Free Science Lessons: Understanding Elements, Compounds, Mixtures, and Molecules

Key Concepts

Element: A substance where all atoms are the same. Represented in the periodic table.
- Example: Magnesium (Mg), Sulfur (S).
- Each element has a unique symbol starting with a capital letter.
Compound: A substance formed when two or more different elements are chemically combined in fixed proportions.
- Example: Magnesium Sulfide (MgS).
- Compounds have different properties from their constituent elements.
- They require chemical reactions to be separated back into elements.
Mixture: A combination of different elements or compounds that are not chemically combined.
- Can be separated by physical means (e.g., filtration, distillation, crystallization, chromatography).
Molecule: Formed when two or more atoms are chemically combined.
- Can be the same element (e.g., O2, Cl2) or different elements (compounds, e.g., H2O, CH4).

Element: A pure substance with all atoms identical.
Compound: Consists of two or more elements in fixed proportions.
Mixture: Contains elements or compounds not chemically bonded.
Molecule: Two or more atoms chemically bonded, can be elements or compounds.

Elements:
- Magnesium (Mg): All magnesium atoms are the same.
- Sulfur (S): All sulfur atoms are the same.
Compounds:
- Magnesium Sulfide (MgS): Equal proportions of magnesium and sulfur atoms.
- Properties differ from elemental magnesium and sulfur.
Mixtures:
- Combination of magnesium, sulfur, and magnesium sulfide can be separated physically.
Molecules:
- Methane (CH4), Water (H2O), Ammonia (NH3) are compounds and molecules.
- Chlorine (Cl2) and Oxygen (O2) are molecules but not compounds.