Question 1

Which of the following best explains why atomic radius decreases from left to right across a period in the periodic table?

Accepted Answer

The number of protons increases, increasing the effective nuclear charge and pulling electrons closer to the nucleus.

Question 2

Which electron configuration represents a ground-state atom of calcium (Ca, Z = 20)?

Accepted Answer

$$1s^2$$ $$2s^2$$ $$2p^6$$ $$3s^2$$ $$3p^6$$ $$4s^2$$

Question 3

A certain element has the electron configuration $$[Ar]4s^1$. Which element is this, and what is its group in the periodic table?

Accepted Answer

Potassium (K), Group 1

Question 4

Which of the following ions is isoelectronic with neon ($$Ne$$)?

Accepted Answer

$$F^-$$

Question 5

Which of the following statements about the Pauli exclusion principle is correct?

Accepted Answer

No two electrons in an atom can have the same set of four quantum numbers.

Question 6

Which of the following elements would you expect to have the highest first ionization energy?

Accepted Answer

Neon ($$Ne)$$

Question 7

Which of the following best describes the trend in electron affinity across a period from left to right?

Accepted Answer

Electron affinity generally becomes more positive (more energy released) as you move from left to right.

Question 8

Which of the following ions has the largest radius in the isoelectronic series: $$O^{2-}$$, $$F^-$$, $$Na^+$$, $$Mg^{2+}$$?

Accepted Answer

$$O^{2-}$$

Question 9

A transition metal ion is formed by removing electrons from which subshell first?

Accepted Answer

The outermost s subshell

Question 10

Which of the following is the correct condensed electron configuration for $$Pb ($$lead, $$Z = 82$$)?

Accepted Answer

$$[Xe]6s^2$ 4f$$^{14}$$ 5d$$^{10}$$ 6p^2$

Question 11

Which of the following best explains why the second ionization energy of sodium ($$Na) is $$much higher than the first?

Accepted Answer

The second electron is removed from a core shell, which is closer to the nucleus and experiences a higher effective nuclear charge.

Question 12

Which of the following elements is most likely to form a $$2+$$ cation?

Accepted Answer

Magnesium ($$Mg)$$

Question 13

Which of the following statements about Hund’s rule is correct?

Accepted Answer

Electrons will occupy degenerate orbitals singly before pairing up.

Question 14

Which of the following best describes the trend in metallic character as you move down a group in the periodic table?

Accepted Answer

Metallic character increases as you move down a group.

Question 15

Which of the following is a property of metalloids?

Accepted Answer

They have properties intermediate between metals and nonmetals.

Question 16

Which of the following electron configurations represents a paramagnetic atom?

Accepted Answer

$$1s^2$$ $$2s^2$$ $$2p^4$$

Question 17

Which of the following best describes the effective nuclear charge ($$Z_{eff}) $$experienced by a valence electron?

Accepted Answer

$$Z_{eff} = Z - \sigma$$

Question 18

Which of the following is a correct statement about the electron configuration of $$Cr ($$chromium, $$Z = 24$$)?

Accepted Answer

$$[Ar]4s^1$ 3d^5$ is more stable than $$[Ar]$$4s^2$$ $$3d^4$$ due to half-filled d subshell stability.

Question 19

Arrange the following elements in order of increasing atomic radius: $$O$$, $$S$$, $$P.$$

Accepted Answer

$$O \u003c S \u003c P$$

Question 20

Which of the following best explains why the electron affinity of oxygen for a second electron is negative (unfavorable)?

Accepted Answer

Adding a second electron to $$O^-$$ creates strong electron-electron repulsion, requiring energy input.

Periodic Trends and Electron Configurations: Study Guide for Introductory Chemistry

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