Chemistry: Atomic Structure & Periodicity Flashcards

A sample of an element contains isotopes of mass 10.0 amu (20%) and 11.0 amu (80%). What is the average atomic mass?

Answer: 10.8 amu

Weighted average = (0.20)(10.0) + (0.80)(11.0) = 10.8 amu.

Which set of quantum numbers (n, l, ml, ms) is NOT allowed?

Answer: 2, 2, 1, +1/2

For n=2, l can only be 0 or 1, so l=2 is forbidden.

How many electrons in a ground-state phosphorus atom (Z=15) are unpaired?

Answer: 3

The 3p^3 configuration places one electron in each of three p orbitals by Hund's rule, giving 3 unpaired electrons.

Which species is isoelectronic with the neon atom?

Answer: Na+

Na+ has 10 electrons, the same as neon.

What is the maximum number of electrons that can occupy the n=3 principal energy level?

Answer: 18

The capacity is 2n^2 = 2(3)^2 = 18 electrons.

An electron transition releasing the highest-energy photon in a hydrogen atom would be from n=4 to which level?

Answer: n=1

A drop to n=1 spans the largest energy gap, so it emits the highest-energy photon.

Which subshell is filled immediately after the 4s subshell according to the aufbau principle?

Answer: 3d

After 4s, the 3d subshell fills next due to its lower energy.