AP Chemistry: Atomic Structure & Periodicity 2

Question 1

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

Accepted Answer

10.8 amu

Answer

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

Question 2

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

Accepted Answer

2, 2, 1, +1/2

Answer

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

Question 3

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

Accepted Answer

3

Answer

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

Question 4

Which species is isoelectronic with the neon atom?

Accepted Answer

Na+

Answer

Na+ has 10 electrons, the same as neon.

Question 5

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

Accepted Answer

18

Answer

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

AP Practice Test