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Atomic Structure

Protons, Neutrons, Electrons & Isotopes

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Learning Objectives

Describe the structure of an atom

Identify subatomic particles and their properties

Understand proton number, neutron number, and mass number

Explain electronic configuration

Define and explain isotopes

Calculate relative atomic mass from isotope data

Atomic Structure Development

Atoms consist of a nucleus (protons & neutrons) surrounded by electrons in shells.

Subatomic Particles

Proton: Positive charge, mass ≈ 1 amu, in nucleus
Neutron: No charge, mass ≈ 1 amu, in nucleus
Electron: Negative charge, negligible mass, in shells around nucleus

Identifying Atoms

Proton number (Z): Number of protons in nucleus

Neutron number (N): Number of neutrons in nucleus

Mass number (A): Total of protons + neutrons

Formula: A = Z + N

Atomic Notation

Atoms are represented as: ¹²₆C (Carbon-12: 6 protons, 6 neutrons)

¹H: Hydrogen with 1 proton, 0 neutrons
¹⁶₈O: Oxygen with 8 protons, 8 neutrons
¹⁴₇N: Nitrogen with 7 protons, 7 neutrons

Electron Configuration & Shells

Electrons occupy shells (energy levels) in specific orders: 2, 8, 8, 18...

First shell: max 2 electrons
Second shell: max 8 electrons
Third shell: max 8 electrons (for atoms up to Ar)
Electrons fill lowest energy levels first

Writing Electronic Configuration

Method 1: Shell notation → 2,8,7 (for Chlorine)

Method 2: Full notation → 1s² 2s² 2p⁶ 3s² 3p⁵
Method 3: Simplified → [Ne] 3s² 3p⁵

Configuration Examples

Hydrogen (H): 1

Carbon (C): 2,4

Oxygen (O): 2,6

Sodium (Na): 2,8,1

Chlorine (Cl): 2,8,7

Isotopes

Isotopes: Atoms of the same element with different numbers of neutrons (same Z, different A)

Same element = same proton number
Different mass numbers
Same chemical properties
Different nuclear properties

Examples of Isotopes

Carbon-12 (¹²C): 6 protons, 6 neutrons

Carbon-14 (¹⁴C): 6 protons, 8 neutrons (radioactive, used in dating)

Chlorine-35 & Chlorine-37: Both exist naturally

Relative Atomic Mass (Ar)

When isotopes exist, the relative atomic mass is a weighted average of all isotope masses.

Formula: Ar = Σ(isotope mass × abundance %)
Example: Boron has ¹⁰B (19%) and ¹¹B (81%)
Ar = (10 × 0.19) + (11 × 0.81) = 1.9 + 8.91 = 10.81

Practice: Calculate Ar

Boron has two isotopes:

¹⁰B with abundance 19%

¹¹B with abundance 81%

Ar = (10 × 0.19) + (11 × 0.81) = 10.81

Ions & Charged Atoms

Ion: An atom with unequal numbers of protons and electrons.

Cation: Loses electrons → positive charge (e.g., Na⁺, Ca²⁺)
Anion: Gains electrons → negative charge (e.g., Cl⁻, O²⁻)
Charge = Protons − Electrons

Lesson Summary

Atoms = nucleus (protons + neutrons) + electrons in shells

Proton number defines the element

Mass number = protons + neutrons

Isotopes have same Z, different A

Ar is weighted average of isotope masses

Ions are charged atoms