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💡 Counting the heavy bits
Electrons are almost weightless. So an atom’s mass is really just a headcount of the heavy particles in the nucleus — the nucleons.
What is mass number (A)?
- Total number of protons + neutrons (called nucleons ).
- Denoted by A : A = p + n .
- Electrons are so light their mass is ignored .
Standard notation writes the mass number (A) as a superscript and atomic number (Z) as a subscript before the symbol, e.g. carbon: 12 6 C .
| Element | Protons (p + ) | Neutrons (n 0 ) | Mass number (A) |
|---|---|---|---|
| Hydrogen | 1 | 0 | 1 |
| Helium | 2 | 2 | 4 |
| Lithium | 3 | 4 | 7 |
⏸▶ Pause and Ponder
- 10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons. → Z = 26, so protons = 26 and electrons = 26 . Nucleons = A = 56, so neutrons = 56 − 26 = 30 .
- 11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it. → neutrons = A − Z = 41 − 20 = 21 .
- 12. An atom has 18 neutrons and an atomic number of 17. What is its mass number? → A = protons + neutrons = 17 + 18 = 35 .
- 13. An atom 23 A has 11 electrons. Find the number of neutrons in it. → electrons = protons = 11, so Z = 11 and A = 23. Neutrons = 23 − 11 = 12 .
NCERT Question 8 — Magnesium is essential for many
Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.
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NCERT Question 11 — An atom 70X has 31
An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
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NCERT Question 12 — An atom has 79 protons
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
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NCERT Question 13 — Complete the Table 8.5
Complete the Table 8.5:
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| Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the elements |
|---|---|---|---|---|---|
| 5 | – | 6 | – | – | – |
| – | 14 | – | – | 7 | Nitrogen |
| – | 24 | – | 12 | – | – |
| 15 | – | 16 | – | – | – |
| – | 1 | 0 | – | – | – |
✅ Test Yourself
-
Write the formula for mass number.
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Mass number (A) = number of protons + number of neutrons. -
What are nucleons?
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The protons and neutrons taken together in the nucleus. -
Lithium has 3 protons and 4 neutrons. What is its mass number?
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A = 3 + 4 = 7 . -
Why is the electron ignored while calculating mass?
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Its mass is almost negligible compared with a proton or a neutron.
Important Definitions
- Mass number (A) — the total number of protons and neutrons in the nucleus.
- Nucleons — the protons and neutrons present together in the nucleus.
