Atoms and Moles
Chapter 4
• Approximately 2500 years ago
• Philosophers
• How many times can one divide
something until there is nothing left?
DEMOCRITUS
Eventually you would
have the smallest
possible piece of
matter….hmm?
The Atom
atom (n) Gr. Atomos
meaning
indivisible.
• Early Greek
philosopher who did
NOT believe that
there was a limit to
the number of times
matter could be
divided.
• Thought all matter
consisted of four
elements
Aristotle’s Four Basic
Elements
Law of Definite Proportions
• No matter the amount
of a compound, the
element proportions
are exactly the same
throughout.
• Joseph Proust (17541826) French chemist
Law of Conservation of Mass
• Matter can neither be
created nor destroyed.
• Antoine Lavoisier
(1743-1794)
• French Chemist
Law of Multiple Proportions
• The Law of Multiple Proportions is
the third postulate of Dalton's
atomic theory. It states that the
masses of one element which
combine with a fixed mass of the
second element are in a ratio of
whole numbers.
JOHN DALTON
• Early 1800s
English chemist
• Developed the
“Atomic Theory”
in 1803
The Atomic Theory
1) All matter is composed of atoms.
2) Atoms of the same element are exactly
alike
3) Atoms of different elements are different
4) Compounds are formed by the joining of
atoms of 2 or more elements
JJ ThOMSON
• 1897 English
scientist
• Proved atom could
be divided into
smaller parts
• Discovered small
negatively charged
particles he called
“corpuscles”
The Plum Pudding Model
ERNEST RUTHERFORD
• 1908 British
physicist
• Tested Thomson’s
model
• click to view
biography
Thomson vs Rutherford
Rutherford’s Gold Foil Experiment
What should happen?
The particles should pass
relatively easily and evenly
through the foil.
The Rutherford Model
• A small dense
positively charged
center called the
“nucleus”
• The electrons are
located somewhere
around the nucleus
NIELS BOHR
• 1913 Danish Scientist
• Said “electrons move
in definite orbits
around the nucleus”
• Often known as the
“solar system model”
• click to view
biography
Modern Atomic Model
• It is impossible to determine the exact
location of an electron
• Particles are explained according to wave
mechanics and quantum mechanics
• Max Planck (1858-1947) considered the
“father of quantum mechanics”
• click to view biography
Protons
• Sub-atomic particle that is
positively charged
• Mass of 1 amu (a small unit
used to measure the masses of
atoms)
• Found in the nucleus
Electrons
• Negative charge
• Very small 1/2000 the
mass of a proton
• In a neutral atom
number of electrons
same as number of
protons
• Moving very fast
Neutrons
• Sub-atomic particle that has
NO CHARGE. Neutral.
• Very similar in mass to
protons; slightly larger
• Found in the nucleus
Atomic Number
• Same as the number
of protons in an atom.
• Determines the
identity of the
element.
• Basis for the
arrangement of
periodic table order.
6
C
12.011
Isotopes
•
= atoms of the same
element that have the same
number of protons but
different number of neutrons
• The arrangement of nucleus determines the
isotope:
Mass Number
• =the sum of the PROTONS and
NEUTRONS in the nucleus of an atom
• Example: What is the mass number of a
nitrogen atom with 8 neutrons?
• Answer: 15
• MASS NUMBER = Atomic number + # of
neutrons
Atomic Mass
• The average
mass of all the
isotopes for a
particular
element.
• always a
decimal
6
C
12.011
Atomic mass of an element:
Electron Cloud
• PROBABLE space
around the nucleus in
which electrons are
most likely found.
• Similar to bees around
a beehive
Electron Cloud continued:
• Location of electron depends on how much
energy it possesses
• Arranged in energy levels
• Lowest energy level closest to nucleus
• Highest energy level furthest away
• Each energy level can only hold a specific
number of electrons.
• 1st level holds 2; 2nd and 3rd hold 8
Electron Configuration
• The arrangement of the electrons in an
orbital
• Ground state : when all of the
electrons in an atom have the lowest
possible energies.
Assignment