Development of the Atomic Theory

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ATOMIC MODELS• In ancient Greece Democritus

argued:

– Atoms were the smallest particles of matter.

– “Atom”: (from Greek atomos, meaning “not able to be divided”

Toga!Toga!

4th Century

B.C.

John Dalton’s model:

• Matter is made up of atoms

• Atoms cannot be divided into smaller pieces

• All atoms of an element are exactly alike

• Different elements are made of different atoms

**An atom was a hard sphere that was the same throughout

1803

FIRST ATOMIC THEORY:

William Crookes’ experiment • Cathode ray tube• Electrode = metal that

conducts electricity• Anode (+) Cathode (-)• Battery connected,

greenish glow appeared in tube

• Shadow of cross fell at other end of tube

• Proof that something was traveling in a straight line through the tube and was deflected by the cross

JJ Thomson Model

• Discovered electrons!!• Thomson thought the

electrons were evenly mixed throughout the atom

The CRT particles were negatively charged.These particles – electrons – are part of every kind of atom

1897

Ernest Rutherford• Rutherford discovered that atoms contain a

small, dense, positively charged center called the nucleus.

1911

Neils Bohr: Improvements on Rutherford’s model

• 1920 – protons were identified (Rutherford)

• 1932 - neutrons were discovered because of inconsistencies in the mass of atoms (Chadwick)

Electrons

+

Neutrons

Protons

1913 Electrons move around nucleus in

different energy levels. Valence electrons are in outermost

level. They give an atom its chemical properties/reactivity

Niels Bohr

Electrons

Nucleus

Electrons

Bohr suggested that electrons (which have a negative charge) moved around the nucleus at certain fixed distances.Defined energy levels where electrons will likely orbit the nucleus.

Current Atomic Theory• According to the

current atomic theory, electrons are most likely to be found in the electron cloud around the nucleus.

Bohr’s Model!!!!!!!!!!!!!!!

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.

Parts of the Atom• Proton: a positively (+) charged particle of the

nucleus.• Neutron: a neutral particle of the nucleus.• Electron: a negatively (-) charged particle of

the nucleus.

Electron (-)

Neutron (neutral) (no charge)

Proton (+)

Nucleus(98% of themass of atom)

Electron Cloud (levels/orbits)

• 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. (P+)• Mass Number: number of protons and neutrons in an atom. (P+ + No)• Atomic Mass: the weighted average of the masses of all the naturally occurring isotopes of an element.

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

What are atoms made of?• Protons

– Found in nucleus– Positive charge (+)

• Neutrons– Found in nucleus– NO charge (0) –

neutral

• Electrons– Orbits around the

nucleus– Negative charge (-)

P+No

2e-

8e-

8e-

18e-32 e-

8 (18) e-

8 e-

2 e-