Question 1

Which of the following best explains why the electron configuration of chromium (Cr, Z = 24) is $[Ar]4s^1 3d^5$ instead of the expected $[Ar]4s^2 3d^4$?

Accepted Answer

Because a half-filled d sublevel ($3d^5$) provides extra stability due to exchange energy and symmetry.

Question 2

Which of the following elements has the highest effective nuclear charge ($Z_{\mathrm{eff}}$) for its valence electrons?

Accepted Answer

Chlorine (Cl)

Question 3

Which of the following statements correctly describes the periodic trend for atomic radius among main-group elements?

Accepted Answer

Atomic radius decreases from left to right across a period and increases from top to bottom within a group.

Question 4

Which of the following ions is isoelectronic with argon ($[Ne]3s^2 3p^6$)?

Accepted Answer

$\mathrm{Cl}^-$

Question 5

A neutral atom of vanadium (V, $Z=23$) has the electron configuration $[Ar]4s^2 3d^3$. How many unpaired valence electrons does it have?

Accepted Answer

Three

Question 6

Which of the following best describes the trend in first ionization energy ($IE_1$) across a period from left to right?

Accepted Answer

$IE_1$ increases due to increased effective nuclear charge.

Question 7

Which of the following ions is most likely to form from a Group 2A (alkaline earth metal) element, and what is its electron configuration?

Accepted Answer

$\mathrm{M}^{2+}$; [noble gas]$

Question 8

Which of the following best explains why the second ionization energy ($IE_2$) of sodium is much higher than its first ionization energy ($IE_1$)?

Accepted Answer

Because the second electron is removed from a noble gas core, which is much more stable.

Question 9

Which of the following is the correct condensed electron configuration for the $\mathrm{Fe}^{2+}$ ion?

Accepted Answer

$[Ar]3d^6$

Question 10

Which of the following best describes the octet rule?

Accepted Answer

Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons.

Question 11

Which of the following is the empirical formula for hydrogen peroxide, given its molecular formula is $\mathrm{H}_2\mathrm{O}_2$?

Accepted Answer

$\mathrm{HO}$

Question 12

Which of the following statements about the formation of ions is correct?

Accepted Answer

Metals tend to lose electrons to form cations, and nonmetals tend to gain electrons to form anions.

Question 13

Which of the following best describes the difference between empirical and molecular formulas?

Accepted Answer

Empirical formulas show the lowest whole-number ratio of atoms, while molecular formulas show the actual number of atoms in a molecule.

Question 14

Which of the following best explains why noble gases are generally unreactive?

Accepted Answer

They have a filled valence shell (octet), making them especially stable.

Question 15

Which of the following is the correct Lewis symbol for sulfur (S)?

Accepted Answer

S with 6 dots around it

Question 16

Which of the following best describes the process of drawing a Lewis symbol for a main-group element?

Accepted Answer

Place one dot for each valence electron, pairing them only after each side has one dot.

Question 17

Which of the following statements about transition metal cations is correct?

Accepted Answer

Transition metals lose their ns electrons before their (n-1)d electrons.

Question 18

Which of the following best explains why atomic radius increases down a group?

Accepted Answer

Because additional principal energy levels are added, increasing the distance between the nucleus and valence electrons.

Question 19

Which of the following ions is most likely to form from tin (Sn, Z = 50), and what is its condensed electron configuration?

Accepted Answer

$\mathrm{Sn}^{2+}$; [Kr]5s^2 4d^{10}$

Question 20

Which of the following best describes the type of bonding in sodium chloride ($\mathrm{NaCl}$)?

Accepted Answer

Ionic bonding due to electrostatic attraction between oppositely charged ions.

Periodic Properties of the Elements and Electron Configurations

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