ATOMS AND MOLECULES

Question 1

Name the scientist who established the 2 important laws of chemical combination?

Answer 1

Lavoisier and Proust

Question 2

State the law of conservation of mass.

Answer 2

Mass can neither be destroyed nor created in a chemical reaction.

Question 3

State the law of constant proportions.

Answer 3

In a chemical substance elements are present in definite proportions by mass.

Question 4

State the postulates of Daltons atomic theory?

Answer 4

1. Matter is composed of tiny particles called atoms which participate in chemical reactions.
2. Atoms are indivisible particles which cannot be created nor destroyed in a chemical reaction.
3. Atoms of a given element have identical mass and chemical properties.
4. Atoms of different elements have different mass and chemical properties.
5. Atoms are combined in ratios of small whole numbers to form compounds.
6. The relative number and the kind of atoms are constant in a given type of compound.

Question 5

Significance of the formula of a substance.

Answer 5

• Formula represents the name of the substance.
• It gives the names of all the elements present in the molecule.
• It gives the number of atoms of each element present in the molecule.
• It represents a definite mass of the substance(molecular mass in grams)

Question 6

Define Atom.

Answer 6

It is the smallest unit of a matter that may or may not exist independently and always take part in chemical reactions.

Question 7

Define molecule

Answer 7

The smallest particle of an element or compound that is capable of existing independently and shows all teh properties of that substance.

Question 8

Define Gram Atomic mass

Answer 8

The atomic mass of an element expressed in terms of grams.

Question 9

Molu

Question 10

Define Molecular mass.

Answer 10

The sum of atomic masses of all the atoms in a molecule of the substance. The number of times a molecule of a pure substance is heavier than 1/12th part of C-12 isotope of carbon.

Question 11

Define atomicity

Answer 11

The number of atoms constituting a molecule is known as atomicity

Question 12

Define Valency

Answer 12

The combining capacity of an element and is defined as the number of hydrogen atoms which can combine directly or indirectly with one atom of an element so as to form a chemical compound.

Question 13

Define variable valency

Answer 13

It has been found that certain metals exhibit more than one valency . They are said to exhibit variable valency.
EXAMPLE: - Iron, Copper

Question 14

Define Molecular formula

Answer 14

The symbolic representation of the kind and the actual number of atoms in one molecule of a pure substance which may be an element or a compound.

Question 15

Define Chemical formula

Answer 15

The chemical formula of a compound is the symbolic representation of its composition.

Question 16

Define Ion.

Answer 16

Compounds composed of metals and non-metals contain charged species. These species are known as ions. Ions may consist of single charged atom or a group of atoms that have a net electric charge on them.. A negative charged ion is called anion and a positively charged ion is known as cation.

Question 17

Define Radical (polyatomic ion)

Answer 17

A group of atoms of different elements having some charge (definite positive or negative charge )which can retain its identity in a chemical change.

Question 18

Steps of writing chemical formula

Answer 18

Step1: Write the symbol of a positive ion/radical to the left and for the negative ion/radical on the right.

Step2: Put the valency number for the ion/radical on its top right.

Step3: Divide the valency number by the H.C.F.(if any) to get a simple ratio.

Step4: Cross the valency i.e. shift the valency number by interchanging the valency number of ion/radical.

Question 19

Define Formula unit mass

Answer 19

The formula unit mass of a substance is the sum of atomic masses of all the atoms in a formula unit of a compound.

Question 20

Define mole.

Answer 20

The mole is the amount of substance of a system that contain 6.02214076 ⅹ 10²³ specified elements.
- One mole contains exactly 6.02214076 ⅹ 10²³ elementary entities.

Question 21

Who introduced mole?

Answer 21

• Wilhelm Ostwald

Question 22

Define Avogadro number.

Answer 22

N is the Avogadro constant. When the Avogadro constant is in the form of mol⁻¹, then it
A is called Avogadro number.