Physics Wallah is an Indian edtech platform that provides accessible & comprehensive learning experiences to students from Class 6th to postgraduate level. We also provide extensive NCERT solutions, sample paper, NEET, JEE Mains, BITSAT previous year papers & more such resources to students. Physics Wallah also caters to over 3.5 million registered students and over 78 lakh+ Youtube subscribers with 4.8 rating on its app.
We Stand Out because
We provide students with intensive courses with India’s qualified & experienced faculties & mentors. PW strives to make the learning experience comprehensive and accessible for students of all sections of society. We believe in empowering every single student who couldn't dream of a good career in engineering and medical field earlier.
Our Key Focus Areas
Physics Wallah's main focus is to make the learning experience as economical as possible for all students. With our affordable courses like Lakshya, Udaan and Arjuna and many others, we have been able to provide a platform for lakhs of aspirants. From providing Chemistry, Maths, Physics formula to giving e-books of eminent authors like RD Sharma, RS Aggarwal and Lakhmir Singh, PW focuses on every single student's need for preparation.
What Makes Us Different
Physics Wallah strives to develop a comprehensive pedagogical structure for students, where they get a state-of-the-art learning experience with study material and resources. Apart from catering students preparing for JEE Mains and NEET, PW also provides study material for each state board like Uttar Pradesh, Bihar, and others
Q.1 : What is the difference between atomic number and mass number?
Ans. Atomic number represents the number of protons in an atom's nucleus, while mass number represents the total number of protons and neutrons in the nucleus.
Q.2 : How do isotopes relate to atomic number and mass number?
Ans. Isotopes are atoms of the same element with the same atomic number but different mass numbers due to varying numbers of neutrons in their nuclei.
Q.3 : What role does mass number play in atomic structure?
Ans. Mass number indicates the total number of particles (protons and neutrons) in an atom's nucleus. It helps differentiate between isotopes of the same element and influences the atom's stability and physical properties.
Q.4 : Can two elements have the same atomic weight?
Ans. Yes, two elements can have the same atomic weight if their isotopic compositions result in similar weighted averages. However, they will have different atomic numbers and chemical properties.
Q.5 : Are atomic number, mass number, and atomic weight constant for all atoms of an element?
Ans. No, these parameters may vary for different isotopes of an element. While atomic number remains constant for all atoms of an element, mass number and atomic weight may differ depending on the specific isotope being considered.
Atomic Number (Z), Mass Number (A), Isotopes And Atomic Weight, Important Topics JEE 2025
Atomic Number : The atomic number is a fundamental property of an element in chemistry, representing the number of protons in the nucleus of an atom. It uniquely identifies an element and determines its position in the periodic table. The atomic number also equals the number of electrons in a neutral atom, influencing its chemical behavior.
Shrivastav 5 Jun, 2024
Share
Atomic Number :
Welcome, eager learners, to the fascinating realm of science! Today, we're going to unravel the mysteries behind three crucial concepts in the world of atoms: atomic number, mass number, and atomic weight. Now, you might be wondering, "What do these terms even mean?" Well, don't worry! By the end of this article, you'll have a clear understanding of how these concepts work and why they're so important in the field of chemistry. So, let's embark on this enlightening journey together and uncover the wonders of atomic structure!
Atomic Number
Every atom has an atomic number that tells us how many protons it has in its nucleus. Protons are positively charged particles found in the centre of an atom. For example, hydrogen, the simplest atom, has an atomic number of 1 because it has one proton in its nucleus. Each element on the periodic table has a different atomic number, which helps us identify and distinguish between them.
It is represented by “Z”
For neutral Atom : Number of Protons = Number of Electrons
For an ion : Number of Electrons = Z – (Charge on ion)
Z = Number of protons only
Nuclear charge = Z
e
Mass Number
This tells us the total number of protons and neutrons in an atom's nucleus. Neutrons are neutral particles found alongside protons in the nucleus. The mass number is like a sum of protons and neutrons. For example, carbon, a common element, has a mass number of 12 because it has 6 protons and 6 neutrons in its nucleus. It's important to note that different isotopes of the same element can have different mass numbers because they have different numbers of neutrons.
It is represented by “A”
Formula
A = Number of Protons + Number of Neutrons
Number of Neutrons = A-Z
Note:
A is always a whole number.
An atom of the element is represented by
Where,
X = Symbol of Element
Z = Atomic Number = Number of Protons = Number of Electron (If atom is neutral)
A = Mass Number = Number of Neutron + Atomic Number
Isotopes
These are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have different mass numbers but the same atomic number. Isotopes can have slightly different properties because of their varying numbers of neutrons. For example, carbon has three isotopes: carbon-12, carbon-13, and carbon-14. They all have 6 protons (since they're all carbon atoms) but different numbers of neutrons, giving them different mass numbers.
Another Example Of Isotopes
Neutron is not available in Protium
No. of Nucleons = No. of Neutrons + No. of Protons = n + p
Note:
Isotopes do note have the same value of e/m.
Another Weight
This is the average weight of all the isotopes of an element, taking into account their relative abundance in nature. In other words, it's the weighted average of the masses of all the isotopes of an element.
Atomic weight is expressed in atomic mass units (amu).
If element have three isotopes y
1
, y
2
and y
3
and their isotopic weights are w
1
, w
2
, w
3
and their percentage/ possibility/probability/ratio of occurrence in nature are x
1
, x
2
, x
3
respectively, then the average atomic weight of element is
Average atomic weight =
Average atomic weight =
Mass no. [A] and atomic weight (amu = atomic mass unit)
Mass of Proton
(m
p
)
Mass of Neutron (m
n
)
Mass of Electron (m
e
)
1.672 × 10
–27
kg
1.675 × 10
–27
kg
9.1 × 10
–31
kg
1.672 × 10
–24
g
1.675 × 10
–24
g
9.1 × 10
–28
kg
1.00727 amu
1.00866 amu
0.000549 amu
[m
p
≃
m
n
] [m
n
> m
p
]
Method for Analysis of atomic weight
e.g.
p → 6
Weight of Proton = 6 × 1.00727
n → 6
Weight of Neutron = 6 × 1.00866
e → 6
Weight of Electron = 6 × 0.000549
--------------------------------------------
Weight of
Atom = 12.099 a.m.u.
Mass no. of
atom = 12
Note:
Mass no. of atom is always a whole no. but atomic weight may be in decimal.
1.
If no. of protons in X
–2
is 16 then no. of electron in X
+2
will be-
(1)
14
(2)
16
(3)
18
(4)
None
No. of protons in X
–2
is = 16
No. of electrons in X
+2
is = 14
2.
An element have three isotopes and their isotopic weights are 11, 12, 13 unit and their percentage of occurrence in nature is 85, 10, 5 respectively then calculate the average atomic weight of element.
Average Atomic weight =
Average weight =
3.
If mass of neutron is doubled, mass of proton is halved and mass of electron is doubled then find out the new atomic weight of
.
Step-
1
If mass of neutron is doubled, mass of proton is halved and mass of electron is doubled then new atomic mass will be:
Step-
2
Talk to a counsellorHave doubts? Our support team will be happy to assist you!
