3.3 Proton or Atomic Number

This topic explains atomic number (also called proton number) — the number that identifies an element and determines its position in the periodic table.

Definition

Proton number refers to the number of protons in the nucleus of an atom. It is also known as the atomic number and is indicated by the symbol Z.

Simple explanation: The atomic number tells you how many protons are in an atom's nucleus.

Main Idea

What determines the identity of an element? The answer is the number of protons.

• Every atom of a given element has the same number of protons.
• Different elements have different numbers of protons.
• The atomic number is unique to each element.

Simple explanation: If you know the atomic number, you know exactly which element you are dealing with.

Core Concept

Protons have a positive electrical charge. In neutral atoms, the number of protons is equal to the number of electrons. This balances the positive charge of the protons.

This means that the proton number also indicates the number of electrons in the atom.

Simple explanation: In a neutral atom: Number of protons = Number of electrons = Atomic number

Detailed Explanation

What Is Atomic Number?

The atomic number (Z) is the number of protons in the nucleus of an atom.

Examples:

• Hydrogen (H): 1 proton → atomic number = 1
• Helium (He): 2 protons → atomic number = 2
• Carbon (C): 6 protons → atomic number = 6
• Nitrogen (N): 7 protons → atomic number = 7
• Oxygen (O): 8 protons → atomic number = 8
• Sodium (Na): 11 protons → atomic number = 11

Simple explanation: The atomic number is like the element's ID card number — no two elements have the same one.

Atomic Number and Electrons

In a neutral atom, the number of protons equals the number of electrons.

So, atomic number also tells us: - Number of electrons in a neutral atom

• Carbon has atomic number 6 → 6 protons, 6 electrons (in a neutral atom).
• Sodium has atomic number 11 → 11 protons, 11 electrons (in a neutral atom).

Why Atomic Number Is Important

The atomic number is the fundamental property of an element because:

• It identifies the element.
• It determines the element's position in the periodic table.
• It determines the chemical properties of the element.
• It tells us the number of electrons in a neutral atom.

Uniqueness of Proton Number

Each element has a unique proton number that distinguishes it from other elements. It determines the various properties of an element and its position in the periodic table.

In the periodic table, elements are arranged based on their atomic or proton number. Therefore, the number of protons is related to the position of the element in the periodic table.

Thus, the number of protons determines each particular element. This will tell you what element you are talking about.

Examples from the book:

• If an atom has a proton number of 6, it must be carbon.
• If an atom has 11 protons, it must be sodium.
• Similarly, each nitrogen atom has 7 protons.
• Each oxygen atom has 8 protons.

You can identify each atom by the number of protons.

Intermediate Level

Atomic Number and the Periodic Table

The periodic table is arranged in order of increasing atomic number:

• Hydrogen (Z = 1) is first.
• Helium (Z = 2) is second.
• Lithium (Z = 3) is third.
• And so on...

This is why the atomic number is so important — it determines where an element sits in the periodic table and how it behaves chemically.

Atomic Number and Neutral Atoms

For a neutral atom:

Number of protons = Number of electrons = Atomic number (Z)

• 6 protons (in nucleus)
• 6 electrons (in shells)
• Net charge = 0 (neutral)

Atomic Number and Ions

• The number of protons stays the same (it is still the same element).
• The number of electrons changes.

• Na⁺: 11 protons, 10 electrons (lost 1 electron)
• Cl⁻: 17 protons, 18 electrons (gained 1 electron)

The atomic number is still 11 for Na and 17 for Cl — because atomic number depends only on protons.

Atomic Number and Isotopes

• Same atomic number (same number of protons)
• Different mass numbers (different numbers of neutrons)

• All have 6 protons (atomic number = 6)
• They differ in neutrons: 6, 7, 8 respectively

So the atomic number is the same for all isotopes of an element.

Important Points

• - Proton number (atomic number, Z): number of protons in the nucleus of an atom.
• - In neutral atoms, number of protons = number of electrons = atomic number.
• - Each element has a unique atomic number.
• - Atomic number determines the identity of the element.
• - Elements are arranged in the periodic table by increasing atomic number.
• - Atomic number determines the element's position in the periodic table.
• - Examples:
• - Hydrogen: Z = 1
• - Helium: Z = 2
• - Carbon: Z = 6
• - Nitrogen: Z = 7
• - Oxygen: Z = 8
• - Sodium: Z = 11
• - All isotopes of an element have the same atomic number.

Remember

• - Atomic number (Z) = number of protons.
• - In a neutral atom: protons = electrons = Z.
• - Unique to each element.
• - Determines identity and position in periodic table.
• - Elements are arranged by increasing atomic number.
• - **Examples:** H = 1, He = 2, C = 6, N = 7, O = 8, Na = 11.
• - Isotopes have the same atomic number but different mass numbers.

Common Mistakes

• 1. Thinking atomic number = mass number. Atomic number = protons; mass number = protons + neutrons.
• 2. Thinking atomic number changes in ions. It does not — only electrons change.
• 3. Confusing atomic number with number of neutrons. Neutrons = mass number − atomic number.
• 4. Thinking different isotopes have different atomic numbers. They have the same atomic number.
• 5. Forgetting that atomic number identifies the element. This is its most important function.
• 6. Thinking atomic number is the number of electrons. In a neutral atom it is equal to electrons, but the definition is protons.

Quick Revision

• Proton number (atomic number, Z): number of protons in the nucleus of an atom.
• In neutral atoms, number of protons = number of electrons = atomic number.
• Each element has a unique atomic number.
• Atomic number identifies the element and determines its position in the periodic table.
• Elements are arranged in order of increasing atomic number.
• Examples: Hydrogen = 1, Helium = 2, Carbon = 6, Nitrogen = 7, Oxygen = 8, Sodium = 11.
• Isotopes have the same atomic number but different mass numbers.

60-Second Revision

• Atomic number (Z) = number of protons.
• In neutral atom: protons = electrons = Z.
• Unique to each element → identifies the element.
• Elements arranged by increasing atomic number.
• **Examples:** H = 1, He = 2, C = 6, N = 7, O = 8, Na = 11.
• Warning: atomic number ≠ mass number.