SolveItClass 9 · NCERT

NCERT Solutions · Class 9 Science Journey Inside the Atom

33 questions · 19 still being checked

Pause and Ponder 8.11–8.12 (part 17 of 23)

  1. Exercise 8.11

    The nucleus of an atom contains 20\displaystyle 20 protons. If its mass number is 41\displaystyle 41, find the number of neutrons in it.
    NCERT’s answer
    Neutrons $\displaystyle 21$
    $\displaystyle 21$ neutrons.
    NCERT_Solution_Class9_Science_Ch8_PP_Q8-11
    Protons \(\displaystyle = 20\), so \(\displaystyle Z = 20\); mass number \(\displaystyle A = 41\).
    \(\displaystyle A = \text{protons} + \text{neutrons}\)
    \(\displaystyle \text{neutrons} = A - Z = 41 - 20 = 21\)

  2. Exercise 8.12

    An atom has 18\displaystyle 18 neutrons and an atomic number of 17. What is its mass number?
    NCERT’s answer
    Mass number $\displaystyle 35$
    Mass number \(\displaystyle = 35\).
    NCERT_Solution_Class9_Science_Ch8_PP_Q8-12
    Atomic number \(\displaystyle Z = 17\), so the nucleus holds $\displaystyle 17$ protons.
    \(\displaystyle A = \text{protons} + \text{neutrons} = 17 + 18 = 35\)
    (An atom with \(\displaystyle Z = 17\) is chlorine.)