Year 9 Physics: Mass Number, Atomic Number and Isotopes

Introduction

Welcome to our lesson on mass number, atomic number, and isotopes! These concepts are fundamental in understanding the building blocks of matter. By the end of this lesson, you’ll be able to explain what these terms mean and how they are related. Let’s get started!

What is Atomic Number?

Definition

The atomic number is the number of protons in the nucleus of an atom. It is represented by the letter Z.

Key Points

• The atomic number determines the element. For example:
  • Hydrogen has an atomic number of 1 (1 proton).
  • Carbon has an atomic number of 6 (6 protons).
• It also tells you how many electrons an atom has when it is neutral (not charged).

Example

If an atom has an atomic number of 8, it is oxygen, which has 8 protons and 8 electrons.

What is Mass Number?

Definition

The mass number is the total number of protons and neutrons in the nucleus of an atom. It is represented by the letter A.

Key Points

• Mass number = Number of protons + Number of neutrons
• Neutrons have no charge and are found in the nucleus along with protons.

Example

If an atom has 6 protons (like carbon) and 6 neutrons, its mass number would be:

$$

A = 6 \text{ (protons)} + 6 \text{ (neutrons)} = 12

$$

So, carbon-12 has a mass number of 12.

What are Isotopes?

Definition

Isotopes are atoms of the same element that have the same number of protons but different numbers of neutrons. This means they have different mass numbers.

Key Points

• Isotopes behave the same in chemical reactions because they have the same number of electrons.
• Different isotopes can have different stability. Some may be stable, while others can be radioactive.

Example

• Carbon has two stable isotopes:
  • Carbon-12 (6 protons, 6 neutrons: $$A = 12$$ )
  • Carbon-14 (6 protons, 8 neutrons: $$A = 14$$ )

Summary

1. Atomic Number (Z): Number of protons in an atom.
2. Mass Number (A): Sum of protons and neutrons in an atom.
3. Isotopes: Atoms of the same element with different mass numbers due to different neutron counts.

Questions

Easy Level Questions

1. What is the atomic number of carbon?
2. How many neutrons are in an atom of oxygen with mass number 16?
3. What does the mass number represent?
4. How many protons are in an atom of sodium (Na)?
5. What is the atomic number of hydrogen?
6. If an element has 12 protons, what is its atomic number?
7. What is the mass number of an element with 5 protons and 5 neutrons?
8. How many isotopes does carbon have?
9. What is the atomic number of an element with 20 protons?
10. How many neutrons does carbon-14 have?

Medium Level Questions

11. If an atom has a mass number of 23 and 11 protons, how many neutrons does it have?
12. What are isotopes?
13. Name an isotope of hydrogen.
14. How is the atomic number related to the number of electrons in a neutral atom?
15. What is the mass number of chlorine with 17 protons and 20 neutrons?
16. If an element has 10 neutrons and an atomic number of 5, what is its mass number?
17. Which is heavier: carbon-12 or carbon-14?
18. Why do isotopes of the same element behave similarly in chemical reactions?
19. What do you call an atom with the same atomic number but different mass numbers?
20. How many protons are in an atom of potassium (K) with an atomic number of 19?

Hard Level Questions

21. If an element has an atomic number of 26 and a mass number of 56, how many neutrons does it have?
22. Explain the difference between stable and radioactive isotopes.
23. A particular isotope of uranium has 92 protons and 143 neutrons. What is its mass number?
24. How can isotopes be used in medicine?
25. Why do isotopes have different mass numbers?
26. If an atom has 30 protons and 30 neutrons, what is its atomic number and mass number?
27. Give an example of an element with multiple isotopes and their mass numbers.
28. How are atomic number and mass number represented in notation?
29. If an atom has an atomic number of 15 and a mass number of 31, how many neutrons does it have?
30. Describe how the concept of isotopes is important in carbon dating.

Answers

Easy Level Answers

1. 6
2. 8
3. Total number of protons and neutrons.
4. 11
5. 1
6. 12
7. 10
8. Two (Carbon-12 and Carbon-14)
9. 20
10. 8

Medium Level Answers

11. 12 neutrons.
12. Atoms of the same element with different numbers of neutrons.
13. Deuterium or Tritium.
14. They are the same; atomic number = number of protons = number of electrons (if neutral).
17. Carbon-14 is heavier.
18. They have the same number of electrons.
19. Isotopes.
20. 19 protons.

Hard Level Answers

21. 30 neutrons.
22. Stable isotopes do not change over time, while radioactive isotopes decay and can emit radiation.
24. They can be used for diagnosis and treatment (e.g., cancer).
25. Different numbers of neutrons.
26. Atomic number is 30, mass number is 60.
27. Chlorine has Chlorine-35 and Chlorine-37.
28. Element notation: A (mass number) over Z (atomic number).
29. 16 neutrons.
30. Isotopes help estimate the age of ancient organic materials.

Feel free to ask questions if you’re unsure about any concepts!