Atomic Structure & the Periodic Table

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Quick Summary

Elements: Pure substances made of one type of atom (same number of protons)
Compounds: Two or more elements chemically joined; can't be separated by physical means
Mixtures: Two or more substances NOT chemically joined; can be separated physically
Atomic Structure: Nucleus (protons + neutrons) surrounded by electrons in shells
Atomic Number (Z): Number of protons = number of electrons in neutral atoms
Mass Number (A): Total protons + neutrons
Electronic Configuration: Shows how many electrons in each shell (e.g., 2,8,2)
Periodic Table Position: Number of shells = period; outer electrons = group
Isotopes: Same element, different number of neutrons; same chemistry, different mass
Relative Atomic Mass: Weighted average of all isotopes of an element

1. Elements, Compounds & Mixtures

What's the Difference?

All matter can be sorted into three categories. Think of it like a hierarchy of purity:

🔹 Elements

Definition: A pure substance made of atoms that all contain the same number of protons and cannot be split into anything simpler by chemical means.

Key fact: The atomic number (number of protons) defines which element it is. All carbon atoms have 6 protons. All oxygen atoms have 8 protons. There are about 118 elements in the Periodic Table.

Examples: Pure iron (Fe), pure oxygen gas (O₂), pure copper (Cu)

Why "can't be split by chemical means"? If you try to break down an element, you hit atoms. And atoms are made of protons, neutrons, and electrons held together by forces that chemical reactions can't overcome. That's the wall—you can't go simpler without nuclear reactions.

🔹 Compounds

Definition: A pure substance made up of two or more elements chemically combined. The elements are held together by chemical bonds.

Key fact: Compounds have unlimited number of possibilities—you can combine elements in countless ways. Once atoms are chemically bonded, they can only be separated by chemical reactions, not physical ones (no amount of stirring, filtering, or evaporating will split a compound back into elements).

Examples: Water (H₂O = 2 hydrogen atoms + 1 oxygen atom), carbon dioxide (CO₂ = 1 carbon + 2 oxygen), table salt (NaCl = sodium + chlorine)

Exam trick: "Can it be separated by physical means?" If yes, it's a mixture. If no, it's a compound.

🔹 Mixtures

Definition: A combination of two or more substances (elements and/or compounds) that are NOT chemically combined.

Key fact: The substances in a mixture keep their individual properties. You can separate them using physical methods: filtering, evaporation, magnetic separation, distillation, etc.

Examples: Sand and water, oil and water, air (mixture of nitrogen, oxygen, argon, CO₂), salt solution (salt + water)

Analogy: Think of a compound like a married couple (chemically bonded—they're a new thing together). A mixture is like a crowd at a party—everyone's there, but not bonded. You can ask people to leave and separate the crowd.

Practice: Elements, Compounds & Mixtures

Q1: Is sea water an element, compound, or mixture? Explain your reasoning.

4. Isotopes

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Also in the full note

2. Atomic Structure
3. Electronic Configuration
What to Memorise
Concepts Checklist
Exam Tips & Common Pitfalls
Final Revision Summary
The Basic Idea
Subatomic Particles

What's inside

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Atomic Structure & the Periodic Table