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Is the smallest particle of an element tht retains its identity in a chemical reaction.

Atomic mass

An element is a weighted average mass of the atoms in a naturally occurring sample of the element.

Atomic mass unit (amu)

Is defined as one twelfth of the mass of a carbon-12 atom.

Atomic number

It is the number of nucleus of protons in the nucleus of an atom of the element

Cathode ray

A stream of electrons produced at the negative electrode of a tube containing a gas at low pressure.

Dalton's atomic theory

The 1st theory to relate chemical changes to events at the atomic level.


A negative charged subatomic particle.


A vertical colum of elements in the periodic table; the constituent elements of a group have similar chemical and physical properties.


Atoms that have the same number of protons but different numbers of neutrons.

Mass number

The total number of protons and neutrons in an atom.


Subatomic particles with no charge but with a mass nearly equal to that of a proton.


The tiny central core of an atom and is composed of protons and neutrons.


Each horizontal row of the periodic table.

Periodic table

Is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties.


A positively charged subatomic particle found in the nucleis of an atom.

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