## Atomic Structure Questions and Answers Part-4

1. The number of electrons in $\left[\begin{array}{c}40\\19\end{array}K\right]^{1-}$ is
a) 20
b) 40
c) 18
d) 19

Explanation: 19 + 1e = 20 electrons.

2. Which of the following statements is incorrect?
a) Cathode rays carry negative charge
b) Frequency of X-rays is higher than that of microwaves
c) Unit for wave number is $m^{-1}$
d) Threshold frequency is the same for all metals

Explanation: Threshold frequency is not the same for all metals.

3. Which is not true with respect to cathode rays?
a) A stream of electrons
b) Charged particles
c) Move with speed same as that of light
d) Can be deflected by magnetic fields

Explanation: incorrect

4. Rutherford’s $\alpha$-particle dispersion experiment concludes
a) all positive ions are deposited at small part
b) all negative ions are deposited at small part
c) proton moves around the electron
d) neutrons are charged particles

Explanation: All positive ions are deposited at small part. (nucleus of atom).

5. Ratio of mass of proton and electron is
a) infinite
b) $1.8 ×10^{3}$
c) 1.8
d) None of these

Explanation: Ratio of mass of proton and electron is approx. 1837 = 1.8 × 103

6. Which of the following is the main cause of late discovery of neutron?
a) Neutron is chargeless particle
b) Neutron is highly unstable particle
c) Neutron in the nucleus moves very fast
d) All of these

Explanation: Neutron being chargless particle could not effect certain properties.

7. Which of the following does not contain number of neutrons equal to that of $\begin{array}{c}40\\18\end{array}Ar$
a) $\begin{array}{c}41\\19\end{array}K$
b) $\begin{array}{c}43\\21\end{array}SC$
c) $\begin{array}{c}40\\21\end{array}SC$
d) $\begin{array}{c}42\\20\end{array}Ca$

Explanation: $_{18}Ar^{40}$ contains 22 neutrons and $_{21}Sc^{40}$ contains 19 neutrons. The number of neutrons = (A – Z)

8. Rutherford’s experiment which established the nuclear model of the atom used a beam of.
a) $\beta$-particles which impinged on a metal foil and got absorbed
b) $\gamma$-rays which impinged on a metal foil and ejected electrons
c) helium atoms, which impinged on a metal foil and got scattered
d) helium nuclei, which impinged on a metal foil and got scattered

Explanation: Rutherford used doubly charged helium particle. (a - particle)

9. Number of protons, neutrons and electrons in the element $_{89}X^{231}$ is
a) 89, 89, 242
b) 89, 142, 89
c) 89, 71, 89
d) 89, 231, 89