Atomic Number and Mass Number – Long Answer Questions
Medium Level (Application & Explanation)
Q1. Differentiate between atomic number and mass number. Explain with suitable examples.
Answer:
The atomic number (Z) is the number of protons in the nucleus. It fixes the identity of the element.
The mass number (A) is the total number of protons + neutrons in the nucleus.
Thus, Z tells “which element,” and A tells “how heavy the nucleus is.”
Example: Carbon has Z = 6. If it has 6 neutrons, then A = 12.
Another example: Aluminium has Z = 13 and A = 27. So it has 14 neutrons.
So, remember: Z = protons, A = protons + neutrons, and neutrons = A − Z.
Q2. Given atomic symbols, find the number of protons, neutrons, and electrons in neutral atoms: 14/7 N, 27/13 Al, and 24/12 Mg.
Answer:
For 14/7 N: Z = 7, so protons = 7. A = 14, so neutrons = 14 − 7 = 7. In a neutral atom, electrons = protons = 7.
For 27/13 Al: Z = 13, so protons = 13. A = 27, so neutrons = 27 − 13 = 14. Electrons = 13 if neutral.
For 24/12 Mg: Z = 12, so protons = 12. A = 24, so neutrons = 24 − 12 = 12. Electrons = 12 if neutral.
Key idea: Z gives protons. A minus Z gives neutrons.
In a neutral atom, electrons = protons.
This helps you decode any nuclear symbol quickly.
Q3. Explain what isotopes are. How are isotopes of the same element similar and different?
Answer:
Isotopes are atoms of the same element with the same atomic number (Z) but different mass numbers (A).
They have the same number of protons but different numbers of neutrons.
Because Z is the same, isotopes have the same chemical properties.
Their mass and some physical properties can be different because neutrons change A.
Example: Carbon-12 and Carbon-14 both have Z = 6, but A = 12 and A = 14.
So, isotopes share identity but differ in nuclear composition.
Q4. How does atomic number decide an element’s position and basic behavior in the periodic table?
Answer:
The atomic number (Z) decides the element’s place in the periodic table.
Z equals protons, and in a neutral atom, protons equal electrons.
Electrons decide how atoms bond and react. So Z controls chemical behavior.
Elements are arranged in increasing Z. This makes a pattern of properties.
Example: Oxygen with Z = 8 will always be oxygen, with 8 protons.
So, Z fixes identity, position, and basic properties of an element.
Q5. Decode the nuclear symbol A/Z X. What information can you extract from it? Illustrate with 14/7 N and 27/13 Al.
Answer:
In A/Z X, X is the element symbol, Z is the atomic number, and A is the mass number.
Z = protons. A = protons + neutrons.
So, neutrons = A − Z.
For 14/7 N: Z = 7 ⇒ protons = 7. A = 14 ⇒ neutrons = 7.
For 27/13 Al: Z = 13 ⇒ protons = 13. A = 27 ⇒ neutrons = 14.
This symbol helps you find protons, neutrons, and identity quickly.
High Complexity (Analysis & Scenario-Based)
Q6. A student says, “If two atoms have the same mass number, they must be the same element.” Analyze and correct this statement with examples.
Answer:
The statement is incorrect. Mass number (A) alone does not fix identity.
Atomic number (Z) decides the element because Z = protons.
Two atoms can have the same A but different Z. They are different elements.
For example, if Atom 1 has A = 14, Z = 7 (nitrogen), and Atom 2 has A = 14, Z = 6 (carbon-14), they are not the same.
They only share total nucleons (protons + neutrons), not identity.
So, always check Z to identify an element, not just A.
Q7. You receive three samples labeled X: 12/6 C, Y: 13/6 C, and Z: 14/6 C. Compare their particles and predict which properties are similar or different.
Answer:
All three have Z = 6. So each has 6 protons.
Their neutrons are different: X has 6, Y has 7, Z has 8 neutrons.
They are isotopes of carbon. Same element, different mass numbers.
Their chemical properties are similar because protons and electrons control bonding.
Their masses differ, so physical properties like mass-related behavior can differ.
So, identity stays the same, but nuclear composition and mass change.
Q8. A neutral atom becomes an ion by losing or gaining electrons. Explain what happens to atomic number and mass number in this process.
Answer:
When an atom becomes an ion, only electrons change.
The atomic number (Z) does not change because protons do not change.
The mass number (A) also does not change because protons + neutrons stay the same.
So, ions have the same Z and A as the neutral atom.
What changes is only the charge and the number of electrons.
Thus, ion formation does not affect identity or nuclear mass.
Q9. A researcher claims: “Neutrons are not important because they have no charge.” Use atomic structure ideas to evaluate this claim.
Answer:
The claim is wrong. Neutrons are very important.
Neutrons add to mass and help stabilize the nucleus.
Protons are all positively charged and repel each other.
Neutrons reduce this repulsion and hold the nucleus together.
Changing neutron number changes the mass number (A) and can affect stability.
So, even without charge, neutrons are key to a stable nucleus.
Q10. You are given data for an unknown atom: protons = 17, neutrons = 18. Identify Z and A. Decide the particle counts in neutral and ionic forms. Explain each step.
Answer:
Z = protons = 17. So the atomic number is 17.
A = protons + neutrons = 17 + 18 = 35.
In a neutral atom, electrons = protons = 17.
If it forms an ion by gaining 1 electron, electrons = 18, but Z = 17 and A = 35 remain the same.
If it loses 1 electron, electrons = 16, still Z = 17, A = 35.
So, identity (by Z) and nuclear mass (by A) stay fixed while charge changes.
