Chapter 4.1 (Introduction to Atoms) Study Guide

Democritus
440 B.C.: proposed that eventually you will end up with an uncuttable particle. He called this particle an atom (coming from the Greek “atomos” meaning uncuttable)

John Dalton
1700s: published his own atomic theory after many experiments; Atoms cannot be created, destroyed, or divided, Atoms of the same element are exactly alike, atoms of different elements are different, Atoms join with others to make new substances.

J.J. Thompson
1897: discovered small particles in the atom. Used a cathode-ray tube and electrically charged plates to realize that atoms have a negative charge. Called the small negatively charged particles in the atom corpuscles ELECTRONS). Made the plum pudding model as a theory as to the arrangement of the electrons.

Ernest Rutherford
1909: Designed an atomic shooting gallery to test Thompson’s theory. Shot atoms at gold foil, most went through, but some bounced back. Determined that atoms are mostly empty space and that there is a positive charge in the atom. Presented a new atomic model. Proposed that there is a positively charged nucleus at the center of the atom with electrons traveling through empty space around the nucleus.

Niels Bohr
1913: Danish scientist who proposed that there were paths that electrons traveled on. They were different distances from the nucleus. Electrons could jump from path to path but couldn’t hang out in the space between.

Electron clouds
The modern atomic theory states that electrons can be found in these. The exact path of a moving electron cannot be predicted, but they are likely to be in regions that are related to the atomic paths that Bohr suggested.

Atom
The smallest particle into which an element. can be divided and sill be the same substance. They have a diameter of 0.00000003 cm. They are considered to have a neutral charge.

Electrons
Negatively charged particles that are found in all atoms. Can be found in electron clouds outside the nucleus. Their mass is almost zero amu.

Model
A representation of an object or system. Presents a picture of what the theory explains.

Nucleus
rutherford infered, an extremely dense, positively charged reigon at the center of the atom. Composed of protins and neutrons.

Protons
Positively charged particles that are found in the nucleus of an atom. 1 amu. All protons are identical.

Neutrons
Uncharged particles found in the center of the atom. 1 amu. All neutrons are identical.

Atomic Mass Unit (amu)
The SI unit to measure the masses of particles in atoms.

Atomic Number
The number of protons found in the nucleus of the atom. NEUTRONS AND ELECTRONS DO NOT COUNT.

Isotope
An atom that has the same number of protons but a different number of neutrons.

Mass number
The sum of protons and neutrons of an atom. ELECTRONS DO NOT COUNT.

Atomic Mass
The weighted average of the masses of all the naturally occuring isotopes of an element.