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💡 A weighty mystery
A helium atom has 2 protons like “double hydrogen”, yet it is about 4 times heavier, not 2 times. Something extra must be sitting in the nucleus.
Why is a helium atom heavier than expected?
- Helium has 2 protons (hydrogen has 1).
- By protons alone, helium should be about twice as heavy.
- But helium is about 4 times heavier — so the nucleus holds extra mass with no extra charge.
❓ What if…
What if an atom had no empty space ? Then matter would be unimaginably dense and objects would shrink enormously in size, because almost all of an atom is actually empty space.
NCERT Question 15 — In an atom, there are
In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom’s: (i) Atomic number (ii) Atomic mass (iii) Mass number (iv) Overall charge
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Important Definitions
- Mass of an atom — almost all of it is concentrated in the nucleus (protons and neutrons); electrons are too light to count.
