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Chemistry Cheat Sheet (DRAFT) by

GCE O Levels Chemistry Sheet

This is a draft cheat sheet. It is a work in progress and is not finished yet.

Atoms, Elements and Compounds

Atoms
Smallest particle of an element that has the same chemical property of that element
Element
Pure substance that cannot be broken down into simpler substances by chemical process
Compound
Pure substance that are made up of two or more elements chemically combined in a fixed ratio
Molecule
Group of two or more atoms chemically bonded together
Mixture
Two or more substances that are not chemically combined (physical)

Atom

An atom consists of a nucleus (conta­­ining protons and neutrons) surrounded by electrons in different energy shells.
Protons are positively charged (+1), neutrons have no charge (0), and electrons are negatively charged (-1).
  

Periodic Table of Elements

• An element consists of only one type of atom and cannot be broken down into simpler substances by chemical means.
• Periodic table organizes elements based on their atomic number and proper­ties.

Compound

• A compound is formed when two or more different elements chemically combined in a fixed ratio.
• Example: Water (H₂O) consists of 2 hydrogen atoms and 1 oxygen atom.
• Chemical bonds (ionic or covalent) hold the atoms together.
  

Element, Compound and Mixture

Element, Compound and Mixture

Property
Compound
Mixture
Separation
Cannot be separated physically
Can be separated by physical means
Compos­ition
Fixed ratio
Variable
Properties
Has different properties from its components
Retains properties of individual components
Example
Water (H₂O)
Air (O₂, N₂, CO₂)

Active Recall

1. What is a compound? How is it different from an element?
•A compound is a substance made up of two or more different elements chemically combined in a fixed ratio. Unlike elements, compounds can be broken down into simpler substa­nces.
2. What is the difference between a molecule and a compound?
•A molecule is a group of atoms chemically combined together (can be the same two or more atoms bonde or different elements).
•A compound is a molecule that contains at least two different elements.