What is Atomic structure & isotopes on the Chemistry Regents?

Atomic structure & isotopes is a Atomic Theory topic on the Chemistry Regents. RegentsLab has 36 practice questions on it, each with a full worked solution.

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Atomic structure & isotopes: Chemistry Regents Practice

Practice Atomic structure & isotopes for the Chemistry Regents with real-style questions and full worked solutions. Miss one? Read the solution — the exam reuses the same ideas every year.

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Question 3 · 1 pt

What does the number of protons in a neutral atom determine?

(A) the number of neutrons
(B) the mass number
(C) the charge of the atom
(D) the identity of the element ✓

Worked solution

$Z = \text{number of protons}$ Z is the atomic number.

$Z \Rightarrow \text{which element}$

Answer: \text{the element's identity}

The atomic number (proton count) uniquely identifies the element; changing it changes the element.

Question 43 · 2 pts

An atom of an isotope of chlorine has a mass number of 37. How many neutrons are in the nucleus of this atom?

(A) 20 ✓
(B) 17
(C) 54
(D) 37

Worked solution

$\text{atomic number of Cl} = 17 \quad (\text{protons})$ From the Periodic Table on the Reference Tables.

$\text{neutrons} = \text{mass number} - \text{atomic number}$

$\text{neutrons} = 37 - 17 = 20$

Answer: 20 neutrons

The mass number equals protons plus neutrons, so the number of neutrons is found by subtracting the atomic number (protons) from the mass number.

Question 44 · 2 pts

Two atoms are isotopes of the same element. Which statement must be true about these two atoms?

(A) They have the same number of neutrons but different numbers of protons.
(B) They have the same mass number but different atomic numbers.
(C) They have the same number of protons but different numbers of neutrons. ✓
(D) They have different numbers of protons and different numbers of electrons.

Worked solution

$\text{isotopes: same element} \Rightarrow \text{same atomic number} \Rightarrow \text{same protons}$

$\text{different mass number} \Rightarrow \text{different number of neutrons}$

Answer: They have the same number of protons but different numbers of neutrons.

Isotopes are atoms of the same element, so they have identical numbers of protons (same atomic number) but differ in their number of neutrons and therefore their mass number.

Question 1 · 2 pts

An atom of chlorine-37 has 17 protons. How many neutrons does this atom contain?

(A) 17
(B) 37
(C) 20 ✓
(D) 18

Worked solution

$\text{Mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$37 = 17 + n$

$n = 37 - 17 = 20$

Answer: 20

The mass number (37) equals the number of protons plus neutrons. Since chlorine has 17 protons, subtracting gives 20 neutrons. A common error is confusing mass number with atomic number and answering 17 or 37.

Question 16 · 2 pts

An atom of a certain element contains 17 protons and 18 neutrons. What is the mass number of this atom?

(A) 17
(B) 18
(C) 36
(D) 35 ✓

Worked solution

$\text{mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$17 + 18 = 35$

Answer: 35

The mass number of an atom equals the sum of its protons and neutrons (17 + 18 = 35); the number of protons alone (17) gives only the atomic number, and 18 is just the neutron count.

Question 34 · 2 pts

An atom of a certain element has 17 protons and a mass number of 35. How many neutrons are in this atom?

(A) 35
(B) 17
(C) 52
(D) 18 ✓

Worked solution

$\text{mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$35 = 17 + \text{neutrons}$

$\text{neutrons} = 35 - 17 = 18$

Answer: 18

The mass number equals the sum of protons and neutrons in the nucleus, so subtracting the number of protons (17) from the mass number (35) gives 18 neutrons. A common error is confusing mass number with neutron number directly, or adding protons and mass number instead of subtracting.

Question 47 · 2 pts

An atom of a certain element contains 17 protons and 18 neutrons in its nucleus. What is the mass number of this atom?

(A) 35 ✓
(B) 17
(C) 1
(D) 18

Worked solution

$\text{Mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$\text{Mass number} = 17 + 18 = 35$

Answer: 35

The mass number of an atom equals the sum of its protons and neutrons; here 17 protons plus 18 neutrons gives a mass number of 35. Choosing just the proton count (17) or neutron count (18) alone is a common error that ignores the definition of mass number.

Question 59 · 4 pts · constructed response

A neutral atom of potassium-39 has an atomic number of 19. State the number of protons, the number of neutrons, and the number of electrons in this atom.

Model answer: Protons = 19, neutrons = 20, electrons = 19

Worked solution

$\text{Protons} = \text{atomic number} = 19$

$\text{Neutrons} = \text{mass number} - \text{atomic number} = 39 - 19 = 20$

$\text{Electrons} = \text{protons (neutral atom)} = 19$

Answer: Protons = 19, neutrons = 20, electrons = 19

In a neutral atom the atomic number gives the number of protons and, since the atom has no net charge, the number of electrons equals the number of protons. The number of neutrons is found by subtracting the atomic number from the mass number.

Question 75 · 4 pts · constructed response

An atom of chlorine has an atomic number of 17 and a mass number of 37. Determine the number of neutrons in this atom. Then determine the number of electrons in a Cl⁻¹ ion formed from this atom.

