Development of the
Atomic Theory
History of the Atom
• Democritus argued that atoms were the
smallest particles of matter.
• The atom comes from the Greek word atomos
that means “not able to be divided”.
John Dalton
• Dalton proposed the first atomic theory.
1) All matter is made of atoms. Atoms are
indivisible and indestructible.
2) All atoms of a given element are identical in mass
and properties
3) Compounds are formed by a combination of two
or more different kinds of atoms.
4) A chemical reaction is a rearrangement of
atoms.
J.J. Thomson
• Thomson discovered negatively charged
particles called electrons.
• Designed the “plum pudding” model.
Ernest Rutherford
• Rutherford discovered that atoms contain a
small, dense, positively charged center called
the nucleus.
Niels Bohr
• Bohr suggested that electrons (which have a
negative charge) moved around the nucleus at
certain fixed distances.
Electrons
Nucleus
Electrons
• According to the current atomic theory,
electrons are most likely to be found in the
electron cloud around the nucleus.
Electron Cloud
Dmitri Mendeleev
• Russian chemist
• Created the periodic table
Matter
What is Matter?
• Matter is anything that has mass and takes up
space.
• The basic building blocks of all matter are
atoms.
• An atom is the smallest particle into which an
element can be divided and still be the same
substance.
Molecules
• A particle made of two or more atoms bonded
together forms a molecule.
• For example, two hydrogen atoms join with
one oxygen atom to form a water molecule.
Elements
• An element is the simplest form of matter that
cannot be changed into another simpler form
by ordinary means.
• Examples of elements: Oxygen, Helium, Gold,
Silver….
Compounds
• A substance made of two or more elements
chemically combined form a compound.
• Example: NaCl, H2O, CO2
Parts of the Atom
• Proton: a positively (+) charged particle of the
nucleus.
• Neutron: a neutral particle of the nucleus.
• Electron: a negatively (-) charged particle
around the nucleus.
Electron (-)
Neutron (neutral)
Nucleus
(98% of the mass of
atoms)
Proton (+)
Electron (Shell) Cloud
(levels or orbitals)
• Atomic Mass Unit (amu): the SI unit for the
masses of particles in atoms.
• Atomic Number: the number of protons in the
nucleus of an atom.
• Mass Number/Atomic Mass: the number of
protons and neutrons in an atom.
• Atomic Mass: the weighted average of the
masses of all the naturally occurring isotopes
of an element.