# Atomic Theory

## Vocabulary

### alpha particle

particle identical to a helium ion (He2+) that is emitted during the decay of radioactive elements

### angstrom (Å)

unit of length equal to 10–10 meter

### anion

atom or group of atoms that has more electrons than protons, giving it an overall negative charge

### atom

smallest particle of an element that has the properties of that element

### atomic mass unit (amu)

mass of a single proton or neutron, equal to 1 gram per mole (g/mol), defined as one-twelfth the mass of a carbon-12 atom

### atomic number (Z)

number of protons in each atom of an element

### atomic weight

average mass of all the isotopes of an element, based on the relative abundance of each isotope

distance between the nucleus and the electron in a hydrogen atom

### cathode ray

beam of electrons emitted from a negatively charged conducting plate in a vacuum chamber containing very little gas

### cation

atom or group of atoms that has more protons than electrons, giving it an overall positive charge

### chemical symbol

letter or letters used to represent an element

### electron

negatively charged subatomic particle

### electron cloud

arrangement of electrons moving around an atomic nucleus based on probabilities of their locations

### fundamental unit of charge (e)

charge of an electron, $1.6021766\times{10}^{-19}\;\rm{C}$ (coulomb)

### isotope

one of two or more atoms of an element that have the same number of protons but different numbers of neutrons

### law of conservation of mass

law that states that matter is neither created nor destroyed during a chemical reaction

### law of definite proportions

law that states that compounds have the same proportions of masses of their constituent elements regardless of how small they are broken down

### law of multiple proportions

law that states that when two elements combine to form more than one compound, the mass of one element combines with a fixed mass of the other in a ratio of small whole numbers

### mass number (A)

sum of the number of protons and neutrons in a nucleus

### neutron

subatomic particle that has a neutral charge in the nucleus of an atom

### nucleus

positively charged center of an atom containing protons and neutrons

### orbital

region in which an electron has a high probability of being located. Orbitals are described by the quantum numbers s, p, d, and f, which differ from one another by their shapes.

### proton

positively charged subatomic particle in the nucleus of an atom