Chemistry

Atomic Number

The number of protons in the nucleus of an atom; determines the element's identity.

Mass Number

The total number of protons and neutrons in an atom's nucleus.

Subatomic Particles

The three main components of an atom: protons (+), neutrons (0), and electrons (-).

Proton

A positively charged subatomic particle found in the nucleus with a relative mass of 1.

Neutron

A neutral subatomic particle found in the nucleus with a relative mass of 1.

Electron

A negatively charged subatomic particle orbiting the nucleus with almost negligible mass.

Isotope

Atoms of the same element with the same number of protons but a different number of neutrons.

Electronic Configuration

The arrangement of electrons in electron shells around an atom's nucleus (e.g., 2,8,8,2).

Valence Electrons

Electrons located in the outermost energy level/shell of an atom.

Group

A vertical column in the Periodic Table; elements in the same group have the same number of valence electrons and similar chemical properties.

Period

A horizontal row in the Periodic Table; elements in the same period have the same number of electron shells.

Alkali Metals

Group 1 elements; highly reactive metals that have 1 valence electron and readily form +1 ions.

Halogens

Group 17 elements; highly reactive non-metals that have 7 valence electrons and readily form -1 ions.

Noble Gases

Group 18 elements; extremely unreactive (inert) gases with full outer electron shells.

Reactivity Trend (Metals)

Metal reactivity increases moving DOWN a group and to the LEFT across a period.

Reactivity Trend (Non-metals)

Non-metal reactivity increases moving UP a group and to the RIGHT across a period (excluding Noble Gases).

Chemical Bonding

The electrostatic attraction between atoms or ions that binds them together to form stable compounds.

Ion

A charged particle formed when an atom gains or loses electrons to achieve a full outer shell.

Cation

A positively charged ion formed when an atom (usually a metal) loses electrons.

Anion

A negatively charged ion formed when an atom (usually a non-metal) gains electrons.

Ionic Bonding

The strong electrostatic force of attraction between oppositely charged ions (metals + non-metals) involving the transfer of electrons.

Covalent Bonding

The electrostatic attraction formed when non-metal atoms SHARE pairs of electrons to achieve full valence shells.

Polyatomic Ion

A charged group of covalently bonded atoms that acts as a single ion unit (e.g., SO4^2-, NO3^-).

Valency

The combining power of an element, determined by the number of electrons lost, gained, or shared to achieve a full shell.

Chemical Formula

A notation using chemical symbols and numerical subscripts to show the types and ratios of atoms/ions in a compound.

Law of Conservation of Mass

Matter cannot be created or destroyed in a chemical reaction; total mass of reactants equals total mass of products.

Reactants

The starting substances in a chemical reaction, written on the left side of the equation.

Products

The new substances formed during a chemical reaction, written on the right side of the equation.

Balanced Chemical Equation

An equation where the total number of atoms for each element is equal on both sides of the reaction arrow.

Exothermic Reaction

A chemical reaction that releases thermal energy to its surroundings, causing the temperature to rise ($\Delta H < 0$).

Endothermic Reaction

A chemical reaction that absorbs thermal energy from its surroundings, causing the temperature to drop ($\Delta H > 0$).

Activation Energy

The minimum amount of energy required for reacting particles to collide successfully and initiate a chemical reaction.

Synthesis Reaction

A reaction where two or more simple substances combine to form a single complex product ($A + B \rightarrow AB$).

Decomposition Reaction

A reaction where a single compound breaks down into two or more simpler products ($AB \rightarrow A + B$).

Combustion Reaction

A rapid reaction between a fuel and oxygen gas that produces heat, light, carbon dioxide, and water.

Displacement Reaction

A reaction where a more reactive element displaces a less reactive element from a compound ($A + BC \rightarrow AC + B$).

Precipitation Reaction

A double displacement reaction occurring in aqueous solution that forms an insoluble solid (precipitate).

Acid

A substance that releases hydrogen ions ($H^+$) in aqueous solution; turns universal indicator red/orange/yellow ($pH < 7$).

Base

A substance that releases hydroxide ions ($OH^-$) or accepts $H^+$ ions in aqueous solution; turns universal indicator blue/purple ($pH > 7$).

Alkali

A soluble base that dissolves in water to release $OH^-$ ions.

pH Scale

A logarithmic measure ranging from 0 to 14 indicating how acidic or basic a solution is.

Neutralization Reaction

A chemical reaction between an acid and a base producing a neutral salt and water ($Acid + Base \rightarrow Salt + Water$).

Acid-Metal Reaction

Reaction yielding a salt and hydrogen gas ($Acid + Metal \rightarrow Salt + H_2$).

Acid-Carbonate Reaction

Reaction yielding a salt, water, and carbon dioxide gas ($Acid + Carbonate \rightarrow Salt + H_2O + CO_2$).

Rate of Reaction

The speed at which reactants are consumed or products are formed in a chemical reaction over time.

Collision Theory

States that for a reaction to occur, particles must collide with sufficient energy (equal to or greater than activation energy) and correct orientation.

Surface Area Effect

Increasing surface area exposes more reactant particles, increasing collision frequency and raising reaction rate.

Concentration Effect

Higher concentration means more reactant particles per unit volume, increasing the frequency of collisions.

Temperature Effect

Higher temperature increases particles' kinetic energy, causing more frequent collisions and a higher proportion of collisions exceeding activation energy.

Catalyst

A substance that increases the rate of a chemical reaction by providing an alternative pathway with a lower activation energy, without being consumed itself.