Class 9 Science

Chapters

# Atomic Number

• Numerical problems on atomic number
• Numerical problems on mass number

Atomic number is the fundamental properties of an atom. Every atom is identified by its unique atomic number. Atomic number is denoted by āzā.

Atomic number is equal to the number of protons present in an atom.

Since an atom is electrically neutral, thus number of protons and number of electrons are equal to make an atom electrically neutral.

Atomic number = Number of protons = Number of electrons

Example:

The atomic number of Hydrogen is 1, helium is 2, lithium is 3, beryllium is 4, boron is 5, carbon is 6, nitrogen is 7, oxygen is 8, etc.

Sample exercise:

(1) Atomic number of calcium is 20. Calculate the number of electrons and protons in calcium.

Solution:

Since, Atomic number = Number of protons = Number of electrons
Therefore,
Number of electrons in calcium = 20
Number of protons in calcium = 20

(2) Number of protons in sodium atom is 11, find the atomic number and number of electrons in a sodium atom.

Solution:

Since, Atomic number = Number of protons = Number of electrons
Therefore,
Atomic number of sodium = 11
Number of electrons in sodium = 11

## Mass Number or Atomic Mass

Mass number of an atom is defined as the sum of the number of protons and number of neutrons. Mass number is nearly equal to the atomic mass of an atom. Since, protons and neutrons reside in the nucleus, thus they are also known as nucleons.

This means

Mass number of an atom = Number of protons + Number of neutrons

Example

(1) Atomic mass of aluminium is 27 u and atomic number is 13, find the number of protons and number of neutrons in aluminium.

Solution:
Since,
Atomic number = 13
Therefore, number of proton = 13

We know that; Atomic mass (Mass number) = Number of protons + Number of neutrons
Therefore,
27 u = 13 + n
Or, n = 27 ā 13 = 14
Therefore, number of proton = 13 and number of neutron = 14

(2) The atomic number of carbon is 6 and number of neutron is equal to 6. Find the atomic mass or mass number of carbon.

Solution:
Since atomic number of carbon = 6
Therefore, number of proton = 6
Now, Atomic mass = number of proton + number of neutron
Or, Atomic mass or mass number = 6 + 6 = 12 u
Thus, mass number or atomic mass of carbon = 12u

(3) The atomic number of oxygen is 8 and number of neutron is equal to 8. Find the atomic mass or mass number of oxygen.

Solution:
Since atomic number of oxygen = 8
Therefore, number of proton = 8
Now, Atomic mass = number of proton + number of neutron
Or, Atomic mass or mass number = 8 + 8 = 16 u
Thus, mass number or atomic mass of oxygen = 16u

## Quiz

1: (b) 20, 2: (d) 11, 3: (c) Number of protons + Number of neutrons, 4: (b) 14, 5: (a) 13, 6: (c) 12, 7: (b) Two, 8: (d) Eight, 9: (c) Seven, 10: (c) Six

## Matter in Our Surroundings

Anything that has both mass and volume is called matter. You can also say that anything which has mass and which occupies space is called matter.

## Is Matter Around Us Pure?

Elements and compounds are pure substances. All other substances are mixtures which means they are not pure substances.

## Atoms and Molecules

Read about law of conservation of mass, law of constant proportions and Dalton's atomic theory.

## Structure of Atoms

Atom is made of three particles; electron, proton and neutron. These particles are called fundamental particles of an atom or sub atomic particles.

## Cell: The Fundamental Unit of Life

A cell is capable of independent existence and can carry out all the functions which are necessary for a living being.

## Tissue

A groups of cells which is meant for a specific task is called tissue. Tissues are the first step towards division of labour in complex organisms.

## Diversity in Living Organisms

Without proper classification, it would be impossible to study millions of organisms which exist on this earth.

## Motion

If an object changes its position with respect to a reference point with elapse of time, the object is said to be in motion.

## Force & Laws of Motion

Force has numerous effects. Force can set a stationary body in motion. Force can stop a moving body.

## Gravitation

Earth attracts everything towards it by an unseen force of attraction. This force of attraction is known as gravitation or gravitation pull.

## Work & Energy

When force is exerted on an object and object is displaced, work is said to be done. It means work is the product of force and displacement.

## Sound

Sound is a type of energy. Sound travels in the form of wave from one place to another.

## Why Do We Fall Ill

Health is a state of physical, mental and social well being. A condition in which the affected person is unable to carry out normal activities is termed as disease.

## Natural Resources

Resources which are obtained from nature are called natural resources. Examples: Air, water, soil, wood, etc.

## Improvement in Food Production

Food security is said to exist when all people, at all times, have physical and economic access to sufficient, safe and nutritious food to meet their dietary needs and food preferences for an active and healthy life.