Structure of Matter
4. Atomic Number and Mass Number
Learning outcomes
- I can define atomic number and mass number.
- I can determine the number of protons, neutrons, and electrons in an atom using atomic notation.
- I can interpret standard nuclear notation for atoms.
- I can explain why atomic number identifies an element.
- I can calculate the number of neutrons from atomic number and mass number data.
Introduction
Every atom has its own unique identity. Just as every person has a unique fingerprint, every element is identified by the number of protons in its atoms. Chemists use two important numbers—the atomic number and the mass number—to describe the structure of atoms and distinguish one element from another.
Understanding these numbers allows us to determine how many protons, neutrons, and electrons an atom contains. This information is essential for studying isotopes, chemical bonding, nuclear reactions, and the periodic table.
What Is the Atomic Number?
The atomic number is the number of protons in the nucleus of an atom.
Every element has its own unique atomic number.
Definition:
The atomic number is the number of protons in the nucleus of an atom.
For example:
| Element | Atomic Number | Number of Protons |
|---|---|---|
| Hydrogen | 1 | 1 |
| Carbon | 6 | 6 |
| Oxygen | 8 | 8 |
| Sodium | 11 | 11 |
| Chlorine | 17 | 17 |
Because no two elements have the same number of protons, the atomic number uniquely identifies an element.
Why Does Atomic Number Identify an Element?
Changing the number of protons changes the element itself.
For example:
- 6 protons → Carbon
- 7 protons → Nitrogen
- 8 protons → Oxygen
Even adding or removing one proton creates a completely different element.
This is why the atomic number is sometimes called an atom's identity number.
What Is the Mass Number?
The mass number is the total number of:
- protons
- neutrons
inside the nucleus.
Definition:
Mass number = Number of protons + Number of neutrons
Electrons are not included because their mass is extremely small compared with protons and neutrons.
Example
Carbon-12 contains:
- 6 protons
- 6 neutrons
Mass number:
12
Comparing Atomic Number and Mass Number
| Property | Atomic Number | Mass Number |
|---|---|---|
| Symbol | Z | A |
| Counts | Protons | Protons + Neutrons |
| Changes the Element? | Yes | No |
| Found in the Nucleus? | Yes | Yes |
Remember:
Atomic number identifies the element.
Mass number tells us the total number of particles in the nucleus.
Standard Nuclear Notation
Chemists often write atoms using standard nuclear notation.
It has the form:
\( {A \brack Z} X \)
where:
- A = mass number
- Z = atomic number
- X = chemical symbol
For example:
\( {23 \brack 11} Na \)
means:
- mass number = 23
- atomic number = 11
- element = sodium
Determining the Number of Subatomic Particles
Once the atomic number and mass number are known, the numbers of protons, neutrons, and electrons can be determined.
Step 1 – Number of Protons
Protons = Atomic Number
Step 2 – Number of Electrons
For a neutral atom:
Electrons = Protons
Step 3 – Number of Neutrons
Neutrons = Mass Number − Atomic Number
Worked Example 1
An atom has:
- Atomic number = 8
- Mass number = 16
Determine:
- protons
- neutrons
- electrons
Solution
Protons:
8
Electrons:
8
Neutrons:
16 − 8 = 8
Answer
- Protons = 8
- Neutrons = 8
- Electrons = 8
Worked Example 2
Interpret the notation:
\( {27 \brack 13} Al \)
Solution
Atomic number:
13
Therefore:
Protons = 13
Neutral atom:
Electrons = 13
Neutrons:
27 − 13 = 14
Answer
- Protons = 13
- Neutrons = 14
- Electrons = 13
Another Example
Worked Example 3
Interpret:
\( {40 \brack 20} Ca \)
Atomic number:
20
Therefore:
Protons = 20
Electrons = 20
Neutrons:
40 − 20 = 20
Atoms and Ions
For ions, the number of electrons is different from the number of protons.
Example:
Na⁺
Atomic number:
11
Therefore:
- Protons = 11
- Electrons = 10
The mass number remains unchanged because only electrons have been gained or lost.
Summary Table
| Quantity | Formula |
|---|---|
| Protons | Atomic number |
| Electrons (neutral atom) | Atomic number |
| Neutrons | Mass number − Atomic number |
This table is one of the most useful summaries in introductory chemistry.
Common Mistakes
Students often make these errors.
❌ Confusing atomic number with mass number.
✔ Atomic number counts protons only.
✔ Mass number counts protons and neutrons.
❌ Including electrons in the mass number.
✔ Electrons contribute almost no mass and are not included.
❌ Forgetting to subtract when finding neutrons.
✔ Use:
Neutrons = Mass Number − Atomic Number
❌ Assuming ions have the same number of electrons as protons.
✔ Only neutral atoms have equal numbers of protons and electrons.
Real-World Connection
Doctors use atomic notation when working with radioactive isotopes in medical imaging and cancer treatments. Archaeologists use carbon-14 to determine the age of ancient artefacts, while nuclear engineers rely on atomic and mass numbers to identify isotopes used in reactors. Every element listed on the periodic table is organised according to its atomic number, making this concept one of the foundations of modern chemistry.
Did You Know?
The periodic table is arranged in increasing atomic number, not by mass. Early versions of the periodic table were organised mainly by atomic mass, but scientists later discovered that the number of protons is the true property that determines an element's identity and its chemical behaviour.
Key Terms
- Atomic number (Z) — the number of protons in an atom.
- Mass number (A) — the total number of protons and neutrons in an atom.
- Nuclear notation — a standard way of writing an atom using its mass number, atomic number, and chemical symbol.
- Proton — a positively charged particle found in the nucleus.
- Neutron — a neutral particle found in the nucleus.
- Electron — a negatively charged particle found in the electron cloud.
- Neutral atom — an atom with equal numbers of protons and electrons.
- Ion — an atom that has gained or lost electrons and has an overall charge.
Key Takeaways
- The atomic number equals the number of protons and uniquely identifies an element.
- The mass number equals the total number of protons and neutrons.
- Standard nuclear notation shows both the atomic number and the mass number of an atom.
- For a neutral atom, the number of electrons equals the number of protons.
- The number of neutrons is found by subtracting the atomic number from the mass number.
- Understanding atomic number and mass number is essential for studying isotopes, ions, the periodic table, and nuclear chemistry.