Transcript Chapter 7 - TeacherWeb

Chapter 7
Section 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