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

This table is one of the most useful summaries in introductory chemistry.

 

6

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.