CART - Chemistry Acceleration Readiness Atomic Structure and Periodicity Questions and Answers ā Questions and Answers
Question 1: Which of the following sets of quantum numbers (n, l, m_l, m_s) is invalid for an electron in an atom?
- 3, 2, -1, +1/2
- 2, 0, 0, -1/2
- 4, 3, -4, +1/2 (Correct answer)
- 1, 0, 0, +1/2
Correct answer: 4, 3, -4, +1/2
The magnetic quantum number, m_l, can only have integer values from -l to +l. In the set (4, 3, -4, +1/2), l=3, so the possible values for m_l are -3, -2, -1, 0, 1, 2, and 3. The value -4 is not allowed, making this set of quantum numbers invalid.
Question 2: An atom has an atomic number of 15 and a mass number of 31. How many protons, neutrons, and electrons does a neutral atom of this element have?
- 15 protons, 15 neutrons, 16 electrons
- 15 protons, 16 neutrons, 15 electrons (Correct answer)
- 16 protons, 15 neutrons, 16 electrons
- 31 protons, 15 neutrons, 31 electrons
Correct answer: 15 protons, 16 neutrons, 15 electrons
The atomic number (Z) is equal to the number of protons. For a neutral atom, the number of electrons is equal to the number of protons. The mass number (A) is the sum of protons and neutrons. Therefore, protons = 15, electrons = 15, and neutrons = Mass Number - Protons = 31 - 15 = 16.
Question 3: Consider the elements Fluorine (F), Carbon (C), and Oxygen (O). Which of the following correctly lists these elements in order of increasing atomic radius?
- C, O, F
- F, C, O
- O, F, C
- F, O, C (Correct answer)
Correct answer: F, O, C
Atomic radius generally decreases from left to right across a period in the periodic table. This is because as the atomic number increases across a period, the effective nuclear charge increases, pulling the electron shells closer to the nucleus. Fluorine, Oxygen, and Carbon are all in Period 2. Carbon (Group 14) is furthest to the left, followed by Oxygen (Group 16), and then Fluorine (Group 17). Therefore, the order of increasing atomic radius is F < O < C.
Question 4: What is the ground-state electron configuration for a sulfide ion (SĀ²ā»)?
- 1sĀ²2sĀ²2pā¶3sĀ²3pā“
- 1sĀ²2sĀ²2pā¶3sĀ²3pĀ²
- 1sĀ²2sĀ²2pā¶3sĀ²3pā¶ (Correct answer)
- 1sĀ²2sĀ²2pā¶3sĀ²
Correct answer: 1sĀ²2sĀ²2pā¶3sĀ²3pā¶
A neutral sulfur atom (S) has an atomic number of 16, so its electron configuration is 1sĀ²2sĀ²2pā¶3sĀ²3pā“. The sulfide ion (SĀ²ā») has gained two electrons. These two electrons are added to the outermost shell, which is the 3p subshell. This fills the 3p subshell completely, resulting in the electron configuration 1sĀ²2sĀ²2pā¶3sĀ²3pā¶, which is isoelectronic with the noble gas Argon.
Question 5: Which of the following statements best describes the trend of first ionization energy on the periodic table?
- It decreases down a group and increases from left to right across a period. (Correct answer)
- It increases down a group and decreases from left to right across a period.
- It decreases down a group and decreases from left to right across a period.
- It increases down a group and increases from left to right across a period.
Correct answer: It decreases down a group and increases from left to right across a period.
First ionization energy is the energy required to remove the outermost electron from a neutral atom. It decreases down a group because the valence electrons are in higher energy levels, are farther from the nucleus, and experience more shielding, making them easier to remove. It increases from left to right across a period because the increasing effective nuclear charge holds the electrons more tightly, requiring more energy to remove one.
Question 6: An unknown element has two naturally occurring isotopes. The first isotope has a mass of 10.0129 amu and an abundance of 19.9%. The second isotope has a mass of 11.0093 amu and an abundance of 80.1%. What is the average atomic mass of this element?
- 10.51 amu
- 10.81 amu (Correct answer)
- 10.01 amu
- 11.01 amu
Correct answer: 10.81 amu
The average atomic mass is the weighted average of the masses of its isotopes. To calculate it, multiply the mass of each isotope by its fractional abundance and then sum the results: (10.0129 amu * 0.199) + (11.0093 amu * 0.801) = 1.9925671 amu + 8.8184493 amu = 10.8110164 amu. This value, rounded to two decimal places, is 10.81 amu (which corresponds to Boron).
CART - Chemistry Acceleration Readiness Atomic Structure and Periodicity Questions and Answers
Which of the following sets of quantum numbers (n, l, m_l, m_s) is invalid for an electron in an atom?