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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: \( {}^{12}_{6}\text{C} \).
| Element | Protons (p + ) | Neutrons (n 0 ) | Mass number (A) |
|---|---|---|---|
| Hydrogen | 1 | 0 | 1 |
| Helium | 2 | 2 | 4 |
| Lithium | 3 | 4 | 7 |
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 by writing the missing values in each row.
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| Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the elements |
|---|---|---|---|---|---|
| 5 | 11 | 6 | 5 | 5 | Boron |
| 7 | 14 | 7 | 7 | 7 | Nitrogen |
| 12 | 24 | 12 | 12 | 12 | Magnesium |
| 15 | 31 | 16 | 15 | 15 | Phosphorus |
| 1 | 1 | 0 | 1 | 1 | Hydrogen |
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.
