📝 Atomic number vs atomic mass (11 MCQs)

📖 From Campbell Biology • 2. The Chemistry of Life • 11 questions available

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What is Atomic number vs atomic mass?

Definition:
Atomic number is the number of protons in the nucleus of an atom, which defines the element and determines its identity and position on the periodic table, while atomic mass (or atomic weight) is the total mass of protons and neutrons in the nucleus, typically expressed in atomic mass units (amu), and it is an average of the masses of the naturally occurring isotopes, with atomic number being an integer and atomic mass often a decimal due to isotopic abundance.

Working:
The atomic number ( $Z$ ) is the same for all atoms of an element and equals the number of protons, and it also equals the number of electrons in a neutral atom, which determines chemical properties; atomic mass ( $A$ ) is calculated as the sum of protons and neutrons ( $A = Z + N$ ), and the average atomic mass is a weighted average based on isotopic abundance, calculated by $\text{Average Mass} = \sum (\text{Isotopic Mass} \times \text{Fractional Abundance})$ , and these two numbers are used to identify and characterize elements.

Example:
A simple example is carbon, which has an atomic number of 6 (6 protons) and an atomic mass of about 12.01 amu, reflecting the average of isotopes carbon-12 (98.9%) and carbon-13 (1.1%); another example is chlorine, atomic number 17, atomic mass 35.45 amu, due to isotopes Cl-35 and Cl-37, illustrating the difference between atomic number and mass.

Reason:
Understanding atomic number vs atomic mass is crucial for chemistry and biology, as it helps identify elements, calculate molar masses, and understand isotopic effects in biological and environmental studies.

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📝 All Atomic number vs atomic mass MCQs

Q1. An atom has 17 protons and 18 neutrons. Another atom has 17 protons and 20 neutrons. Which conclusion best explains their relationship?

A.They are different elements because their masses differ

B.They are isotopes of the same element because they have the same atomic number ✅

C.They are ions because their neutron numbers differ

D.They are different elements because their neutron numbers differ

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Both atoms contain 17 protons, so they have the same atomic number and therefore belong to the same element. Their different neutron counts change their mass numbers, making them isotopes rather than different elements or ions.

Q2. A student claims that an atom with atomic number 12 and mass number 24 must contain 24 protons. Which reasoning correctly identifies the mistake?

A.Atomic number counts electrons, so the atom has 24 electrons

B.Mass number represents only neutrons, so it has 24 neutrons

C.Atomic number identifies protons, while mass number represents protons plus neutrons ✅

D.Atomic number and mass number always have identical meanings

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: The atomic number specifically equals the number of protons, so the atom has 12 protons. The mass number is the total of protons and neutrons, meaning this atom has 12 neutrons as well.

Q3. Two atoms are measured: Atom X has 8 protons and 9 neutrons, while Atom Y has 9 protons and 8 neutrons. Both have a mass number of 17. What is the most important difference between them?

A.They are isotopes of the same element

B.They must have identical chemical identities because their mass numbers match

C.They are different elements because their atomic numbers differ ✅

D.They contain different numbers of neutrons but the same atomic number

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Mass number alone does not determine an element's identity. Atom X has atomic number 8, whereas Atom Y has atomic number 9. Because their proton numbers differ, they represent different elements even though their mass numbers are equal.

Q4. A laboratory identifies an unknown neutral atom containing 19 electrons and 20 neutrons. Which combination correctly identifies its atomic number and mass number?

A.Atomic number 19; mass number 39 ✅

B.Atomic number 20; mass number 39

C.Atomic number 39; mass number 19

D.Atomic number 19; mass number 20

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: For a neutral atom, the number of electrons equals the number of protons, so 19 electrons indicate atomic number 19. Adding its 19 protons and 20 neutrons gives a mass number of 39.

Q5. An unknown atom has a mass number of 56. A second measurement shows that it contains 30 neutrons. If the atom is electrically neutral, which additional information can be determined immediately?

