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)

Proton (+)

Nucleus(98% of the mass of atoms)

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.