The Atom, Atoms, History of the Atom

Atomic Number

Equal to the number of protons

Mass Number

Equal to the number of protons + number of neutrons

Protons

Positively charged particles located in the nucleus, responsible for the identity of the atom

Neutrons

Neutral particles located in the nucleus, responsible for the stability of the nucleus

Electrons

Negatively charge particles located in the electron cloud, responsible for chemical properties

Nucleus

Located at the center of the atom, holds the protons and the neutrons

Isotope

Same number of protons, different number of neutrons

Valence Electrons

Electrons in the outermost energy level

Dalton

Atomic Theory

Atomic Number

Number of protons in an atom

Atomic Mass

total weight of protons and neutrons

atom

smallest unit of matter

nucleus of atom

the positively charged dense center of an atom

neutron

A subatomic particle that has no charge and that is found in the nucleus of an atom

proton

A subatomic particle that has a positive charge and that is found in the nucleus of an atom

electron

A subatomic particle that has a negative charge

Periodic Table

A chart of the elements showing the repeating pattern of their properties

Democritus

460 BC; There is a limit to how many times one can break matter into smaller pieces; eventually one reaches the smallest particle.

Aristotle

300 BC; All matter composed of fire, water, earth, and air. Founder of alchemy.

John Dalton

1803; Atomic Theory: all matter made of atoms; all atoms of an element are alike, but are different from other elements; elements can be combined to make compounds; chemical reactions are a rearrangement of elements, but do not change the elements themselves.

JJ Thomson

1897; finds the electron of an atom using a cathode ray tube; also said the electron must be balanced by positive charge; proposed the plum pudding model.

Ernest Rutherford

1911; performed the gold foil experiment; proved that most of the atom is empty space and the existence of the nucleus.

proton

A subatomic particle that has a positive charge and that is found in the nucleus of an atom

neutron

A subatomic particle that has no charge and that is found in the nucleus of an atom

nucleus

Center of an atom

electron

A subatomic particle that has a negative charge

james chadwick

1932-Discovered the neutron and that its mass is the same as the proton. This led to development of atomic energy.

Niels Bohr

Electrons should move around the nucleus but only in prescribed orbits