Chapter 2: Atoms, Molecules and Ions

Element

A substance consisting of atoms with the same atomic number; historically thought to be indivisible, but modern view defines elements by atomic number and composition.

Atomic number

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

Isotope

Atoms of the same element with different numbers of neutrons, giving different masses.

Allotrope

Different structural forms of the same element in the same phase (e.g., carbon: diamond, graphite, fullerenes).

Diadatomic molecule

A molecule composed of two atoms, such as H2, O2, or N2.

Atom

The basic unit of matter; consists of a positively charged nucleus surrounded by negatively charged electrons.

Nucleus

The positively charged center of an atom, containing protons and neutrons.

Proton

A positively charged subatomic particle in the nucleus; mass similar to a neutron; charge +e.

Neutron

An electrically neutral subatomic particle in the nucleus.

Electron

A negatively charged subatomic particle orbiting the nucleus; very light; e/m ratio is about -1.76 × 10^8 C/g; discovered by J. J. Thomson.

Cathode ray

A beam of electrons produced in a vacuum tube that Thomson used to demonstrate the existence of electrons.

Law of conservation of mass

Mass is neither created nor destroyed during a chemical reaction.

Law of definite proportions

In a pure compound, the constituent elements are present in definite mass ratios.

Law of constant composition

All samples of a given compound have the same elemental composition.

Law of multiple proportions

When two elements form a series of compounds, the masses of one element that combine with a fixed mass of the other are in small whole-number ratios.

Dalton's atomic theory

Elements consist of tiny indivisible atoms; atoms of the same element are identical; different elements have different atoms; compounds form when atoms combine in simple whole-number ratios; atoms retain their identities in reactions.

Avogadro1 Hypothesis

Equal volumes of different gases, at the same temperature and pressure, contain equal numbers of particles.

Gay-Lussac's Law of Combining Volumes

Gases react in definite simple integer ratios by volume when measured at the same T and P.

Cannizzaro

Chemist who, using Avogadro's hypothesis, established consistent relative atomic and molecular masses and helped settle atomic mass determinations.

Relative molecular mass

The mass of a molecule relative to a standard (often based on equal volumes; hydrogen is used as a reference with assigned mass).

Relative atomic mass

The mass of an atom relative to a standard reference used to compare atomic masses.

Mixture

A combination of two or more elements or compounds that retains its own properties and can be separated by physical means.

Homogeneous mixture

A mixture with uniform composition throughout (e.g., NaCl in water, air).

Heterogeneous mixture

A mixture with nonuniform composition and distinct phases (e.g., Cu(NO3)2·6H2O mixed with CdS).

Pure substance

A substance that is either an element or a compound (not a mixture).

Compound

A substance that contains two or more elements; can be separated into its constituent elements (e.g., H2O).

Binary compound

A compound that contains two elements (e.g., H2O).

Ternary compound

A compound that contains three elements (e.g., C2H5OH).

Quaternary compound

A compound that contains four elements (e.g., CF3CH2OH).

Allotropy (recap)

The phenomenon of different structural forms of the same element; examples include carbon allotropes and oxygen (O2 vs O3).

Oxygen allotropes

Different forms of oxygen, notably O2 (dioxygen) and O3 (ozone).

Carbon allotropes

Forms of carbon such as diamond, graphite, and fullerenes (e.g., C60).

Diamond

A hard crystalline allotrope of carbon with a tetrahedral lattice.

Graphite

A layered carbon allotrope with strong in-plane bonds and weaker interlayer interactions.

Fullerenes (C60)

Spherical carbon molecules (buckyballs) consisting of carbon atoms arranged in a hollow sphere.

Amorphous carbon

Carbon without long-range crystalline order (e.g., char, soot, coal).

Polymorphs

Different structural forms of a compound in the same phase (e.g., TiO2 and SiO2).