Chemistry - Atoms and Isotopes

Democritus (400 B.C.)

Matter is made of atoms that are solid, homogeneous, indivisible, and differ in size and shape.

Dalton (1803)

Proposed that all matter is made of atoms, atoms of the same element are identical, and atoms combine in whole-number ratios.

Thomson (1897)

Discovered the electron using cathode ray experiments; found that atoms are divisible and contain negatively charged particles.

Cathode Ray Tube Experiment

Thomson used this to show that “cathode rays” were streams of negatively charged particles (electrons).

Rutherford (1911)

Used the gold foil experiment to show that atoms have a small, dense, positively charged nucleus and are mostly empty space.

Bohr (1913)

Proposed that electrons orbit the nucleus in fixed energy levels.

Modern Model (1935–present)

Quantum Mechanical Model—electrons exist in regions of probability around the nucleus, not fixed paths.

Subatomic Particles

Protons (positive), Neutrons (neutral), and Electrons (negative).

Neutron Role

Neutrons add mass to atoms and stabilize the nucleus.

Nanotechnology

The manipulation of individual atoms to form structures and machines on the atomic scale.

Atomic Number

The number of protons in an atom; defines the element.

Isotopes

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

Mass Number

The total number of protons and neutrons in an atom.

Atomic Mass

Weighted average mass of all the isotopes of an element.

Proton

Positively charged particle found in the nucleus.

Neutron

Neutral particle found in the nucleus.

Electron

Negatively charged particle found in the electron cloud.

Atomic Mass Unit (amu)

Unit used to measure atomic mass; 1 amu ≈ mass of 1 proton or neutron.

Ion

An atom that has gained or lost electrons.

p+, n0, e- symbols

p+ = proton, n0 = neutron, e- = electron.

Radioactive Decay

Spontaneous emission of radiation from unstable nuclei to gain stability.

Radiation

Rays and particles emitted from unstable nuclei.

Alpha Radiation

Positively charged particle with 2 protons and 2 neutrons; a helium nucleus.

Beta Radiation

Negatively charged particle; a high-speed electron emitted from the nucleus.

Gamma Radiation

Pure energy with no charge or mass; often emitted with alpha or beta particles.

Nuclear Reaction

A process in which an atom’s nucleus changes, transforming it into a different element.

Alpha Decay

Result: mass number decreases by 4 and atomic number decreases by 2.

Beta Decay

Result: neutron changes to a proton, increasing atomic number by 1.