MCQ practice · SSC Chemistry Board questions · Chapter 3: Structure of Matter
In the symbol ³⁹₁₉K, for the element:
- i. Number of protons 19
- ii. Mass number 39
- iii. Number of electrons 20
Think it through first, then pick an option.
- a i and ii
- b i and iii
- c ii and iii
- d i, ii and iii
Answer and explanation
Correct answer: a i and ii
The proton number 19 is the lower-left number and the mass number 39 the upper-left one; both are right.
Why the other options are wrong
- b i and iii: iii is false: a neutral K atom has electrons = protons = 19; 20 is the neutron count (39 − 19).
- c ii and iii: iii is false: there are 19 electrons, not 20; and the true statement i is left out.
- d i, ii and iii: iii is false: 20 is the neutron count; the electrons are 19.
More board questions on this topic
- How many protons does the Al³⁺ ion have?
- How many electrons does the ion ²⁷₁₃Al³⁺ have?
- Which carries the true identity of an atom?
- How many protons are in the nucleus of helium?
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