Unit 2 (Atoms, Elements, and Compounds) Chemistry

Atom

The smallest particle of an element that can exist and take part in chemical reactions

Element

A pure substance made of only one type of atom

Chemical symbol

One- or two-letter notation that represents an element (e.g., Na, O)

Molecule

Two or more atoms covalently bonded as a single particle

Molecule of an element

A molecule containing only one type of atom (e.g., O₂, Cl₂)

Diatomic elements

Elements that exist as two-atom molecules: H₂, N₂, O₂, F₂, Cl₂, Br₂, I₂

Monatomic element

An element that exists as single atoms (e.g., noble gases like He, Ne)

Allotrope

Different structural forms of the same element in the same physical state (e.g., diamond/graphite)

Metal (element)

Typically shiny, good conductor of heat/electricity, malleable, forms basic oxides

Non-metal (element)

Typically dull, poor conductor, brittle (if solid), forms acidic/neutral oxides

Compound

A substance formed when atoms of two or more elements are chemically combined in fixed ratios

Molecule of a compound

A neutral group of different atoms bonded together (e.g., H₂O, CO₂)

Chemical formula

Shows the types and numbers of atoms in a substance (e.g., C₆H₁₂O₆)

Fixed composition in compounds

The elements in a compound are present in definite proportions by mass

Ionic compound

Made of positive and negative ions held by electrostatic attraction in a lattice

Covalent compound

Made of non-metal atoms sharing pairs of electrons to form molecules or giant networks

Lattice

A regular, repeating 3D arrangement of particles (common in ionic solids and some covalent solids)

Mixture

Two or more substances physically combined, not chemically bonded

Homogeneous mixture

Uniform composition throughout (e.g., salt solution)

Heterogeneous mixture

Non-uniform composition with visibly different parts (e.g., oil and water)

Solution

A homogeneous mixture of a solute dissolved in a solvent

Solute

The substance that is dissolved

Solvent

The substance that does the dissolving (often water)

Solubility

Maximum mass of solute that dissolves in a given mass/volume of solvent at a stated temperature

Saturated solution

A solution that cannot dissolve more solute at that temperature

Filtration

Separation of an insoluble solid from a liquid using filter paper

Simple distillation

Separation of a dissolved solid from a liquid, or liquids with very different b.p.s

Fractional distillation

Separation of a mixture of miscible liquids with closer boiling points using a fractionating column

Paper chromatography

Separation of substances by differing solubilities and attractions to the paper and solvent

Rf value

Ratio: distance moved by substance ÷ distance moved by solvent front

Inside atoms

Atoms consist of a tiny nucleus (protons + neutrons) surrounded by electrons in shells

Proton

Subatomic particle with charge +1 and relative mass 1, found in the nucleus

Neutron

Subatomic particle with charge 0 and relative mass 1, found in the nucleus

Electron

Subatomic particle with charge −1 and negligible relative mass (~0), in shells around the nucleus

Nucleus

Dense central region of the atom containing protons and neutrons

Relative charges and masses

Proton +1 (mass 1), neutron 0 (mass 1), electron −1 (mass ~0)

Atomic number (proton number)

Number of protons in the nucleus; identifies the element

Mass number (nucleon number)

Total number of protons + neutrons in the nucleus

Isotope

Atoms of the same element with the same proton number but different numbers of neutrons

Relative atomic mass (Aᵣ)

Weighted average mass of an element's isotopes compared to ¹²C = 12

Electron shell (energy level)

Regions around the nucleus where electrons are likely to be found

Electronic configuration

Distribution of electrons in shells (e.g., Na: 2,8,1)

Valence (outer-shell) electrons

Electrons in the outermost shell; largely determine chemical properties

Ion

A charged particle formed when an atom gains or loses electrons

Cation

Positively charged ion formed by electron loss (often metals)

Anion

Negatively charged ion formed by electron gain (often non-metals)

Noble gas stability

Atoms tend to form ions or share electrons to achieve a full outer shell like noble gases

Oxidation

A reaction involving gain of oxygen, loss of hydrogen, or loss of electrons

Reduction

A reaction involving loss of oxygen, gain of hydrogen, or gain of electrons

Redox reaction

A reaction in which oxidation and reduction occur simultaneously

Oxidising agent

Substance that causes oxidation (and is reduced itself)

Reducing agent

Substance that causes reduction (and is oxidised itself)

OIL RIG

Mnemonic: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons)

Combustion

Rapid oxidation that releases heat and light; forms oxides (e.g., C + O₂ → CO₂)

Rusting of iron

Slow oxidation of iron in the presence of oxygen and water forming hydrated iron(III) oxide (rust)

Metal + oxygen reaction

General form: metal + oxygen → metal oxide