Model answer: The atom has 20 neutrons, and the Cl⁻¹ ion has 18 electrons.

Worked solution

$\text{neutrons} = \text{mass number} - \text{atomic number}$ Definition of neutron number

$\text{neutrons} = 37 - 17 = 20$

$\text{neutral atom electrons} = 17 \text{ (equal to protons)}$

$\text{Cl}^{-1} \text{ gains 1 electron} \Rightarrow 17 + 1 = 18 \text{ electrons}$

Answer: The atom has 20 neutrons, and the Cl⁻¹ ion has 18 electrons.

The mass number equals the sum of protons and neutrons, so subtracting the atomic number (protons) from the mass number gives the neutron count. A negative ion forms when a neutral atom gains one or more electrons, so a Cl⁻¹ ion has one more electron than the neutral atom's proton count.

Question 1 · 2 pts

Which subatomic particle has a mass of approximately 1 atomic mass unit (amu) and carries no electrical charge?

(A) electron
(B) neutron ✓
(C) alpha particle
(D) proton

Worked solution

$\text{Proton: charge } +1, \text{ mass} \approx 1\ \text{amu}$ has mass but also has charge, so it is eliminated

$\text{Electron: charge } -1, \text{ mass} \approx 0\ \text{amu}$ negligible mass, eliminated

$\text{Neutron: charge } 0, \text{ mass} \approx 1\ \text{amu}$ matches both conditions

Answer: neutron

The neutron is the only subatomic particle with a mass close to 1 amu and zero charge; protons have similar mass but a +1 charge, while electrons have negligible mass.

Question 14 · 2 pts

An atom of a certain element has 17 protons and a mass number of 37. How many neutrons are in this atom?

(A) 20 ✓
(B) 37
(C) 54
(D) 17

Worked solution

$\text{neutrons} = \text{mass number} - \text{protons}$ Mass number = protons + neutrons

$\text{neutrons} = 37 - 17 = 20$

Answer: 20

The mass number equals the sum of protons and neutrons in an atom's nucleus. Subtracting the number of protons (17) from the mass number (37) gives 20 neutrons. A common error is confusing mass number with neutron number directly.

Question 26 · 2 pts

An atom of chlorine-37 has an atomic number of 17. What is the total number of neutrons in this atom?

(A) 17
(B) 37
(C) 20 ✓
(D) 54

Worked solution

$\text{neutrons} = \text{mass number} - \text{atomic number}$ Definition of mass number

$\text{neutrons} = 37 - 17 = 20$

Answer: 20

The mass number (37) equals the sum of protons and neutrons, while the atomic number (17) equals the number of protons. Subtracting gives the number of neutrons: 37 minus 17 equals 20.

Question 40 · 2 pts

An atom of potassium-39 has an atomic number of 19. What is the total number of neutrons in this atom?

(A) 20 ✓
(B) 19
(C) 39
(D) 58

Worked solution

$\text{mass number} = \text{protons} + \text{neutrons}$

$39 = 19 + \text{neutrons}$

$\text{neutrons} = 39 - 19 = 20$

Answer: 20

The mass number (39) equals the number of protons plus neutrons. Since potassium has 19 protons (its atomic number), the number of neutrons is 39 minus 19, which equals 20. Choosing 19 confuses neutrons with protons, and 58 mistakenly adds instead of subtracting.

Question 52 · 4 pts · constructed response

An atom of chlorine-37 has an atomic number of 17. Determine the number of neutrons contained in this atom.

Model answer: The atom contains 20 neutrons.

Worked solution

$\text{mass number} = \text{protons} + \text{neutrons}$

$\text{neutrons} = 37 - 17 = 20$

Answer: The atom contains 20 neutrons.

The mass number equals the sum of protons and neutrons. Subtracting the atomic number (number of protons) from the mass number gives the number of neutrons.

Question 84 · 4 pts · constructed response

An atom of potassium has a mass number of 39 and an atomic number of 19. Determine the number of neutrons in this atom of potassium.

Model answer: 20 neutrons

Worked solution

$\text{number of neutrons} = \text{mass number} - \text{atomic number}$

$39 - 19 = 20$

Answer: 20 neutrons

The mass number equals the sum of protons and neutrons, while the atomic number equals the number of protons, so subtracting atomic number from mass number gives the number of neutrons.

Question 14 · 2 pts

An atom of chlorine-37 has 17 protons. How many neutrons are contained in this atom?

(A) 54
(B) 17
(C) 20 ✓
(D) 37

Worked solution

$\text{Mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$37 = 17 + \text{neutrons}$

$\text{neutrons} = 37 - 17 = 20$

Answer: 20

The mass number (37) equals the number of protons plus neutrons. Subtracting the 17 protons from the mass number of 37 gives 20 neutrons. A common student error is to confuse mass number with proton count and answer 17, or to add protons and mass number instead of subtracting.

Question 30 · 2 pts

Which set of numbers represents the number of protons, neutrons, and electrons, respectively, in a neutral atom of $^{35}_{17}\text{Cl}$?

