IB MYP 4-5 Chemistry -Atomic number and mass number- Study Notes - New Syllabus
IB MYP 4-5 Chemistry -Atomic number and mass number- Study Notes
Key Concepts
- Atomic Number and Mass Number
Atomic Number and Mass Number
Atomic Number and Mass Number
Every atom is characterized by two key values — the atomic number and the mass number. These numbers describe how many subatomic particles (protons, neutrons, electrons) the atom contains and help identify the element and its isotope.
Atomic Number \(( \mathrm{Z} )\)
The atomic number (\( \mathrm{Z} \)) of an element is the number of protons present in the nucleus of each atom of that element.
- It defines the identity of the element.
- In a neutral atom, the number of protons = number of electrons.
- Atomic number determines the position of an element in the periodic table.
\( \mathrm{Z = Number\ of\ Protons = Number\ of\ Electrons\ (in\ a\ neutral\ atom)} \)
Changing the number of protons changes the **element itself**.
For example:
- \( \mathrm{Z = 1} \) → Hydrogen
- \( \mathrm{Z = 2} \) → Helium
- \( \mathrm{Z = 6} \) → Carbon
Examples of Atomic Numbers
| Element | Symbol | Atomic Number (\( \mathrm{Z} \)) | Number of Electrons |
|---|---|---|---|
| Hydrogen | H | 1 | 1 |
| Carbon | C | 6 | 6 |
| Oxygen | O | 8 | 8 |
Mass Number \(( \mathrm{A} )\)
The mass number (\( \mathrm{A} \)) of an atom is the total number of protons and neutrons present in its nucleus.
- Protons and neutrons together are called nucleons.
- Electrons are not included because their mass is negligible.
\( \mathrm{A = Number\ of\ Protons + Number\ of\ Neutrons} \)
Mass number tells us how massive the atom is — the larger the mass number, the heavier the atom.
Example: Carbon-12
For \( \mathrm{^{12}_6C} \):
- Protons = 6
- Neutrons = \( \mathrm{12 – 6 = 6} \)
- Electrons = 6 (if neutral)
So,
- \( \mathrm{A = 12} \)
- \( \mathrm{Z = 6} \)
Relationship Between Atomic and Mass Numbers
The relationship among subatomic particles can be expressed as:
\( \mathrm{A = Z + N} \)
- \( \mathrm{A} \): Mass number
- \( \mathrm{Z} \): Atomic number (protons)
- \( \mathrm{N} \): Number of neutrons
Example
An atom of oxygen has a mass number of 16 and an atomic number of 8. Find the number of neutrons.
\( \mathrm{N = A – Z = 16 – 8 = 8} \)
Answer: Oxygen has 8 neutrons.
Representation of Atoms
Atoms are represented using their atomic and mass numbers as:
\( \mathrm{^{A}_{Z}X} \)
Where:
- \( \mathrm{X} \) = chemical symbol of the element
- \( \mathrm{A} \) = mass number
- \( \mathrm{Z} \) = atomic number
Example:
- \( \mathrm{^{23}_{11}Na} \): Sodium → 11 protons, 12 neutrons, 11 electrons
- \( \mathrm{^{35}_{17}Cl} \): Chlorine → 17 protons, 18 neutrons, 17 electrons
Comparison: Atomic Number vs. Mass Number
| Property | Atomic Number (\( \mathrm{Z} \)) | Mass Number (\( \mathrm{A} \)) |
|---|---|---|
| Definition | Number of protons in the nucleus | Sum of protons and neutrons in the nucleus |
| Symbol | \( \mathrm{Z} \) | \( \mathrm{A} \) |
| Determines | Identity and position of element | Isotope and mass of atom |
| Includes Electrons? | Yes (equal in neutral atom) | No (only nucleons counted) |
| Example | Oxygen → 8 | Oxygen → 16 |
Example :
Find the number of protons, neutrons, and electrons in \( \mathrm{^{37}_{17}Cl} \).
▶️ Answer / Explanation
Step 1: Atomic number = 17 → 17 protons, 17 electrons.
Step 2: Mass number = 37 → neutrons = 37 − 17 = 20.
Final Answer: Protons = 17, Neutrons = 20, Electrons = 17.
Example :
Two isotopes of oxygen are \( \mathrm{^{16}_{8}O} \) and \( \mathrm{^{18}_{8}O} \). What do they have in common, and what is different?
▶️ Answer / Explanation
Common: Both have 8 protons and 8 electrons (same element).
Difference: Number of neutrons — \( \mathrm{^{16}O} \) has 8, \( \mathrm{^{18}O} \) has 10.
Final Answer: Isotopes have the same atomic number but different mass numbers due to varying neutrons.
Example :
Explain why atomic number, not mass number, determines the chemical properties of an element.
▶️ Answer / Explanation
Step 1: Atomic number = number of protons = number of electrons in a neutral atom.
Step 2: Electrons determine how an atom bonds and reacts chemically.
Step 3: Mass number only affects mass and stability, not chemical behavior.
Final Answer: The atomic number defines the electron arrangement, hence the element’s chemical properties.