Atomic Structure Quiz - Answer Key

Section A Counting Subatomic Particles - Answers

Problem 1

Atomic number 53, mass number 127:

a. 53 protons
b. 74 neutrons
c. 53 electrons

Work:
a. Protons = Atomic number = 53
b. Neutrons = Mass number - Atomic number = 127 - 53 = 74
c. Electrons = Protons (neutral atom) = 53

Problem 2

For \({}^{139}_{56}\text{Ba}\):

a. 56 protons
b. 83 neutrons
c. 56 electrons

Work:
a. Protons = Z (bottom number) = 56
b. Neutrons = A - Z = 139 - 56 = 83
c. Electrons = Protons (no charge shown = neutral) = 56

Problem 3

Neutrons in \({}^{108}_{47}\text{Ag}\):

61 neutrons

Neutrons = 108 - 47 = 61

Problem 4

Protons in \({}^{65}_{30}\text{Zn}\):

30 protons

Protons = Z (bottom number) = 30

Problem 5

Electrons in neutral \({}^{27}_{13}\text{Al}\):

13 electrons

Neutral atom: Electrons = Protons = Z = 13

Section B Isotope Notation - Answers

Problem 6

Isotope notation:

a. \({}^{55}_{24}\text{Cr}\) (Chromium-55)
b. \({}^{173}_{75}\text{Re}\) (Rhenium-173)

Work:
a. 24 protons → Z = 24 → Chromium (Cr)
   Mass number = 24 + 31 = 55
   24 electrons = 24 protons → neutral atom (no charge)

b. 75 protons → Z = 75 → Rhenium (Re)
   Mass number = 75 + 98 = 173
   75 electrons = 75 protons → neutral atom

Problem 7

Which pair represents isotopes?

b. \({}^{7}_{3}\text{Li}\) and \({}^{8}_{3}\text{Li}\)

Analysis:
a. Different atomic numbers (10 vs 11) → different elements, NOT isotopes
b. Same atomic number (3), different mass numbers (7 vs 8) → ISOTOPES!
c. These are incorrectly written (one has Z > A which is impossible)
d. Different atomic numbers (23 vs 24) → different elements

Problem 8

Each atom of a specific element has the same:

a. number of protons

The number of protons (atomic number) defines an element. All atoms of the same element MUST have the same number of protons.

- Atomic mass varies (due to isotopes) ❌
- Neutrons vary (due to isotopes) ❌
- Electrons vary (due to ions) ❌

Section C Ions - Answers

Problem 9

13 protons, 14 neutrons, 10 electrons:

a. Aluminum (Al)
b. Mass number = 27
c. Charge = 3+
d. Cation
e. \({}^{27}_{13}\text{Al}^{3+}\)

Work:
a. 13 protons → Z = 13 → Aluminum
b. Mass number = protons + neutrons = 13 + 14 = 27
c. Charge = protons - electrons = 13 - 10 = +3
d. Positive charge → lost electrons → Cation
e. Combine all information: \({}^{27}_{13}\text{Al}^{3+}\)

Problem 10

Electrons in each ion:

a. Mg²⁺: 10 electrons
b. S^2-: 18 electrons
c. Fe³⁺: 23 electrons
d. Br^-: 36 electrons

Formula: Electrons = Protons - Charge

a. Mg (Z=12): 12 - (+2) = 10 electrons
b. S (Z=16): 16 - (-2) = 18 electrons
c. Fe (Z=26): 26 - (+3) = 23 electrons
d. Br (Z=35): 35 - (-1) = 36 electrons

Problem 11

Isotope notation with charge:

a. \({}^{41}_{18}\text{Ar}^{6-}\)
b. \({}^{123}_{53}\text{I}^{4+}\)

Work:
a. 18 protons → Ar; Mass = 18 + 23 = 41
Charge = 18 - 24 = -6 → \({}^{41}_{18}\text{Ar}^{6-}\)

b. 53 protons → I (Iodine); Mass = 53 + 70 = 123
Charge = 53 - 49 = +4 → \({}^{123}_{53}\text{I}^{4+}\)

