Chemistry MCQs – Atomic Structure

Multiple Choice Questions

1. Which property of an element is most dependent on the shielding effect?
atomic number
atomic mass
atomic radius
Correct. Shielding effect can be defined as a reduction in the effective nuclear charge on the electron cloud, due to a difference in the attraction forces of the electrons on the nucleus. It directly affect the atomic radius.
number of stable isotopes
2. Which element will exhibit the photoelectric effect with light of the longest wavelength?
Correct. Remember, E=hν and ν=c/λ. So, longest wave length present with the lowest frequency, so the lowest energy. Then, we should find which element can emit EM waves with lowest energy. Energy difference decreases when the distance between the energy levels come closer. So, we have to find which element has the closest energy levels. K, Mg and Ca all have four energy levels in total. But Rb has five energy levels. Energy levels significantly come closer when far from the nucleus. So, the closest energy levels are present in Rb which is between the 4th and 5th energy levels. Hence the longest wave length can be achieved by Rb.
3. Magnesium (Z = 12) has isotopes that range from Mg–20 to Mg–31. Only Mg–24, Mg–25, and Mg–26 are not radioactive. What mode of radioactive decay would convert Mg–20, Mg–21, Mg–22, and Mg–23 into stable isotopes most quickly?
electron emission
alpha particle emission
gamma emission
positron emission
Correct. Positron is represented as . Mg has 12 protons in its nucleus. So, for isotopes Mg-20, Mg-21, Mg-22 and Mg-23, to become more stable, either their number of neutrons should increase or the number of protons should decrease. In other words, the mass number should increase or the atomic number should decrease. The second option can be achieved by positron emission. (eg:- )
4. Which region of the electromagnetic spectrum is capable of inducing electron transitions with the greatest energy?
Correct. You have to know that E=hν and which EM wave has the highest frequency.
5. Which experimental evidence most clearly supports the suggestion that electrons have wave properties?
Correct. This is knowledge.
emission spectra
photoelectric effect
deflection of cathode rays by a magnet
6. Which species is most likely to lose a positron (β+)?
Correct. Positron emission is most possible in atomic species with high nuclear charge relative to the mass number.
7. What mode of radioactive decay is most likely for the isotope ?
electron capture
Correct. The given species should do positron emission or electron capture.
8. The first ionization energy of cesium is . What is the minimum frequency of light that is required to ionize a cesium atom?
Correct. E=hν ⇒ ν=E/h.
9. Which nucleus is not radioactive?
Correct. You have to find the most stable nucleus. Easy!!
10. The energy required to ionize a potassium ion is . What is the longest wavelength of light that can cause this ionization?
285 nm
Correct. Simple calculations with Plank’s constant.
216 nm
200 nm
107 nm
11. What is the energy of a photon from a laser that emits light at 632.8 nm?
Correct. E=hν ⇒ E=h(c/λ).
12. What is the frequency of light with a wavelength of 480 nm?
Correct. Simple calculation, ν=c/λ
13. The energies of the 3s, 3p and 3d orbitals in a multielectron atom increase in that order. To which factor(s) can this order be attributed?
I only
II only
Both I and II
Neither I nor II
14. What is the energy of photons with a wavelength of 434 nm?
Correct. Here you can find the energy produced by one photon by the equation E=hν. Then multiply it by the Avogadro constant to find the energy of one mole of photons.

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