📖 Lesson 5: Atomic Number, Mass Number, and Isotopes

👩🏽‍🏫 Introduction

Hello again, atomic thinker! ⚛️

Every object you see—your clothes, books, food, and even YOU—is made up of tiny building blocks called atoms. But not all atoms are exactly the same.

Some atoms are light, some are heavy. Some have twins called isotopes. And to tell them apart, we use something called atomic number and mass number.

In this lesson, you’ll learn how to read and understand these terms. You’ll also be able to calculate neutrons in an atom and explain what makes one atom different from another—even when they belong to the same element!

📘 Core Concepts

🔹 What is an Atom?

An atom is the smallest particle of an element that retains the properties of that element. It is made of:

• Protons (+ charge) – in the nucleus

• Neutrons (no charge) – in the nucleus

• Electrons (– charge) – orbiting the nucleus

🔹 Atomic Number (Z)

The atomic number is the number of protons in an atom.
It also tells you the number of electrons (in a neutral atom).

🧠 Atomic number = Number of protons = Number of electrons

Example:
Hydrogen (H) has 1 proton → atomic number = 1
Carbon (C) has 6 protons → atomic number = 6

🔹 Mass Number (A)

The mass number is the total number of protons and neutrons in an atom’s nucleus.

🧠 Mass number = Protons + Neutrons

Example:
Carbon with 6 protons and 6 neutrons → Mass number = 12

🔹 Neutrons in an Atom

🧠 Neutrons = Mass number – Atomic number

Example:
Chlorine has a mass number of 35 and atomic number 17
Neutrons = 35 – 17 = 18 neutrons

🔹 Isotopes

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

• Same atomic number

• Different mass numbers

Examples:

• Carbon-12 and Carbon-14 are both carbon (Z = 6)

  • C-12 has 6 neutrons

  • C-14 has 8 neutrons

💡 Why Are Isotopes Important?

• Carbon-14 is used in dating fossils (carbon dating)

• Iodine-131 is used to treat thyroid diseases

• Uranium-235 is used in nuclear energy

Visual Summary

🧩 Sample Problem Walkthrough

🧠 Problem:

An atom of element X has an atomic number of 9 and a mass number of 19.
a) How many protons does it have?
b) How many electrons?
c) How many neutrons?

✅ Step-by-Step Solution:

a) Protons = Atomic number = 9
b) Electrons = Protons = 9 (neutral atom)
c) Neutrons = 19 – 9 = 10 neutrons

✔️ Final Answer:

• Protons = 9

• Electrons = 9

• Neutrons = 10

✍🏽 Practice Exercises

Exercise 1: Fill-in-the-Blanks

1. The atomic number is the number of ______ in an atom.

2. Mass number = ______ + ______.

3. Isotopes of the same element have different numbers of ______.

Answers:

1. protons

2. protons, neutrons

3. neutrons

Exercise 2: Multiple Choice

1. Which of the following is true about isotopes?
   A. They have different atomic numbers
   B. They have the same number of neutrons
   C. They are different elements
   D. They have the same number of protons

✅ Answer: D. They have the same number of protons

2. What is the number of neutrons in an atom of oxygen with mass number 18?
   A. 8
   B. 10
   C. 18
   D. 16

✅ Answer: B. 10 (18 – 8)

Exercise 3: Calculate and Complete

Answers:

• Lithium → 7 – 3 = 4 neutrons

• Neon → 20 – 10 = 10 neutrons

• Carbon → 14 – 6 = 8 neutrons

🌀 Recap

Let’s summarize:

• Atomic number = number of protons = number of electrons in a neutral atom

• Mass number = protons + neutrons

• Neutrons = Mass number – Atomic number

• Isotopes = atoms of the same element with different mass numbers (same protons, different neutrons)

💭 Reflection Prompt

Think about your birthday: It happens once a year, but your age changes every time. Can you relate that to isotopes—same identity (element), but different “weights” (mass numbers)?
Now, pick any two isotopes (like Carbon-12 and Carbon-14) and write what makes them similar and what makes them different.