# AZX Notation & Isotopes ## AZX Notation - A **nuclide** is a group of atoms containing the same number of protons and neutrons - For example, 5 atoms of carbon-12 are all the same nuclide but are 5 separate atoms - Atomic symbols are written in a specific notation called **nuclide notation** or **AZX notation**

In AZX notation, $^{A}_{Z}X$ represents a nucleus where $X$ is the element symbol, $A$ is the nucleon number (top), and $Z$ is the proton number (bottom)

- The **top number A** represents the **nucleon number** or the **mass number** - **Nucleon number (A)** = total number of protons and neutrons in the nucleus - The **lower number Z** represents the **proton number** or **atomic number** - **Proton number (Z)** = total number of protons in the nucleus - Note: In Chemistry, the nucleon number is referred to as the mass number and the proton number as the atomic number. The periodic table is ordered by atomic number ## Isotopes - Although all atoms of the same element always have the same number of protons (and hence electrons), the number of neutrons can vary

An **isotope** is an atom (of the same element) that has an equal number of protons but a different number of neutrons

- Hydrogen has two isotopes: **deuterium** and **tritium** - **Hydrogen** ($^{1}_{1}H$): 1 proton, 0 neutrons - **Deuterium** ($^{2}_{1}H$): 1 proton, 1 neutron - **Tritium** ($^{3}_{1}H$): 1 proton, 2 neutrons - The **neutron number** of an atom is found by subtracting the proton number from the nucleon number: - Neutron number = $A - Z$ - Since nucleon number includes the number of neutrons, an isotope of an element will also have a different nucleon/mass number - Since isotopes have an imbalance of neutrons and protons, they are **unstable** - This means they decay and emit radiation to achieve a more stable form - This can happen from anywhere between a few nanoseconds to 100,000 years ## Differences Between Isotopes - The number of neutrons in an atom does **not** affect the chemical properties of an atom, such as its charge, but **only its mass** - This is because neutrons have no charge but do have mass - The charge of the nucleus of a particular element is **always the same** ### Weighted Average Mass Numbers - In the periodic table, the mass number of Chlorine is often given as 35.5 - The mass number of chlorine is given as 35.5 because it has 2 isotopes: - One with a mass number of 35 - The other with a mass number of 37 - Chlorine-35 is about three times more abundant than chlorine-37 - The given mass number of chlorine is closer to 35 than 37 because the mass number is a **weighted average** - Therefore it takes into account the proportion of each isotope present

The number of electrons and protons in different isotopes remains the same

- Some isotopes are unstable as they have an imbalance of protons and neutrons ## Worked Example One of the rows in the table shows a pair of nuclei that are isotopes of one another. | | **Nucleon number** | **Number of neutrons** | |---|---|---| | A | $\frac{39}{35}$ | 19
22 | | B | $\frac{37}{35}$ | 20
18 | | C | $\frac{37}{35}$ | 18
20 | | D | $\frac{35}{35}$ | 20
18 | Which row is correct? **Answer: B** ### Step 1: State the properties of isotopes - Isotopes are nuclei with the same number of protons but different number of neutrons - The nucleon number is the sum of the protons and neutrons - Therefore, an isotope has a different nucleon number too ### Step 2: Calculate number of protons in the first nucleus - Nucleon number: 37 - Neutrons: 20 - Protons = $37 - 20 = 17$ ### Step 3: Calculate number of protons in the second nucleus - Nucleon number: 35 - Neutrons: 18 - Protons = $35 - 18 = 17$ ### Step 4: Conclusion - Therefore, they have the same number of protons but different numbers of neutrons and are isotopes of each other - The correct answer is therefore option **B**