Atomic number, mass number, and isotopes are key concepts in understanding atomic structure. The atomic number represents the number of protons in an atom, while the mass number is the sum of protons and neutrons. Isotopes are atoms of the same element with different numbers of neutrons, leading to varying mass numbers. These concepts are essential for studying the properties and behavior of elements in chemistry.
The atomic number (Z) of an element represents the number of: (a) Neutrons (b) Protons (c) Electrons + Neutrons (d) Electrons in the outer shell
The mass number (A) of an atom is equal to the: (a) Number of protons + electrons (b) Number of neutrons only (c) Number of protons + neutrons (d) Number of electrons only
An isotope of an element must have the same number of: (a) Protons (b) Neutrons (c) Mass number (d) Nuclear particles
If an atom has 17 protons, 17 electrons, and 20 neutrons, its atomic number is: (a) 17 (b) 20 (c) 37 (d) 54
Isotopes of the same element have different: (a) Atomic numbers (b) Mass numbers (c) Chemical symbols (d) Electron configurations
An atom’s mass number is 40, and it has 18 neutrons. How many protons does it have? (a) 18 (b) 22 (c) 40 (d) 58
Atomic number (Z) = 11 and mass number (A) = 23. How many neutrons are present? (a) 11 (b) 12 (c) 23 (d) 34
Which symbol correctly represents an isotope with 8 protons and mass number 16? (a) 16O (b) 8O (c) 16C (d) 24O
Two different atoms with the same atomic number but different mass numbers are called: (a) Isotopes (b) Isomers (c) Allotropes (d) Radicals
Which of the following pairs are isotopes of each other? (a) 35Cl and 37Cl (b) 12C and 12N (c) 1H and 2He (d) 14N and 14O
If an element has 3 isotopes with mass numbers 35, 37, and 38, what stays constant among them? (a) Number of protons (b) Number of neutrons (c) Mass number (d) Atomic mass
Which statement is true regarding atomic number? (a) It changes in different isotopes (b) It is equal to the number of neutrons (c) It is equal to the number of protons (d) It is always zero for radioactive elements
How is the mass number (A) typically denoted in the notation AZ X? (a) A = Z – # of neutrons (b) A = Z + # of electrons (c) A = total nucleons (protons + neutrons) (d) A = atomic weight in grams
Which quantity can differ for two isotopes of the same element? (a) Number of protons (b) Symbol of the element (c) Physical properties like density (d) Their atomic number
Neon (Ne) has isotopes with mass numbers 20, 21, and 22. Each must have the same: (a) Number of nucleons (b) Number of protons (c) Number of neutrons (d) Mass number
Which of the following is not a valid representation of an isotope? (a) 147N (b) 16😯 (c) 3919K (d) 127C
Uranium-238 and Uranium-235 differ by: (a) 3 protons (b) 3 neutrons (c) 3 electrons (d) 3 in atomic number
A certain element X has 15 protons and 16 neutrons. Which symbol is correct? (a) 3115X (b) 1516X (c) 1615X (d) 3116X
Which property remains unchanged when comparing isotopes of the same element? (a) Mass number (b) Atomic mass (c) Number of neutrons (d) Atomic number
If an atom of magnesium has an atomic number of 12 and a mass number of 24, it has: (a) 24 protons and 12 neutrons (b) 12 protons and 24 neutrons (c) 12 protons and 12 neutrons (d) 24 protons and 24 electrons
What does the mass number (A) specifically refer to? (a) Sum of electrons and neutrons (b) Sum of protons and neutrons (c) Sum of protons and electrons (d) Total mass in grams
Two atoms with atomic numbers 8 and 9 cannot be isotopes because: (a) They have different proton counts (b) They have the same neutron count (c) Their mass number is the same (d) They belong to the same element
Which statement is correct about chlorine-35 and chlorine-37? (a) Same number of neutrons, different protons (b) Same proton count, different neutron count (c) Different chemical symbols (d) They have identical mass numbers
Isotopes of hydrogen include protium (H-1), deuterium (H-2), and tritium (H-3). What do they share in common? (a) Same neutron count (b) Same electron count (c) Same mass number (d) Same atomic number
If an isotope of sulfur has mass number 34, how many neutrons does it have? (Sulfur’s atomic number is 16.) (a) 16 (b) 18 (c) 34 (d) 50
Which of the following must be the same for any two isotopes of the same element? (a) Mass number (b) Proton number (c) Neutron number (d) Nucleon count
What changes among isotopes of the same element? (a) Number of electrons (b) Number of protons (c) Number of neutrons (d) Both protons and electrons
An atom with atomic number 26 and mass number 56 has how many neutrons? (a) 26 (b) 30 (c) 56 (d) 82
Atoms X and Y each have 9 protons but different neutron counts. Which statement is true? (a) They are different elements (b) They have the same mass number (c) They are isotopes of the same element (d) One is a cation, the other is an anion
Two atoms both have 6 protons. One has 6 neutrons, the other has 8. Their mass numbers are: (a) 6 and 8 (b) 12 and 14 (c) 12 and 8 (d) 14 and 14
Answer Key
(b) Atomic number = proton count.
(c) Mass number = protons + neutrons.
(a) Isotopes must share the same proton count.
(a) 17 protons = atomic number 17.
(b) Isotopes differ in mass number (neutrons).
(b) 40 (A) – 18 (n) = 22 protons.
(b) 23 – 11 = 12 neutrons.
(a) 16O indicates mass number 16, atomic number 8.
(a) Same Z, different A = isotopes.
(a) 35Cl and 37Cl differ by neutron count.
(a) Protons remain constant; neutrons vary.
(c) Atomic number = protons.
(c) A = Z + N (protons + neutrons).
(c) Physical properties differ slightly, e.g. density.
(b) Same proton number for all isotopes of neon.
(d) 127C is invalid because 7 protons = N, not C.
(b) 238 – 235 = 3 neutrons difference.
(a) 3115X means 15 protons, 16 neutrons.
(d) Atomic number stays the same among isotopes.
(c) 12 protons, 12 neutrons for Mg (A=24).
(b) Protons + neutrons = mass number.
(a) Different proton counts → different elements. Not isotopes.
(b) Same proton count, different neutron count = isotopes.