Atomic Number (Z), Mass Number (A), Isotopes And Atomic Weight, Important Topics JEE 2025
Shrivastav 5 Jun, 2024
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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”
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”
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
Mass no. of
Note: Mass no. of atom is always a whole no. but atomic weight may be in decimal.
Check : JEE Main Seat Matrix 2024
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
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
Atomic Number FAQs
Q.1 : What is the difference between atomic number and mass number?
Q.2 : How do isotopes relate to atomic number and mass number?
Q.3 : What role does mass number play in atomic structure?
Q.4 : Can two elements have the same atomic weight?
Q.5 : Are atomic number, mass number, and atomic weight constant for all atoms of an element?
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