Atoms and Molecules 9 Science — Revision Notes
Atoms are the smallest units of an element that retain its properties, while molecules are combinations of atoms bonded together. Understanding atomic and
TL;DR: Atoms are the smallest units of an element that retain its properties, while molecules are combinations of atoms bonded together. Understanding atomic…
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Atoms are the smallest units of an element that retain its properties, while molecules are combinations of atoms bonded together. Understanding atomic and
Atoms and Symbols
- Atom: smallest unit of an element, indivisible in chemical reactions
- Element symbols: one or two letter abbreviations (H, C, Fe, Cl)
- Relative atomic mass: compared to 1/12th mass of carbon-12 atom
Molecules and Molecular Mass
- Molecule: group of atoms bonded together; smallest unit of a compound or element
- Molecular mass: sum of atomic masses of all atoms in molecule
- Diatomic molecules: O2, N2, Cl2, H2 (two atoms of same element)
- Polyatomic molecules: CO2, H2O, NH3 (combinations of different elements)
Law of Constant Proportions
- In a compound, elements are always present in same ratio by mass
- Example: water always has 2 parts hydrogen to 16 parts oxygen by mass
- This law explains why compounds have fixed compositions and properties
Valency and Chemical Formulae
- Valency: number of electrons lost, gained, or shared by an atom
- Chemical formula: represents atoms in a molecule using element symbols and numbers
- Ionic compounds: formed by transfer of electrons between atoms
- Covalent compounds: formed by sharing of electrons between atoms
Key Terms
- Atom: Smallest indivisible unit of an element
- Molecule: Group of atoms chemically bonded together
- Valency: Capacity of an atom to combine with other atoms
- Relative Atomic Mass: Mass of atom compared to 1/12th of carbon-12
- Ion: Charged atom or group of atoms with lost or gained electrons
- Molar Mass: Mass of one mole of a substance in grams
Frequently Asked Questions
What is the difference between an atom and a molecule?
An atom is the smallest unit of an element; a molecule is a group of atoms chemically bonded together.
Why do different compounds have different properties even if made of the same elements?
Different compounds have different atomic arrangements and bonding patterns, resulting in different properties.
How is chemical formula written?
Chemical formula uses element symbols with subscript numbers showing the number of each atom in the molecule.
More 9 Science Revision Notes
- Matter in Our Surroundings
- Is Matter Around Us Pure
- Structure of the Atom
- The Fundamental Unit of Life
- Force and Laws of Motion
- Work and Energy
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