Section D Complete the Table - Answers

Problem 12

Completed table:

Element	Symbol	Z	Mass #	Protons	Neutrons	Electrons	Ion Type
Zirconium	Zr²⁺	40	91	40	51	38	Cation
Tellurium	Te^2-	52	128	52	76	54	Anion
Bismuth	Bi	83	209	83	126	83	Neutral
Silver	Ag⁺	47	108	47	61	46	Cation
Silicon	Si	14	28	14	14	14	Neutral
Barium	Ba²⁺	56	137	56	81	54	Cation

Key relationships used:
- Z = Protons (always)
- Neutrons = Mass # - Z
- For neutral atoms: Electrons = Protons
- For ions: Electrons = Protons - Charge
- Cation = positive charge (lost electrons)
- Anion = negative charge (gained electrons)

Section E Average Atomic Mass - Answers

Problem 13

54.00 amu (20.00%) and 56.00 amu (80.00%):

55.60 amu

Work:
\[\text{Avg} = (54.00 \times 0.2000) + (56.00 \times 0.8000)\] \[\text{Avg} = 10.80 + 44.80 = 55.60 \text{ amu}\]

Problem 14

X-23 (63.4%), X-25 (15.2%), X-26 (21.4%):

23.95 amu

Work:
\[\text{Avg} = (23 \times 0.634) + (25 \times 0.152) + (26 \times 0.214)\] \[\text{Avg} = 14.582 + 3.800 + 5.564\] \[\text{Avg} = 23.946 \approx 23.95 \text{ amu}\]

Problem 15

Dwaxium isotopes:

112.23 amu

Work:
\[\text{Avg} = (112)(0.952) + (116)(0.031) + (118)(0.011) + (119)(0.005) + (120)(0.001)\] \[\text{Avg} = 106.624 + 3.596 + 1.298 + 0.595 + 0.120\] \[\text{Avg} = 112.233 \approx 112.23 \text{ amu}\]

Note: The average is very close to 112 because that isotope is 95.2% abundant!

Section F Challenge Problems - Answers

Problem 16

Two isotopes: 120.90 amu (57.4%) and 122.90 amu:

121.75 amu

Work:
First find the abundance of isotope 2:
100% - 57.4% = 42.6%

\[\text{Avg} = (120.90 \times 0.574) + (122.90 \times 0.426)\] \[\text{Avg} = 69.40 + 52.36 = 121.76 \text{ amu}\]

Problem 17

Mass number 80, 45 neutrons, neutral:

Bromine (Br), 35 electrons

Work:
Protons = Mass number - Neutrons = 80 - 45 = 35
Z = 35 → Bromine (Br)
Neutral atom: Electrons = Protons = 35

Problem 18

18 electrons, charge 3-:

15 protons, Nitrogen (N)

Work:
Charge = Protons - Electrons
-3 = Protons - 18
Protons = 18 - 3 = 15

Z = 15 → Nitrogen (N)
This is the N^3- (nitride) ion

Problem 19

Which element is NOT a noble gas?

a. Ra (Radium)

Explanation:
Noble gases are in Group 18: He, Ne, Ar, Kr, Xe, Rn

- Ra (Radium) is in Group 2 → alkaline earth metal, NOT a noble gas ✓
- Xe (Xenon) → noble gas
- He (Helium) → noble gas
- Ar (Argon) → noble gas

Problem 20

Complete isotope notation:

a. \({}^{80}_{35}\text{Br}^{-}\) - Anion
b. \({}^{56}_{26}\text{Fe}^{3+}\) - Cation
c. \({}^{238}_{92}\text{U}\) - Neutral

Work:
a. 35 protons → Br; Mass = 35 + 45 = 80
   Charge = 35 - 36 = -1 → Anion (gained electron)

b. 26 protons → Fe; Mass = 26 + 30 = 56
   Charge = 26 - 23 = +3 → Cation (lost electrons)

c. 92 protons → U (Uranium); Mass = 92 + 146 = 238
   Charge = 92 - 92 = 0 → Neutral atom