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Smallest piece of matter that still retains the property of the element.


Protons and neutrons are found in a small positively charged center of the atom.


Particles with an electrical charge of 1+ (positively charged).


Neutral particles that do not have an electrical charge (no charge).


Particles with an electrical charge of 1- (negatively charged) and are in the cloud outside the nucleus of the atom.


Protons and neutrons are made up of these smaller particles.


Around 400 BC he proposed the idea that atoms make up all substances.

John Dalton

In the 1800's he was able to prove atoms exist.

Niels Bohr

In 1913 he hypothesized that electrons travel in fixed orbits around the atom's nucleus.

Electron Cloud

The area around the nucleus of an atom where its electrons are most likely found.

Atomic Number

Equals the number of protons in an atom.

Mass Number

Sum of the number of protons and neutrons in the nucleus of the atom.


Atoms of the same element that have different numbers of neutrons.

Dmitri Mendeleev

In the 1800's discovered a pattern by arranging the atoms according to the atomic mass.

Henry G.J. Moseley

In 1913 arranged the elements according to their increasing atomic number.

Periodic Table

The elements are arranged by increasing atomic number and by changes in physical and chemical properties.


Vertical Columns in the periodic table.

G.N. Lewis

Created the electron dot diagram.

Electron dot diagram

Uses the symbol of the element and dots to represent the electrons in the outer energy level.


The horizontal rows of elements on the periodic table.

Erwin Shrodinger

Developed the electron cloud model of an atom.

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