objectives: understand the experimental design and
TRANSCRIPT
THE ATOMObjectives: Understand the
experimental design and conclusions
used in the development of modern
atomic theory, including Dalton’s
Postulates, Thomson’s discovery of
electron properties, Rutherford’s
nuclear atom, and Bohr’s nuclear
atom.
Roots of Atomic Theory
• Philosophy: the study of the
nature of knowledge.
• NO EXPERIMENTATION
• Democritus (460-370 B. C.)
• Believed that there was a
smallest piece of matter
possible and called it the
ATOM; which in Greek
means INDIVISIBLE.
“Science” Ignored
• Aristotle (384-322 B. C.)
• Disliked Democritus theory.
• One of the most influential
philosophers of his time.
• Was able to get Democritus theory
rejected.
• This rejection of science: the
existence of the atom went on for
2000 years.
• Thought matter was made of:
– Earth
– Water
– Air
– Fire
John Dalton (1766-1844)
• English School
Teacher
• True Science
• Formation of a
Hypothesis
• Experimentation
• Analysis of Results
• Begin Cycle AGAIN
A New System of Chemical
Philosophy (1808)
Dalton’s Atom Theory1. All matter is made of extremely small particles called
ATOMS.
2. Atoms are indivisible and indestructible.
3. Atoms of the same element are all identical in size mass and chemical properties.
4. Atoms of a specific element are different from other elements.
5. Compounds are formed by a combination of two or more different atoms and they always have the same proportion of elements. THE LAW OF DEFINITE COMPOSITION
6. A chemical reaction is a rearrangement of atoms and the atoms are neither created nor destroyed. THE LAW OF CONSERVATION OF MATTER
Since Dalton’s time experimentation has proven the existence of the atom.
ATOM: smallest particle of matter that RETAINS the properties of the element.
The ELECTRON
• Cathode-Ray Tube
• Glass tube that has most of the air removed by use of a vacuum pump.
• Sir William Crookes discovered THE Cathode-Ray: a green light in the tube.
• Was a stream of negatively charged particles.
J. J. Thomson (1856-1940)
Joseph John Thomson• English physicist who in 1897
used the cathode-ray tube to discovered a particle smaller than the atom; the electron.
• Particle has a negative charge and is much smaller than the atom so must come from the inside of the atom.
• Determined the mass-to-charge ratio of the electron.
• Electrons are scattered around the atom. The positive material is the matrix the negative electrons embedded. (THE PLUM PUDDING MODEL)
Robert Millikan (1868-1953)
• Oil Drop Experiment
• A fine mist of oil droplets is introduced into the chamber.
• The oil is ionized by x-rays.
• The electrons adhere to the oil drops.
• The value for the charge of the electron can be calculated.
• Since Thomson discovered the mass-to-charge ratio the mass could also be calculated.
The Nucleus• Ernest Rutherford (1871-1937)
• New Zealand born physicist; worked in England
• 1911 conducted the “Gold Foil Experiment” the proved the existence of a small positively charged center of the atom.
• Rest of atom is EMPTY SPACE.
• Disproved the “Plum Pudding Model”
• THE NUCLEAR MODEL
• Discovered the proton.
• Thought that the electrons orbited the nucleus like planets orbited the sun.
Rutherford’s Gold
Foil Experiment
Rutherford’s Gold
Foil Experiment
Subatomic Particles
• Objective: Define the characteristics of
the subatomic particles and be able to
“count” the number of protons, neutrons,
and electrons in any atom, ion, or isotope.
Subatomic Particles
symbol locationrelative
charge
relative
mass
actual
mass
(grams)
electron e- electron
cloud 1-1
1840
ZERO
9.11 x
10-28
proton p+ nucleus 1+ 11.673 x
10-24
neutron n0 nucleus 0 11.675 x
10-24
Symbols
Periodic Table Isotopic Symbol
SYMBOL
mass number ion charge
23 +1
Na p+ = 11
11 n0 = 12
atomic number e- = 10
name symbol atomic
number
mass
number
ion
charge
number
of
protons
number
of
neutrons
number
of
electrons
atomic
mass
calcium 20 42 +2 40.08
19 -1
F
9
10 10 10 20.18
238 0
U
92
Radioactivity
• Much of what was discovered about the subatomic
particles was because of radioactivity.
• Will study later in the year.
name symbol atomic
number
mass
number
ion
charge
number
of
protons
number
of
neutrons
number
of
electrons
atomic
mass
potassium 19 40 +1 39.098
amu
15 -2
O
8
18 22 18 39.948
amu
56 0
Fe
26
Objective
• Use isotopic composition to
calculate the average atomic
mass of an element.
Isotopes
Isotopes are atoms of the same element with different
numbers of neutrons.
Same Different
1. protons 1. neutrons
2. chemical properties 2. nuclear properties
3. element 3. radioactivity
Mass Number vs. Atomic Mass
• MASS NUMBER
• Is the sum of the protons and neutrons in the nucleus
• is given for an individual atom
• is given in nuclear symbols
• is always a whole number
• ATOMIC MASS
• is a WEIGHTED
AVERAGE for all the
isotopes of the element
• is the value that is given
on the periodic table
• If you round the average
atomic mass you will
have the MASS
NUMBER of the most
common isotope.
Calculate Atomic Mass• Average Atomic Mass =
• (mass isotope #1 x relative abundance #1)
PLUS
• (mass isotope #2 x relative abundance #2)
PLUS
• (mass isotope #3 x relative abundance #3)
• PLUS (4, 5, etc.)
• Example: YOUR GRADE
• = daily (.50) + exams (.30) + 9 weeks exam (.20)
Average Atomic Mass