(A) 17 protons, 17 neutrons, 18 electrons
(B) 35 protons, 17 neutrons, 18 electrons
(C) 17 protons, 18 neutrons, 17 electrons ✓
(D) 18 protons, 17 neutrons, 17 electrons

Worked solution

$Z = 17$ atomic number = number of protons

$A - Z = 35 - 17 = 18$ mass number minus protons gives neutrons

$\text{electrons} = \text{protons} = 17$ atom is neutral, so electrons equal protons

Answer: 17 protons, 18 neutrons, 17 electrons

The atomic number (17) gives the number of protons and, in a neutral atom, the number of electrons. Subtracting the atomic number from the mass number (35) gives the number of neutrons (18).

Question 54 · 4 pts · constructed response

An atom of a certain element contains 17 protons and 18 neutrons. Determine the mass number of this atom and identify the element by name or symbol.

Model answer: Mass number = 35; the element is chlorine (Cl).

Worked solution

$\text{Mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$\text{Mass number} = 17 + 18 = 35$

$\text{An atom with 17 protons has atomic number 17, which is chlorine (Cl)}$

Answer: Mass number = 35; the element is chlorine (Cl).

The mass number of an atom equals the sum of its protons and neutrons, while the atomic number (number of protons) identifies the element on the periodic table. With 17 protons, the atom must be chlorine, and adding its 18 neutrons gives a mass number of 35.

Question 73 · 4 pts · constructed response

An atom of chlorine-37 has an atomic number of 17. State the number of protons, the number of neutrons, and the number of electrons in a neutral atom of chlorine-37.

Model answer: 17 protons, 20 neutrons, and 17 electrons.

Worked solution

$\text{protons} = \text{atomic number} = 17$

$\text{neutrons} = \text{mass number} - \text{atomic number} = 37 - 17 = 20$

$\text{electrons} = \text{protons (neutral atom)} = 17$

Answer: 17 protons, 20 neutrons, and 17 electrons.

The atomic number equals the number of protons, and in a neutral atom the number of electrons equals the number of protons. The number of neutrons is found by subtracting the atomic number from the mass number given in the isotope name.

Question 81 · 4 pts · constructed response

A neutral atom of an isotope of chlorine has a mass number of 37. Using the Periodic Table, determine the number of neutrons and the number of electrons in this atom.

Model answer: The atom has 20 neutrons and 17 electrons.

Worked solution

$Z = 17$ Atomic number of chlorine from the Periodic Table

$\text{neutrons} = 37 - 17 = 20$

$\text{electrons} = \text{protons} = 17$ Because the atom is neutral

Answer: The atom has 20 neutrons and 17 electrons.

The number of neutrons equals the mass number minus the atomic number (number of protons), and a neutral atom has equal numbers of protons and electrons.

Question 1 · 2 pts

An atom of potassium-39 contains 19 protons. How many neutrons are in this atom?

(A) 20 ✓
(B) 58
(C) 19
(D) 39

Worked solution

$\text{mass number} = \text{protons} + \text{neutrons}$ Definition of mass number

$39 = 19 + n$

$n = 39 - 19 = 20$

Answer: 20

The mass number of an atom equals the number of protons plus the number of neutrons. Subtracting the given number of protons (19) from the mass number (39) gives 20 neutrons. A common student error is to confuse mass number with atomic number and answer 19.

Question 51 · 4 pts · constructed response

An element X has two naturally occurring isotopes: X-63, with an atomic mass of 62.93 u and a relative abundance of 69.15%, and X-65, with an atomic mass of 64.93 u and a relative abundance of 30.85%. Calculate the average atomic mass of element X, to the nearest hundredth.

Model answer: 63.55 u

Worked solution

$\text{Average mass} = (0.6915)(62.93) + (0.3085)(64.93)$ Multiply each isotope's mass by its decimal abundance

$= 43.52 + 20.03$ Add the two weighted masses

$= 63.55\ \text{u}$

Answer: 63.55 u

The average atomic mass of an element is the weighted average of the masses of its naturally occurring isotopes, weighted by their relative (fractional) abundances, which is why the answer is closer to the mass of the more abundant isotope.

Question 60 · 4 pts · constructed response

An atom of bromine has a mass number of 80 and an atomic number of 35. Determine the number of neutrons in this atom of bromine.

Model answer: 45 neutrons

Worked solution

$\text{neutrons} = \text{mass number} - \text{atomic number}$ Definition of mass number

$\text{neutrons} = 80 - 35$

$\text{neutrons} = 45$

Answer: 45 neutrons

The mass number equals the sum of protons and neutrons in the nucleus, so subtracting the atomic number (number of protons) from the mass number gives the number of neutrons.

Question 79 · 4 pts · constructed response

An atom of potassium-39 has an atomic number of 19. Determine the number of neutrons in this atom.

Model answer: 20 neutrons

Worked solution

$\text{neutrons} = \text{mass number} - \text{atomic number}$

$39 - 19 = 20$

Answer: 20 neutrons

The number of neutrons in an atom equals its mass number minus its atomic number (number of protons).

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