law of conservation of mass
In a chemical reaction, matter is neither created nor destroyed. (The total mass of the substances involved in the reaction does not change) (Antoine Lavoisier 1743-1794)
law of definite proportions
All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements. (Joseph Proust 1754-1826)
law of multiple proportions
When two elements (A and B) form two different compounds, the masses of element B that combine with 1 g of element A can be expressed as a ratio of small whole numbers.
The theory that each element is composed of tiny indestructible particles called atoms, that all atoms of a given element have the same mass and other properties, and that atoms combine in simple, whole-number ratios to form compounds. (John Dalton 1766-1844, explained in 1808)
A stream of electrons produced when a high electrical voltage is applied between two electrodes within a partially evacuated tube. (J.J. Thomson 1856-1940, experiments conducted in late 1800s)
cathode ray tube
A glass tube from which the air has been partially evacuated.
Fundamental property of some of certain particles that causes them to experience a force in the presence of electric fields.
Negatively charged, lost mass particle present within all atoms. (Discovered by J.J. Thomson 1856-1940)
The emission of small energetic particles from the core of certain unstable atoms. (Discovered at the end of 19th century by Henri Becquerel 1852-1908 and Marie Curie 1867-1934)
The theory that most of the atom's mass and all of its positive charge is contained in a small, dense nucleus. Most of the volume of the atom is empty space, throughout which tiny electrons are dispersed. There are as many electrons that are outside the nucleus as there are protons within the nucleus , so that the atom is electrically neutral. (Ernest Rutherford 1871-1937)
The very small, dense core of the atom that contains most of the atom's mass and all of its positive charge; it is composed of protons and neutrons.
Positively charged subatomic particle found in the nucleus of an atom.
Electrically neutral subatomic particle found in the nucleus of an atom, with a mass almost equal to that of a proton.
atomic mass unit (amu)
A unit used to express the masses of atoms and subatomic particles, as defined as 1/12 the mass of a carbon atom containing six protons and six neutrons.
atomic number (Z)
The number of protons in an atom; the atomic number defines the element.
A one or two letter abbreviation that is listed directly below its atomic number on the periodic table.
atoms with the same number of protons but different number of neutrons.
The relative percentage of a particular isotope in a naturally occurring sample with respect to other isotopes of the same element.
mass number (A)
The sum of the number of protons and neutrons in an atom.
An atom of molecule with a net charge caused by the loss or gain of electrons.
Positively charged ion.
Negatively charged ion.
When the elements are arranged in order of increasing mass, certain sets of properties recur periodically. (Dmitri Mendeleev 1834-1907, discovered in 1869)
A large class of elements that are generally good conductors of heat and electricity, malleable, ductile, lustrous and tend to lose electrons during chemical changes.
A class of elements that tend to be poor conductors of heat and electricity and usually gain electrons during chemical reactions.
A category of elements found on the boundary between metals and nonmetals of the periodic table, with properties intermediate between those of both groups; also called semimetals.
A material with intermediate electrical conductivity that can be changed and controlled.
Those elements found in the s or p blocks of the periodic table, whose properties tend to be predictable based on their position in the table.
transition elements (transition metals)
Those elements found in the d block of the periodic table whose properties tend to be less predictable based simply on their position in the table.
Columns within the main group elements in the periodic table that contain elements that exhibit similar chemical properties.
The group 8A elements, which are largely unreactive (inert) due to their stable filled p orbitals.
Highly reactive metals in group 1A of the periodic table.
alkaline earth metals
Fairly reactive metals in group 2A of the periodic table.
Highly reactive nonmetals in group 7A of the periodic table.
atomic mass (atomic weight)
The average mass in amu of the atoms of a particular element based on the relative abundance of the various isotopes; it is numerically equivalent to the mass in grams of one mole of the element.
A unit defined as the amount of material containing 6.0221421 x 10^23 (Avogadro's number) particles.
The number of ^12C atoms in exactly 12 g of ^12C; equal to 6.0221421 x 10^23.
The mass in grams of one mole of atoms of an element; numerically equivalent to the atomic mass of the element in amu.