³ molecules

Then 0.051g contains:

⇒ Number of Al³⁺ ions = 6.022 × 10²⁰

Thus, 0.051 g of aluminium oxide contains6.022 × 10²⁰ aluminium ions.

(i) 92 grams of sodium:

Given: Mass of sodium = 92g

Molar mass = 23g/mol

To calculate the number of moles, we apply the formula:

⇒

⇒ Number of moles = 4 mol

Thus, the number of moles in92 grams of sodium is 4 mol.

(ii) 108 grams of aluminium:

Given: Mass of aluminium = 108g

Molar mass = 27g/mol

To calculate the number of moles, we apply the formula:

⇒

⇒ Number of moles = 4 mol

Thus, the number of moles in 108 grams of aluminiumis 4 mol.

(i) 1 mole = 6.022 × 10²³

0.1 mole = 0.1 × 6.022 × 10²³ = 6.022 × 10²² molecules

1 mole of sulphur (S₈) contains = 8 atoms

Hence, no of atoms present = 8 × no. of molecules

⇒ Number of atoms present = 8 × 6.022 × 10²² molecules

⇒ Number of atoms present = 4.8 × 10²³ atoms

Thus,number of atoms present in 0.1 mole of sulphur is 4.8 × 10²³atoms.

(ii) 1 mole = 6.022 × 10²³

2 mole = 2 × 6.022 × 10²³ molecules

1 mole of phosphorus (P₄) contains = 4 atoms

Hence, no of atoms present = 4 × no. of molecules

⇒ Number of atoms present = 4 × 2× 6.022 × 10²³ molecules

⇒ Number of atoms present = 4.8 × 10²⁴ atoms

Thus,number of atoms present in 2 mole of phosphorus is 4.8 × 10²⁴atoms.

To calculate the mass of 1 atom of oxygen:

Atomic mass of oxygen = 16

1 mole of atom = 6.022 × 10²³ (Avogadro’s no.)

Mass of 1 oxygen atom = 2.65 × 10²³g

Molar mass of sucrose = C₁₂H₂₂O₁₁

= 12× 12 + 1× 22 + 16 × 11 = 342g/mol

⇒

⇒ Number of moles = 0.01

Sucrose (C₁₂H₂₂O₁₁) contains 11 oxygen atoms

⇒ 11 × 6.022 × 10²³

For 0.01 moles of sucrose

⇒ 0.01 × 11 × 6.022 × 10²³= 6.6 × 10²²

Now, Molar mass of water = H₂O = 2× 1 + 16 = 18g/mol

⇒

⇒ Number of moles = 1

Sucrose (H₂O) contains 1 oxygen atom = 6.022 × 10²³

For 1 mole of water = 6.022 × 10²³

Now, add the both values: 6.6 × 10²²+ 6.022 × 10²³

We get 6.68 × 10²³ atoms. Hence, the option (a) is correct.

A change in the physical state can be brought about only when energy is either given to, or taken out from the system. When the energy is given, it is called endothermic reaction and when energy is released from the system, it is called exothermic reaction.

In water, hydrogen and oxygen are always present in the ratio of 1:8 by mass, whatever the method or source from which water is obtained.

The postulate of Dalton’s atomic theory that “ Atoms of a given element are identical in mass and chemical properties” was challenged by the next generation scientists. They have to prove this theory correct at any cost.

1/12^th of mass of C-12 atom represents 14u. One atomic mass unit is a mass unit equal to one atom of carbon-12.

The symbol of calcium is Ca.

The symbol of carbon is C.

The symbol of copper is Cu.

The unit accepted for taking mass of atom and molecule is milligram.

A. 0.2 mole of sucrose (C₁₂H₂₂O₁₁):

Given: Number of moles = 0.2

Molar mass of C₁₂H₂₂O₁₁ = 342 g/mol

To find out the mass of sucrose, apply the formula given:

⇒

⇒ Mass = 0.2 mol × 342g/mol

⇒ Mass = 68.4g

Thus, the mass of sucrose is 68.4 g.

B. 2 moles of CO₂:

Given: Number of moles = 2

Molar mass of CO₂ = 44 g/mol

To find out the mass of sucrose, apply the formula given:

⇒

⇒ Mass = 2 mol × 44g/mol

⇒ Mass = 88g

Thus, the mass of CO₂ is 88 g.

C. 2 moles of CaCO₃:

Given: Number of moles = 2

Molar mass of CaCO₃ = 40 + 12 + 3× 16 = 100 g/mol

To find out the mass of sucrose, apply the formula given:

⇒

⇒ Mass = 2 mol × 100g/mol

⇒ Mass = 200g

Thus, the mass of CaCO₃ is 200 g.

