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Chemistry
• What is chemistry?
– Chemistry is the study of matter, what it is
made of, and how it changes.
• What is matter?
– Matter is anything that…
• Has mass.
• Takes up space.
What Is Matter?
• Pretty much everything
– Element
• A pure substance that cannot be broken down to anything
simpler by chemical or physical means.
– Compound
• A pure substance that cannot be broken down to anything
simpler by physical means.
– Mixture
• Not a pure substance. Can be broken down by physical means.
What Is Matter?
• Is there a smallest particle of matter?
– Philosophers and scientists have pondered this
question since about 400 B.C.
• Our current theory:
– An atom is the smallest particle of an element
that retains all the properties of the element.
– History of the modern atomic theory.
The History of the Atom
How I Have Been Misunderstood
Who First Thought of Me?
• Democritus
• 400 BC
• Greek Philosopher
credited with coming up
with the idea of a single
indivisible atom that
was the smallest particle
for all matter.
Have I been famous since then?
• No
• Why not?
• Nobody believed
Democritus. He could
not prove his idea.
• Who didn’t believe
him?
• Other famous Greek
dead dudes like…
History of the Atomic Theory
• By 1790, more accurate measurement lead
to three major laws of nature:
– Law of Conservation of Mass
– Law of Definite Proportions (Constant
Composition)-mass ratio
– Law of Multiple Proportions (small whole
numbers)
So what happened?
• The idea of the atom
wallowed in obscurity
for about 2000 years –
not completely
forgotten, but not
believed, until…1808
John Dalton’s Atomic Theory
• 1808
– All matter is composed of small particles called
atoms.
– Atoms of a given element are identical in size,
mass, and properties; atoms of different
elements are different in size, mass, properties.
– Atoms cannot be subdivided, created, or
destroyed (law of conservation of mass).
John Dalton’s Atomic Theory
– Atoms of different elements combine in simple
whole number ratios to form chemical
compounds (Law of definite proportions, Law
of multiple proportions).
– In chemical reactions, atoms are combined,
separated, or rearranged (law of conservation of
mass).
Main Parts of Daltons Theory
• All things are made of
atoms.
• All atoms of the same
element have the same
size, mass, properties.
• Atoms can’t be created
subdivided or destroyed.
• Atoms can combine or
rearrange to form
compounds.
Modern Atomic Theory
• All things are made of
atoms.
• All atoms of the same
element have the same
size, mass, properties.
• Atoms can’t be created
subdivided or destroyed.
• Atoms can combine or
rearrange to form
compounds.
How did we find out about the
parts?
• JJ Thomson - 1897
discovered
• ELECTRONS
• Plum pudding model
Did we keep that model?
• No
• Ernie Rutherford –
1911 gold foil
experiment discovered
• NUCLEUS
Why don’t the electrons get
sucked into the nucleus?
• Electrons have their own energy –
Neils Bohr
• Only can absorb or give off energy in
specific amounts called quanta
Parts of the Atom
• Information on the Periodic Table tells you
about the number of each of the different
parts of any given element.
---------------------
Atomic Number
• The atomic number is the number of
protons in a an atom.
– This determines what the element is. More or
less protons would change it into a different
completely element.
– The number of protons = the total of the
positive charges for an atom – BUT
• An Atom is neutral, so there can’t be an
overall positive or negative charge.
– Therefore, the number of protons also tells
you …
…the total of the negative charges for the atom
.
Atomic Mass
• For a single atom:
– Atomic mass = # of neutrons + # of protons in the atom.
• For a bunch of atoms = average mass of the atoms
(protons and neutrons are the parts with mass).
• Recall that atomic number tells you the number of
protons. Use Atomic Number and Atomic Mass
together to determine…
… the total number of neutrons in the atom.