MCAT Periodic Trends and Electron Configuration Practice Questions

Answer: B. Group 13

The fourth ionization energy is about 4.5 times the third, a jump of 9200 kJ/mol, far larger than the earlier steps of 1100 and 900 kJ/mol. That jump means the fourth electron comes from a core shell, so the atom has three valence electrons and sits in group 13. A group 1 or group 2 element would show the big jump after the first or second electron.

Answer: D. 5

Neutral iron is [Ar]4s²3d⁶. Forming the cation removes both 4s electrons and then one 3d electron, giving [Ar]3d⁵. By Hund's rule each of the five d orbitals holds one electron, so there are 5 unpaired electrons. Choice C is the count for Fe²⁺, which keeps six d electrons.

Answer: A. 5

Chromium shifts one electron from 4s into 3d, giving one 4s electron and five 3d electrons, a half-filled d subshell that is lower in energy than the simple filling-order prediction of two 4s and four 3d electrons. Choice B is that naive prediction, and choices C and D do not fit the 6 electrons beyond the argon core.

Answer: C. Potassium

Potassium is in period 4, so its valence electron is in a higher shell than sodium's, and it lies at the far left of its period, where effective nuclear charge is lowest. Calcium and bromine are in the same period as potassium but have more protons pulling the same shell inward, so both are smaller. Sodium is one shell smaller.

Answer: A. Atomic radius

Across a period, protons are added while the valence shell stays the same, so effective nuclear charge rises and pulls the electron cloud inward, making atoms smaller. That same stronger pull raises electronegativity and moves the elements from metals such as sodium toward nonmetals such as chlorine. Only atomic radius decreases.

Answer: D. One of oxygen's 2p orbitals holds a pair of electrons, and repulsion within that pair makes one of them easier to remove

Nitrogen has three unpaired 2p electrons, one per orbital. Oxygen's fourth 2p electron must pair up in an orbital that already holds one, and the extra repulsion between the paired electrons lowers the energy needed to remove one. Oxygen actually has a higher effective nuclear charge and a slightly smaller radius, and both atoms use the second shell for valence electrons.

Answer: B. Zn²⁺

Zinc is [Ar]4s²3d¹⁰, and losing the two 4s electrons leaves a full 3d¹⁰ subshell with every electron paired, so Zn²⁺ is diamagnetic. Ti²⁺ is d², Mn²⁺ is d⁵ and Cr³⁺ is d³, and each has unpaired d electrons, so each is paramagnetic.

Answer: C. The calcium ion

With the electron count fixed at 18, size is set by nuclear charge. Calcium has 20 protons, the most of the four, so it draws the electron cloud in most tightly and its 2+ ion is smallest. Sulfide, with only 16 protons for 18 electrons, is the largest, and chloride and potassium fall in between.

Answer: A. Sodium

These are the traits of an alkali metal: one valence electron that is lost easily, giving a +1 ion and a fast reaction with water that yields hydrogen and hydroxide. Magnesium forms a +2 ion and reacts only slowly with cold water, aluminum forms a +3 ion and is protected by an oxide layer, and silicon is a metalloid.