D. 10 moles of H₂O:

Given: Number of moles = 10

Molar mass of H₂O = 18 g/mol

To find out the mass of sucrose, apply the formula given:

⇒

⇒ Mass = 10 mol × 18g/mol

⇒ Mass = 180g

Thus, the mass of H₂O is 180 g.

Hence, the option C is correct which has higher mass.

When the mass is same, the number of atoms is inversely proportional to the molecular mass. This means less will be the molecular mass, more will be the number of atoms.

H₂O = 2× 1 + 16 = 18u

O₂ = 2× 16 = 32u

CO₂ = 12 × 2 × 16 = 44u

CH₄ = 12 + 4× 1 = 16u

Hence, 18 g of CH₄ has maximum numbers of atoms.

When the mass is the same, the number of molecules is inversely proportional to the molecular mass. This means less will be the molecular mass, more will be the number of molecules.

CO₂ = 12 × 2 × 16 = 44u

N₂ = 2× 14 = 28u

H₂ = 2× 1 = 2u

CH₄ = 12 + 4× 1 = 16u

Hence, 1 g H₂ has maximum numbers of molecules.

Atomicity: The number of atoms constituting a molecule is known as its atomicity.

Sulphur molecule (S₈) : Atomicity is 8

Ozone molecule (O₃): Atomicity is 3

Phosphorus molecule (P₄): Atomicity is 4

Methane molecule (CH₄): Atomicity is 5

Thus, option C is correct.

Aluminium sulphate

Valency of aluminium = 3

Valency of sulphate = 2

By applying criss-cross method:

Formula: Al₂(SO₄)₃

Thus, option A is correct.

The chemical symbol of nitrogen gas is N₂.

The chemical symbol of sodium is Na. The symbol of sodium was taken from its Latin name “natrium”.

20 moles of water:

Given: Number of moles = 20

Atomic mass of H₂O = 2×1 + 16 = 18g/mol

To find out the mass of water, apply the formula given:

⇒

⇒ Mass = 20 mol × 18g/mol

⇒ Mass = 360 g

1.2044 x 10²⁵ molecules of water:

1 mole = 6.022 × 10²³

⇒ 20 moles

Mass of 20moles of water is 360g.

Thus, option D is correct.

Atoms are not able to exist independently. They may or may not exist independently. Among all the given options, option A is correct.

18 g of H₂O:

⇒

⇒ Number of moles = 1

6.022 x 10²³ molecules of H₂O:

We now that 1 mole contains 6.022 x 10²³ molecules.

Thus, option A is correct.

A molecule is:

⇒ A group of two or more atoms that are chemically bonded together.

⇒ A smallest particle of an element or a compound that is capable of an independent existence and shows all the properties of that substance.

⇒ Substance which is formed by the combination of atoms of the same element or of different elements.

Thus, option D is correct.

2 moles of H₂O has less number of moles as well as less molecular mass among all the given options. Hence, it has maximum mass. Thus, option D is correct.

False

Oxygen molecule do no exists in the form of O₄. It exists in two forms which are O₂ (oxygen gas) and O₃ (ozone).

Hence, the given statement is false.

False

Na shows valency of Na⁺ because it has only one electron in the outermost shell. It cannot show the valency of Na²⁺

Hence, the given statement is false.

True

Iron shows variable valency. It shows valency of Fe²⁺ and Fe³⁺ simultaneously.

For example: In ferrous oxide: Fe²⁺ and in ferric oxide: Fe³⁺

Hence, the given statement is true.

True

Formulafor cupric oxide:

Valency of cupric = 2

Valency of oxide = 2

By applying criss-cross method:

Formula: CuO

Formulafor cuprous oxide:

Valency of cuprous = 1

Valency of oxide = 2

By applying criss-cross method:

Formula: Cu₂O

Hence, the given statement is correct.

True

The elements having one valency are called monovalent. The given statement is correct.

False

A group of atoms carrying a charge is called polyatomic ion. These are a group of atoms carrying either a negative or positive charge. For example: SO₃^2-, OH^- etc.

Hence, the given statement is wrong.

False

Atomicity of phosphorus (P₄) is 4 and atomicity of sulphur (S₈) is 8. Hence, the given statement is wrong.

False

One mole of any species (atoms, ions or molecules) having a that quantity in number having a mass equal to its atomic or molecular mass in grams.

Hence, the given statement is wrong.

True

A mole always represents for gases, 22.4 litres of volume at 0°C and 1 atmospheric pressure which is known as standard temperature and pressure (STP).

The given statement is correct.

False

Molar mass is the mass of one mole of substance. The given statement is wrong.