Chapter 7Section 1- Discovering the parts of the atom

Early Ideas about matter

•Greek Philosophers•Aristotle (384-322 BC)•Matter made up of four Elements

▫Fire, air, water, earth•Democritus ( 460-370 BC)

▫Matter made up of small, solid objects that cannot

▫Be created, destroyed or divided▫Called them atomos (English- atoms)

Democritus

•He had no way to test his ideas•His ideas did not conform to popular

belief•Therefore, he was not believed•Aristotle was believed

John Dalton•English school teacher and scientist- late 1700’s•Revised the idea of the atom•Proposed the atomic theory

▫All matter is made up of atoms that cannot be divided, created, or destroyed

▫During chemical reaction, atoms of one element cannot be converted into elements of another element

▫Atoms combine in specific ratios▫Atoms of one element are identical to each other

but different from atoms of another element

Current theory

•Matter is made of atoms with empty space between and within them

•An atom is the smallest piece of an element that still represents the atom

•Atoms are very small▫7.5 trillion carbon atoms could fit into a

period at the end of a sentence▫Can be viewed with Scanning Tunneling

microscope (STM)▫First observed in 1981

J.J. Thomson

•English Scientist ( 1856-1940)•Studied cathode tubes•In his experiment- placed two plates on

opposite sides of the cathode ray tube•One plate was + charged while the other

was – charged.•The rays bent towards the + charged

plate•He concluded the cathode ray tube was –

charged.

Thomson continued

•He conducted further experiments and found the particles had mass

•He proposed that these negatively charged particles came from the metal atoms in the electrode

•He called these particles electrons▫A particle with a – 1 charge▫He also proposed that atoms must contain

+ charged particles

Thomson cont.

•His model contained both positive and negative charges.

•An atom was a sphere with positive charges spread throughout

•Negative charges were mixed with the positive charges

•Like chocolate chips in cookie dough

Rutherford

•Ernest Rutherford ( 1871- 1937)•Student of Thomson•“Famous Gold Foil Experiment”•He shot alpha particles which dense and

positively charged at gold foil•Most of particles bounced off the gold foil•1 In 10,000 hit the foil and bounced

straight back

Rutherford’s Atomic Model

•Since most of the alpha particles went straight through then the nucleus is mostly empty space

•Those that bounced and did not go in a straight direction hit something positive

•His Model says▫Most of the mass of an atom and the

positive charge is in the center of an atom called the nucleus

▫Protons are positively charged particles

Discovering Neutrons•James Chadwick ( 1891-1974)•He determined that the nucleus also

contained neutrons which are particles with no charge.

Bohr Model

•Niels Bohr ( 1885-1962)•There were limitations of Rutherford’s

model•He could not explain what the electrons

were doing•Bohr proposed that they move in circular

levels or orbits around the nucleus

Modern Atomic Model

•Electrons form an electron cloud around the nucleus

•An electron cloud is an area around an atomic nucleus where an electron is most likely to be located

Quarks•Protons and neutrons are made of smaller

particles known as quarks

•6 types of quarks•Up, down, charm, strange, top, bottom