Understanding Matter: Properties, Composition, and Atomic Structure
Pure Substances and Mixtures
Pure substances are composed of particles that are identical.
- Simple Substances: These cannot be broken down into simpler substances by chemical methods. Their atoms are all the same (e.g., iron, gold, copper, chlorine).
- Composite Substances: These are formed by the union of two or more chemical elements in a fixed proportion. They can be separated into their constituent elements by simple chemical methods (e.g., water, sodium chloride).
Mixtures are combinations of two or more substances that are not chemically combined.
- Heterogeneous Mixtures: These are material systems consisting of several substances. Their structure, composition, and properties vary depending on the elements that form them and their distribution.
- Homogeneous Mixtures: These are composed of different substances but maintain the same properties throughout.
Types of Homogeneous Mixtures
- Solid-Solid: e.g., bronze
- Solid-Liquid: e.g., salt in water
- Liquid-Liquid: e.g., water-alcohol
- Gas-Liquid: e.g., oxygen in water, carbon dioxide in water
- Gas-Gas: e.g., air (a mixture of different gases)
Atomic Theory: Dalton and Rutherford
John Dalton (1803) proposed that:
- Matter is made up of small particles called atoms.
- Atoms are indivisible and cannot be broken down in chemical reactions.
- Atoms of the same chemical element are identical.
- Atoms of different chemical elements are different.
- Atoms combine in simple whole-number ratios to form compounds.
Rutherford discovered that the atom is mostly empty space, consisting of a very small, dense nucleus compared to the rest of the atom.
- The nucleus is at the center of the atom and contains the positive charge and nearly all of its mass.
- Electrons orbit the nucleus in circular paths, occupying most of the atomic volume.
Subatomic Particles
- Electron: A negatively charged subatomic particle with a charge equal in magnitude to that of a proton.
- Proton: A positively charged subatomic particle with a charge equal in magnitude to that of an electron.
- Neutron: A subatomic particle with no charge. The mass of a proton is nearly equal to the mass of a neutron.
Atomic Number, Mass Number, and Isotopes
- Atomic Number (Z): The number of protons in the nucleus of an atom. It is equal to the number of electrons if the atom is neutral. Each element has a unique atomic number.
- Mass Number (A): The sum of the number of protons and neutrons in the atomic nucleus.
- Isotopes: Atoms of the same element (and therefore the same atomic number) that have different numbers of neutrons and therefore different mass numbers.
- Atomic Mass of an Element: The average mass of an element, taking into account the relative abundance of its isotopes in nature. It is calculated from the weighted average of the isotopic masses and their abundances.
Changes of State
Changes of state are the processes by which a substance changes from one state of aggregation to another when heated or cooled.
- Melting: The change from a solid to a liquid state upon the application of heat.
- Solidification (Freezing): The change from a liquid to a solid state upon cooling.
- Evaporation: The change from a liquid to a vapor at any temperature, occurring at the surface of the liquid.
- Boiling: When a liquid is heated sufficiently, its temperature increases until it begins to boil and becomes a gas.
- Condensation and Liquefaction: The process of a gas changing to a liquid upon cooling.
- Sublimation: The direct change from a solid to a gas without passing through the liquid state (e.g., mothballs, dry ice).
- Reverse Sublimation: The direct change from a gas to a solid (e.g., frost).