SMART EDUCATIONS 1

ATOMS AND MOLECULES

Q. What is an atom?

Ans. Atoms are the building block or structural unit of matter.

Q. What is a molecule?

Ans. Molecule is the smallest part of a matter which shows all the

properties of the matter. Two or more atoms chemically combine to form

Q. How big are atoms?

Ans. An atom is so small that we can’t imagine or compare it with anything.

So, it can’t be seen with naked eyes. Size of the atomic radius is measured

in nanometers(nm). 1nm = 10

-9

m. The smallest atom is helium and the

largest atom is francium.

Q. How do atoms exist?

Ans. An Indian philosopher Pakudha Katyayama said that atoms normally

exist in a combined form which gives us various forms of matter.

Most of the atoms of elements are found in the form of molecules or

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compound or a mixture is composed of small particles called atoms. The

postulates of this theory may be stated as follows:

1. All matter is made of very tiny particles called atoms, which

participate in chemical reactions.

2. Atoms are indivisible particles, which cannot be created or destroyed

in a chemical reaction.

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

properties.

4. Atoms of different elements have different masses and chemical

properties.

1. According to him, atoms are indivisible, but later it was found that

atoms are made up of subatomic particles.

2. According to him, atoms of a given element are identical in mass but

the discovery of Isotopes makes him wrong, which says that atoms of

a given element may not be identical in mass like some hydrogen’s

atomic mass has 1u some has 2u and some has 3u.

3. According to him, atoms of different elements have different masses

but the discovery of Isobars makes him wrong, which says that atoms

of different elements may have different masses like Calcium and

Potassium both have mass 40u.

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2. Law of constant proportions, given by Joseph L. Proust.

Q. State the Law of conservation of mass.

Ans. Law of conservation of mass states that “Mass can neither be created

nor destroyed in a chemical reaction.”

Q. State the Law of constant proportions.

Ans. Law of constant proportions states that “In a chemical substance the

elements are always present in definite proportions by mass.”

Q. How do all elements get their names?

Ans. Elements have been named after a number of things including :-

1. The name of the place where they were found for the first time. For

example, the name coppe r was taken from Cyprus .

2. Specific colours, For example, gold was taken from the English word

meaning yellow.

Q. Who starts the symbolisation of an element?

Ans. Dalton was the first scientist to use the symbols for elements in a very

specific sense but it was difficult to use and memorise, then, Berzilius

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name in English.

2. The first letter of a symbol is always written as a capital letter and the

second letter as a small letter

3. Some symbols have been taken from the names of elements in Latin,

German or Greek.

Q. Table of the symbols of elements.

Ans.

Q. Give some examples of elements whose symbols have been made from

Latin names.

Ans. Some examples of them are:-

1. Gold has the symbol Au, derived from its latin name which is Aurum .

2. Silver has the symbol Ag, derived from its latin name which is

Argentium .

3. Iron has the symbol Fe, derived from its latin name which is Ferrum .

4. Sodium has the symbol Na, derived from its latin name which is

Natrium .

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Q. How were the atomic masses calculated?

Ans. Since determining the mass of an individual atom was a relatively

difficult task, relative atomic masses were determined using the laws of

chemical combinations and the compounds formed. For this, in 1961 1/12

part of an atom of carbon-12 isotope was selected by IUPAC, before it was

1/16

th

part of an atom of oxygen.

Q. What is relative atomic mass?

Ans. The relative atomic mass of the atom of an element is defined as the

average mass of the atom, as compared to 1/12

“unified mass” which is denoted as “u”.

Q. Define 1 unified mass.

Ans. 1 unified mass is equal to the mass of one-twelfth the mass of one

atom of carbon-12.

Q. Write atomic masses of some elements.

Ans.

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Q. What is a molecule?

Ans. A molecule is in general a group of two or more atoms that are

chemically bonded together. It is the smallest particle of an element or a

compound that is capable of an independent existence and shows all the

properties of that substance.

Q. How many types of molecules are there?

Ans. There are two types of molecules:-

in definite proportions to form molecules of compounds. For example

:- CO

4

etc.

Q. What is atomicity?

Ans. The number of atoms constituting a molecule is known as its

atomicity. For example:- Atomicity of H

2

= 2, O

3

= 3, P

4

= 4, S

8

=8, H

O = 3,

NH

Q. What does monatomic mean?

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Q. What does diatomic mean?

Ans. The molecules of an element or compound which are constituted by 2

atoms. For example:- H

, Cl

, N

, O

, CO, NaCl etc.

Q. What does triatomic mean?

Ans. The molecules of an element or compound which are constituted by 3

atoms. For example:- O

, H

O etc.

Q. What does tetratomic mean?

Ans. The molecules of an element or compound which are constituted by 4

Ans.

Atoms

8

2

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Ans. The charged particles in a molecule are called ions. There are two

types of ions:-

1. Cation:- Positive charge is called cation. Examples of cations(all

metals) are:- Al

,Ca

,Na

etc.

2. Anion:- Negative charge is called anion. Examples of anions(almost

all non metals) are :- O

2-

, H

-

,S

2-

Q. What does polyatomic ion mean?

Ans. A group of atoms carrying a charge is known as a polyatomic ion. For

example:- SO

2-

etc.

SMART EDUCATIONS 9

Common Name

Chemical Name

Chemical Formula

2

3

.5H

O