# Isotopes and average atomic mass

> Same element, different neutrons


## Objectives

- Define isotopes: same atomic number, different mass numbers (different numbers of neutrons).
- Explain that the atomic mass in the table is the average of the isotopes, weighted by how common each is.
- Estimate an element's atomic mass from two isotopes and their percentages.

## 1. General properties: mass, radius and metallic property




## 2. Half a neutron?

Predict: where does the .5 come from?

- It is a rounding mistake in the table
- Each chlorine atom has 18.5 neutrons
- It is an average of chlorine atoms with different masses ✓

Chlorine atoms do not all have the same mass: chlorine has isotopes. The 35.5 in the table is the average mass of its atoms, not the mass of one atom.

[ar-EG] ذرات الـ chlorine مش كلها ليها نفس الكتلة: الـ chlorine ليه isotopes. والـ 35.5 اللي في الجدول هو متوسط كتلة ذراته، مش كتلة ذرة واحدة.
## 3. Isotopes

Isotopes Atoms of the same element with the same atomic number but different mass numbers, due to different numbers of neutrons. Two isotopes of chlorine Same protons, so same element. Different neutrons, so different masses.


## 4. Mix the isotopes

Natural chlorine is a mix of both isotopes. Change the share of chlorine-35. Find the table's value. Chlorine-35 in the mix Average atomic mass Found it 75% chlorine-35 and 25% chlorine-37 give 35.5, the value in the table.


## 5. An average, not one atom

The atomic mass in the table is the mass number of one atom. The more common isotope pulls the average towards its own mass.


## 6. A new element

Boron has two isotopes: boron-10 (20%) and boron-11 (80%). Its atomic mass is closest to:

- 10.8 ✓
- 11
- 10.5
- 10.2

The atomic mass is an average weighted by how common each isotope is: 0.20 × 10 + 0.80 × 11 = 2 + 8.8 = 10.8. Boron-11 is far more common, so the average lies close to 11.

[ar-EG] الـ atomic mass متوسط بيتحسب حسب نسبة انتشار كل isotope: 0.20 × 10 + 0.80 × 11 = 2 + 8.8 = 10.8. والـ boron-11 منتشر أكتر بكتير، فالمتوسط بييجي قريب من 11.
## 7. Summary

Isotopes: same atomic number, different mass numbers, due to different numbers of neutrons. The atomic mass in the table is the average of the isotopes, weighted by how common each is. Chlorine: 75% Cl-35 + 25% Cl-37 → 35.5, so no atom needs half a neutron. In Chapter 1: cobalt's radioactive isotopes, and nickel's average atomic mass. Can you explain why the atomic mass of boron is not a whole number? Boron is a mix of two isotopes: boron-10 (20%) and boron-11 (80%). Its atomic mass is their weighted average: $0.20 \times 10 + 0.80 \times 11 = 10.8$. Boron-11 is much more common, so the average is close to 11.



Chapter: Transition Elements (https://yaani.ai/ar/learn/chemistry/chem-s3-en.ch1)
Source: https://yaani.ai/ar/lesson/chem-s3-en.lesson-atom.isotopes