2.1.3 Atomic Number and Mass Number

This topic defines atomic number and mass number and explains how to calculate the number of protons, neutrons, and electrons in atoms and ions.

Definition

Atomic Number (Z): The number of protons in the nucleus of an atom.

Mass Number (A): The total number of protons and neutrons in the nucleus. Also called nucleon number.

Main Idea

Atomic number (Z) determines the identity of an element. Mass number (A) is the sum of protons and neutrons. In a neutral atom, the number of electrons equals the number of protons. For ions, the number of electrons is adjusted based on the charge.

Core Concept

The atomic number and mass number together describe the composition of an atom's nucleus. The number of neutrons can be calculated by subtracting the atomic number from the mass number.

Detailed Explanation

Atomic Number (Z)

• The number of protons in the nucleus of an atom.
• Since an atom is neutral, the number of electrons revolving around the nucleus is equal to the number of protons.
• Represented by Z.
• Determines the identity of the element.

Mass Number (A)

• Represents the total number of protons and neutrons in the nucleus.
• Represented by A.
• Also called nucleon number.
• Mass number is approximately equal to the atomic mass of an atom.

Key Formulas

Atomic number + Number of neutrons = Mass number

P + N = A

Number of Protons = Number of Electrons (in a neutral atom)

Number of electrons in a cation = Atomic number - magnitude of charge on cation

Number of electrons in an anion = Atomic number + magnitude of charge on anion

Notation

^A_ZX

• **X** = element symbol
• **A** = mass number
• **Z** = atomic number

Intermediate Level

Atomic Number (Z)

• Number of protons.
• Determines the element.
• In a neutral atom, electrons = protons.

Mass Number (A)

• Protons + Neutrons.
• Approximately equal to atomic mass.

For Ions

• Cation (positive ion): Electrons = Atomic number − charge.
• Anion (negative ion): Electrons = Atomic number + charge.

Example Calculations

• Atomic number = 12 → 12 protons
• Charge = +2
• Electrons = 12 − 2 = **10**
• Mass number = 24 → Neutrons = 24 − 12 = 12

• Atomic number = 17 → 17 protons
• Charge = −1
• Electrons = 17 + 1 = **18**
• Mass number = 35 → Neutrons = 35 − 17 = 18

• Atomic number = 13 → 13 protons
• Charge = +3
• Electrons = 13 − 3 = **10**
• Mass number = 27 → Neutrons = 27 − 13 = 14

• Atomic number = 16 → 16 protons
• Charge = −2
• Electrons = 16 + 2 = **18**
• Mass number = 32 → Neutrons = 32 − 16 = 16

Important Points

• 1. Atomic number = number of protons.
• 2. Mass number = protons + neutrons.
• 3. In a neutral atom, protons = electrons.
• 4. Cation: electrons = Z − charge.
• 5. Anion: electrons = Z + charge.
• 6. Mass number is approximately equal to atomic mass.
• 7. Atomic number determines the element.
• 8. Isotopes have the same Z but different A.

Remember

• - Atomic number (Z): Protons
• - Mass number (A): Protons + Neutrons
• - Cation: Electrons = Z − charge
• - Anion: Electrons = Z + charge
• - Neutrons: A − Z

Common Mistakes

• 1. Confusing atomic number and mass number: Z = protons; A = protons + neutrons.
• 2. Forgetting to adjust electrons for ions: Cations lose electrons; anions gain electrons.
• 3. Mixing up the formulas: Cation = Z − charge; Anion = Z + charge.
• 4. Thinking mass number is the same as atomic mass: Mass number is a whole number; atomic mass is a weighted average.

Quick Revision

• Atomic number = number of protons.
• Mass number = protons + neutrons.
• In a neutral atom, protons = electrons.
• Cation: electrons = Z − charge.
• Anion: electrons = Z + charge.
• Neutrons = A − Z.

60-Second Revision

• Atomic number (Z): Protons
• Mass number (A): Protons + Neutrons
• Cation: Electrons = Z − charge
• Anion: Electrons = Z + charge
• Neutrons: A − Z

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