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NCERT Solutions for Class 9th Science Chapter 8 .5 Atomic Number — In-text Questions

Book page 1508 Updated on2026-09-08

Q1.
Can you now say that elements with different atomic numbers are distinct from each other, and the atomic number uniquely identifies an element?
Answer

Yes. The atomic number Z uniquely identifies an element, and two atoms with different Z are always different elements.

  • Z = 1 → hydrogen; Z = 2 → helium; Z = 11 → sodium. There is no second element with Z = 11.
  • Nothing else works this way. The mass number does not identify an element — 40Ar, 40K and 40Ca all have A = 40 but are three different elements (isobars).
  • The neutron number does not identify it either — 126C, 136C and 146C have 6, 7 and 8 neutrons and are all carbon (isotopes).
Why it happens: in a neutral atom the number of electrons equals the number of protons, so Z fixes the electron count. The electron count fixes the electronic configuration, which fixes the number of valence electrons — and valence electrons decide chemistry. Change Z and you change the chemistry; change only the neutrons and the chemistry stays the same.
Q2.
Observe Fig. 8.10. How many neutrons and protons are present in a lithium atom, and what is its atomic number?
Answer

Reading the enlarged nucleus in Fig. 8.10, the lithium nucleus contains 3 protons (the red spheres) and 4 neutrons (the green spheres) — 7 nucleons in all. Three electrons are shown moving around it.

Number of protons = 3
Number of neutrons = 4
Atomic number, Z = number of protons = 3
Mass number, A = p+ + n0 = 3 + 4 = 7
Symbol: 73Li

This agrees with Table 8.3, which lists lithium as 3 protons, 4 neutrons, mass number 7. Being neutral, the atom also has 3 electrons, arranged as K = 2, L = 1.

Why it happens: notice how Fig. 8.10 zooms in three times — from a piece of lithium metal, to the packed lithium atoms, to one atom, to its nucleus. Each step is about 105 times smaller than the last but one. That is the picture you should carry: solid metal is atoms packed together, and each atom is mostly the space its 3 electrons occupy.
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