Chemistry

1. Development of Atomic Models

• Atom: The smallest particle of an element that retains the properties of that element.

• Dalton’s model: Atoms are solid, indivisible spheres; atoms of the same element are identical.

• Thomson’s model: “Plum pudding” model — atoms contain negatively charged electrons embedded in a positive sphere.

• Rutherford’s model: Atoms have a small, dense, positively charged nucleus surrounded by mostly empty space.

• Bohr’s model: Electrons orbit the nucleus in fixed energy levels (shells).

• Chadwick’s discovery: Identified the neutron, a neutral particle in the nucleus.

2. Atomic Symbols and Representation

Atomic number (Z): Number of protons in an atom; defines the element.

Mass number (A): Total number of protons and neutrons in an atom’s nucleus.

Element symbol: One- or two-letter abbreviation (e.g., Fe for iron).

IUPAC notation: ^A_ZX — shows element symbol X , atomic number Z , and mass number A .

3. Relative Atomic Mass and Isotopes

• Relative atomic mass (Ar): The weighted average mass of an element’s atoms compared to 1/12 the mass of a carbon-12 atom.

• Isotopes: Atoms of the same element with the same number of protons but different numbers of neutrons.

• Isotopic composition: The percentage abundance of each isotope of an element.

• Chemical properties: Determined by electron configuration, so isotopes react the same chemically.

• Physical properties: Vary between isotopes (e.g., density, melting point).

4. Mass Spectrometry

• Ionisation: The process of removing or adding electrons to form charged particles (ions).

• Mass spectrometer: Instrument that measures the mass-to-charge ratio of ions.

• Mass spectrum: Graph showing relative abundance of isotopes based on their masses.

• Relative abundance: The percentage of each isotope present in a sample.

5. Atomic Structure and Electron Configuration

• Nucleus: Central core containing protons and neutrons.

• Proton: Positively charged particle, mass ≈ 1 amu.

• Neutron: Neutral particle, mass ≈ 1 amu.

• Electron: Negatively charged particle with negligible mass.

• Electron configuration: Arrangement of electrons in energy levels (e.g., 2,8,8,2).

• Energy levels (shells): Fixed regions around the nucleus where electrons reside.

• Electrostatic attraction: Force between positive nucleus and negative electrons.

6. Analytical Techniques: Flame Tests and AAS

• Flame test: Heating a sample in a flame causes electrons to jump energy levels; emitted light colour identifies the element.

• Emission spectrum: Coloured lines produced by electrons releasing energy as they return to lower levels.

• Absorption spectrum (AAS): Measures the light absorbed when electrons move to higher energy levels; used to determine element concentration.

7. Periodic Table Trends and Structure

• Periodic table: Arrangement of elements in order of increasing atomic number.

• Group: Vertical column in the periodic table; elements in a group share similar chemical properties.

• Period: Horizontal row; shows gradual changes in properties across the row.

• Atomic radius: Distance from the nucleus to the outermost electrons; decreases across a period, increases down a group.

• Valency: The combining power of an atom — number of electrons lost, gained, or shared in bonding.

• First ionisation energy: Energy required to remove the most loosely bound electron from one mole of gaseous atoms.

• Electronegativity: The ability of an atom to attract electrons in a bond