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Elements and compounds can exist as pure substances, each with a fixed, definite composition. Some compounds, like water, are made of individual molecules. Others, like salt, form crystals—large, repeating 3D patterns without individual molecules. Additionally, some elements pair up as diatomic molecules, such as O₂ and H₂, rather than existing as single atoms. Understanding whether a substance is molecular or crystalline and how diatomic elements differ from compounds is key to classifying matter.

00:00 Intro
00:48 What are crystals?
01:50 Chemical formula of crystals
03:29 General types of compounds
04:03 Diatomic elements
05:11 Summary of pure substances

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