An atom has three electron shells. There are three electrons in the outer shell.
How many protons and how many neutrons are in this atom?
| protons | neutrons | |
|---|---|---|
| A | 13 | 14 |
| B | 13 | 27 |
| C | 14 | 13 |
| D | 21 | 24 |
▶️ Answer/Explanation
Ans: A
To solve this, we need to determine the element based on its electron configuration:
- Three electron shells means it’s in period 3 of the periodic table.
- Three electrons in the outer shell means it’s in Group 13 (or III).
- The element with this configuration is aluminum (Al), which has:
- Atomic number (protons) = 13
- Most common isotope has 14 neutrons (mass number 27)
Looking at the options:
A) Correct – Matches aluminum’s proton and neutron numbers.
B) Incorrect – Neutron number can’t be 27 (that would be the mass number).
C) Incorrect – 14 protons would be silicon (Group 14).
D) Incorrect – 21 protons would be scandium (a transition metal).
What is the nucleon number of an atom?
A) the number of neutrons
B) the number of protons
C) the total number of protons and neutrons
D) the total number of protons and electrons
▶️ Answer/Explanation
Ans: C
The nucleon number (also called mass number) of an atom is defined as the sum of protons and neutrons in the nucleus.
Let’s analyze each option:
A) Incorrect – This describes only the neutron number.
B) Incorrect – This describes the atomic number (proton number).
C) Correct – This is the exact definition of nucleon number.
D) Incorrect – This would include electrons, which are not part of the nucleus (hence not nucleons).
Which information about an element is given by its atomic number?
A) the number of protons in the nucleus of an atom of an element
B) the number of particles in the nucleus of an atom of an element
C) the relative mass of one atom of an element
D) the total number of particles in one atom of an element
▶️ Answer/Explanation
Ans: A
The atomic number of an element specifically refers to the number of protons in the nucleus of its atoms. This is a fundamental definition in chemistry.
The other options are incorrect because:
B: The number of particles in the nucleus would include both protons and neutrons (nucleons), but the atomic number only counts protons.
C: The relative mass is given by the mass number (protons + neutrons), not the atomic number.
D: The total number of particles would include electrons as well, but the atomic number doesn’t account for these.
Remember: Atomic number = number of protons (which also equals number of electrons in a neutral atom).
One atom of an element contains 12 electrons, 12 protons and 13 neutrons.
How many nucleons does this atom contain?
A) 12
B) 13
C) 24
D) 25
▶️ Answer/Explanation
Ans: D
Nucleons are the particles in the nucleus – protons and neutrons.
Number of nucleons = number of protons + number of neutrons
Given: 12 protons + 13 neutrons = 25 nucleons
Note: Electrons are not nucleons as they’re outside the nucleus.
This atom is magnesium-25 (atomic number 12, mass number 25).