Fundamental Chemistry Terms and Definitions
Atomic Mass Unit (amu)
A unit of mass used to express atomic and molecular weights, equal to one-twelfth of the mass of an atom of carbon-12.
Ion
An atom or group of atoms that carries an electric charge.
Mole
The SI unit for the amount of substance, containing approximately 6.022 x 1023 elementary entities (atoms, molecules, ions, etc.).
Molecule
A group of two or more atoms held together by chemical bonds.
Atomic Mass
The mass of an atom, typically expressed in atomic mass units (amu). It is approximately the sum of the masses of the protons, neutrons, and electrons.
Isotopes
Atoms of the same chemical element (same atomic number, Z) that differ in the number of neutrons, and therefore in mass number.
Noble Gases
A group of chemical elements with similar properties; they are odorless, colorless, monatomic gases with very low chemical reactivity. They are stable and do not readily react with other elements. With the exception of Helium, their atoms have 8 electrons in their outermost electron shell.
Molecular Mass
The mass of a molecule, calculated as the sum of the atomic masses of all the atoms in the molecule.
Molecular Formula
A way to represent a chemical compound using chemical symbols to indicate the types of atoms and subscripts to show the number of atoms of each type in one molecule of the compound.
Anion
An ion with a negative electric charge, formed when an atom gains one or more electrons.
Cation
An ion with a positive electric charge, formed when an atom loses one or more electrons.
Chemical Compound
A substance formed from the chemical union of two or more different elements from the periodic table, joined in a fixed ratio.
Heterogeneous Mixture
A mixture in which the composition is not uniform throughout. Its properties vary, and its components can often be visually distinguished and separated.
Homogeneous Mixture (Solution)
A mixture that has uniform composition and properties throughout. Components retain their original properties and can often be separated by simple physical means (like evaporation).
Element
A pure substance consisting only of atoms that all have the same numbers of protons in their atomic nuclei. Cannot be broken down into simpler substances by chemical means.
Proton
A subatomic particle found in the nucleus of every atom, carrying a positive electric charge.
Neutron
A subatomic particle found in the nucleus of most atoms (except hydrogen-1), carrying no electric charge.
Electron
A subatomic particle orbiting the atom’s nucleus, carrying a negative electric charge.
Electromagnetic Spectrum
The entire range of electromagnetic radiation, ordered by frequency or wavelength. It includes radio waves, microwaves, infrared radiation, visible light, ultraviolet radiation, X-rays, and gamma rays.
Metal
A class of elements characterized by physical properties such as: typically solid (except Mercury), possess metallic sheen, are dense, hard, ductile (can be drawn into wires), malleable (can be hammered into thin sheets), good conductors of heat and electricity, generally have high melting and boiling points, and are oxidizable (tend to lose electrons).
Nonmetal
A class of elements that lack the typical properties of metals. They can be solids, liquids, or gases at room temperature. Generally have lower melting and boiling points than metals, are poor conductors of heat and electricity, and are not ductile or malleable.
Periodic Properties
Trends in the chemical and physical properties of elements that repeat periodically as the atomic number increases. These trends are reflected in the arrangement of the periodic table.
Electronegativity
A measure of the tendency of an atom to attract a bonding pair of electrons (electrons shared in a chemical bond).
Acid
A chemical compound that, when dissolved in water, releases protons (hydrogen ions, H+).
Chemical Reaction
A process that involves the rearrangement of the molecular or ionic structure of a substance, as opposed to a change in physical form or a nuclear reaction. It involves the transformation of initial substances (reactants) into different final substances (products) with different compositions and properties.
Chemical Bond
The forces that hold atoms together in molecules, crystals, or other structures. The main types are ionic bonds, covalent bonds, and metallic bonds, forming ionic networks, molecular structures, covalent networks, and metallic networks.
Ionic Bond (E. Ionico)
A type of chemical bond formed through an electrostatic attraction between two oppositely charged ions. It typically occurs between a metal (which loses electrons to become a cation) and a nonmetal (which gains electrons to become an anion).
Properties:
- Typically solid at room temperature.
- High melting and boiling points.
- Conduct electricity when molten or dissolved in water (electrolytes) because the substance dissociates into mobile ions.
- Often soluble in water.
Covalent Bond (E. Covalente)
A chemical bond that involves the sharing of electron pairs between atoms. These electron pairs are known as shared pairs or bonding pairs, and the stable balance of attractive and repulsive forces between atoms, when they share electrons, is known as covalent bonding. It typically occurs between nonmetal atoms, often those with similar electronegativity.
Properties (Molecular Covalent Compounds):
- Often liquids or gases at room temperature (though some are solids).
- Relatively low melting and boiling points due to weak intermolecular forces.
- Generally poor electrical conductors in any phase.
- Solubility varies greatly depending on polarity.
Properties (Network Covalent Compounds):
- Typically very hard solids.
- Very high melting and boiling points.
- Generally insoluble in common solvents.
- Electrical conductivity varies (e.g., diamond is an insulator, graphite is a conductor).
Metallic Bond (E. Metalico)
A type of chemical bond formed between positively charged atoms in which the free electrons are shared among a lattice of cations. Metals are joined together to form a metallic lattice where positive metal ions occupy fixed positions surrounded by a ‘sea’ of delocalized electrons that belong to the entire network.
Properties:
- Do not dissolve in typical solvents but can form alloys (solid solutions with other metals).
- Ductile and malleable.
- Excellent electrical and thermal conductors due to mobile electrons.
- Generally high melting and boiling points.
Intermolecular Forces
Attractive forces between molecules, responsible for many of the bulk properties of matter (like boiling points and melting points). These forces hold together the molecules in molecular covalent compounds. The main types include:
- Hydrogen Bonds: A special type of dipole-dipole interaction between a hydrogen atom bonded to a highly electronegative atom (like N, O, or F) and another nearby electronegative atom.
- Van der Waals Forces: Weaker forces including dipole-dipole interactions and London dispersion forces (temporary induced dipoles).