# Atomic number and mass number

> What the 26 in ₂₆Fe and the 60 in cobalt 60 mean


## Objectives

- Give the number of protons and electrons of an atom from its atomic number (Z).
- Calculate the mass number and the number of neutrons.
- Read the notation $^{A}_{Z}\text{X}$.

## 1. Atomic number and mass number: from zero




## 2. Two numbers, two meanings

Predict: in $_{26}\text{Fe}$, what does 26 count?

- The protons and the neutrons together
- The protons and the electrons together
- The protons in the nucleus ✓

In $_{26}\text{Fe}$, 26 is the atomic number: the number of protons in the nucleus. Protons and neutrons together give the mass number.

[ar-EG] في $_{26}\text{Fe}$، الـ 26 هو الـ atomic number: عدد الـ protons اللي في الـ nucleus. أما الـ protons والـ neutrons مع بعض فهما الـ mass number.
## 3. Atomic number (Z)

Atomic number The number of protons in the nucleus of an atom. In a neutral atom: number of electrons = number of protons Each element has its own atomic number. Scandium is 21 and zinc is 30. Neutral atoms only This holds for a neutral atom only. $\ce{Fe^2+}$ has lost 2 electrons: still 26 protons, but 24 electrons.


## 4. Mass number

Mass number The sum of the numbers of protons and neutrons in the nucleus. A = mass number, Z = atomic number, N = number of neutrons Worked example: cobalt 60 Z = 27, so 27 protons. A neutral atom, so 27 electrons. Neutrons = 60 − 27 = 33. The mass number counts the electrons too.


## 5. Build a nucleus

Change the protons and the neutrons. Watch what changes. Protons (atomic number Z) Neutrons Z = 21: this is **scandium, Sc**. Z = 22: this is **titanium, Ti**. Z = 23: this is **vanadium, V**. Z = 24: this is **chromium, Cr**. Z = 25: this is **manganese, Mn**. Z = 26: this is **iron, Fe**. Z = 27: this is **cobalt, Co**. Z = 28: this is **nickel, Ni**. Z = 29: this is **copper, Cu**. Z = 30: this is **zinc, Zn**. Mass number (A) Electrons in the neutral atom Change only the neutrons. Does the element change? No. The element is fixed by Z, the number of protons. Changing the neutrons changes only the mass number.


## 6. Count the particles

A copper atom has 29 protons and 34 neutrons. What is its mass number?

- 63 ✓
- 92
- 29
- 5

Mass number = protons + neutrons = 29 + 34 = 63. The electrons are not counted, because they are too light.

[ar-EG] الـ mass number = protons + neutrons = 29 + 34 = 63. والـ electrons مش بتتحسب، عشان خفيفة جدًا.
## 7. Summary

Atomic number (Z) = protons = electrons in a neutral atom: $_{26}\text{Fe}$ has 26. Mass number (A) = protons + neutrons: cobalt 60 has A = 60. Neutrons = A − Z: 60 − 27 = 33. Use the **atomic number** to count the electrons before writing an **electron configuration** in Ch.1. Can you explain why $\ce{Fe^2+}$ has 24 electrons but is still iron? $\ce{Fe^2+}$ has lost 2 electrons, so it has 24 electrons. It still has 26 protons, and the element is fixed by Z, the number of protons.



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