A.It has 26 protons and 26 electrons ✅

B.It has 30 protons and 26 electrons

C.It has 56 protons and no electrons

D.It has 26 protons and 30 electrons

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Mass number equals the combined number of protons and neutrons. Subtracting 30 neutrons from 56 gives 26 protons. Because the atom is neutral, it must also contain 26 electrons.

Q6. A researcher compares three atoms: P has 6 protons and 6 neutrons, Q has 6 protons and 8 neutrons, and R has 8 protons and 6 neutrons. Which pair should be grouped together based on element identity?

A.P and Q ✅

B.P and R

C.Q and R

D.All three

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Element identity depends on proton number rather than neutron number. P and Q each have six protons, so they are the same element with different mass numbers. R has eight protons and therefore represents another element.

Q7. A student calculates an atom's mass number by averaging its proton and neutron counts. For an atom containing 11 protons and 12 neutrons, the student reports 11.5. What correction should be made?

A.The mass number should be 1 because protons and neutrons cancel

B.The mass number should be 11 because only protons contribute

C.The mass number should be 12 because neutrons determine mass

D.The mass number should be 23 because protons and neutrons are added ✅

💡 Difficulty: easy | ✅ Correct: D

📖 Explanation: Mass number is a whole-number count of the particles in the nucleus that contribute most of the atom's mass. Therefore, 11 protons plus 12 neutrons gives a mass number of 23, not an average.

Q8. An instrument produces the following measurements for three particles: A = 10 protons, 10 neutrons; B = 10 protons, 12 neutrons; C = 11 protons, 11 neutrons. A student groups B and C together because both contain 12 total nuclear particles. What is wrong with this grouping?

A.Equal mass numbers do not guarantee the same element ✅

B.Neutron number always determines element identity

C.Particles with equal numbers of nuclear particles must be isotopes

D.Atomic number is determined by the total nuclear particles

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: B has 10 protons and 12 neutrons, giving mass number 22. C has 11 protons and 11 neutrons, also giving mass number 22. Equal mass numbers do not make atoms isotopes; isotopes must have the same proton number.

Q9. A graph plots mass number on the vertical axis against neutron number on the horizontal axis for atoms that all have 12 protons. The plotted points rise in a straight line with slope 1. Which interpretation best explains the graph?

A.Each additional neutron increases mass number by one while proton number remains constant ✅

B.Each additional neutron changes the atomic number by one

C.Mass number decreases as neutron number increases

D.The graph proves that electrons determine mass number

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: With proton number fixed at 12, mass number equals 12 plus neutron number. Therefore, every additional neutron increases the mass number by one, producing a linear relationship with slope 1.

Q10. Two samples contain atoms with the following compositions: Sample 1 contains 75% atoms with 10 neutrons and 25% atoms with 12 neutrons, all having 8 protons. Sample 2 contains 50% atoms with 10 neutrons and 50% atoms with 12 neutrons, also having 8 protons. Which statement is best supported?

A.The samples contain different elements

B.Both samples contain the same element but can have different average atomic masses ✅

C.Sample 1 must have twice the atomic number of Sample 2

D.The neutron percentage changes the atomic number

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Both samples contain atoms with eight protons, so the element is the same in each sample. However, their isotope proportions differ, so the weighted average atomic mass of the samples can also differ.

Q11. An unknown neutral atom has twice as many neutrons as protons, and its mass number is 36. Which conclusion follows from these conditions?

A.It has 9 protons, 18 neutrons, and atomic number 9

B.It has 12 protons, 24 neutrons, and atomic number 12 ✅

C.It has 18 protons, 18 neutrons, and atomic number 18

D.It has 24 protons, 12 neutrons, and atomic number 24

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Let the number of protons be $p$ . The neutrons are $2p$ , so the mass number is $p+2p=3p$ . Since the mass number is 36, $3p=36$ , giving 12 protons and 24 neutrons.

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