Atoms and
Molecules
Atoms
 Atoms
are the smallest whole particles of
an element.
 Atoms are made of protons (+) and
neutrons (0) in the nucleus and electrons
(-) buzzing around the nucleus.
 Atoms are mostly empty space but the
electrons move so fast atoms act like a
solid, like the blades of a fan when it’s on.
Atomic Theory
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The theories about atoms have changed a lot
since Democritus in 400 B.C. He said atoms of a
substance have the properties of that substance.
Like cheese particles that are squishy and yummy.
In 1803 A.D. John Dalton developed what we now
call atomic theory. That all matter is made of
atoms, atoms cannot be created or destroyed in
a chemical reaction, all atoms of a certain
element are exactly the same and compounds
are made from multiple atoms of different
elements.
What caused the theory of atomic structure to
change?
Atomic Theory
 Thomson
used a vacuum tube or
cathode ray tube, to discover electrons
that he called corpuscles. He said that
electrons are distributed in a positive mass
like blueberries in a muffin. This was called
the plum pudding model.
Atomic Theory
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A few years later Ernest Rutherford did the
gold foil experiment.
In this experiment he shot positive alpha
particles at a thin sheet of gold atoms. He
expected that they would all just pass through
in straight line if Thomson was right. What he
discovered was that some particles bounced
back or in other directions. That must mean
there is a positive nucleus that is tightly
packed in the center of an atom.
Atomic Theory
 Neils
Bohr worked with Rutherford and
developed the explanation that electrons
travel in rings like the layer of an onion
and electrons in these rings have different
energy levels.
 Several years later the cloud model of the
atoms was developed using the newly
discovered principles of quantum physics.
This states that electrons can be in one
place or the other but never in between
Atomic Theory
 In
1932 James Chadwick discovered
another particle in the nucleus called the
neutron (it was called neutron because it
is neutral, meaning it has no electrical
charge).
 What is the name of the model we now
consider to be closest to the truth about
how atoms look?
Bonding
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As we’ve discovered by building and presenting
our atoms, many atoms do not have full outside
rings. Usually it is missing a few electrons out of the
possible electrons on the outside ring.
These lonely electrons on the outside ring are
called valence electrons.
For example Carbon has 6 electrons total, so there
are 2 on the inside ring and 4 on the second ring.
Those 4 electrons are the valence electrons.
What are valence electrons?
Bonding
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Sometimes when atoms bond together they
give up or gain these electrons and sort of
pretend that they have what they need to be
stable when they do that. (Atoms don’t think
about this and wonder who to give electrons
to like a person would).
Once an atom is bonded to another atom it is
called a molecule. When the atoms that are
bonded together are different types of atoms
that molecule is referred to as a compound.
Bonding
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Atoms give up an electron (or more if
needed) when they have 3 or less electrons.
When an atom gives up electrons it gains a
positive charge because it now has more
protons than it does electrons.
When an atoms has 5 or more electrons it
usually gains electrons from other atoms and
becomes negative because it now has more
negative electrons than it does positive
protons
Bonding
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When an atom has a negative or positive
charge it is called an ion. If an atom has to
give up electrons to be stable it is a positive
ion, when it gains electrons to be stable it is a
negative ion.
Usually negative and positive ions come
together to make neutral molecules and
compounds. But occasionally groups of
atoms come together and still have a
negative or positive charge (these are called
polyatomic ions). Poly means many.
When is an atom called an ion?
Bonding
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Polyatomic ions combine with other ions or
polyatomic ions to became stable
compounds.
When atoms in the form of an ion give up
electrons to become molecules and
compounds the bond they have is called an
ionic bond because the atoms have become
ions in order to form a bond to make
molecules and compounds.
What is a polyatomic ion?
Chemical Formulas
 Chemical
formulas tell us what types of
atoms and how many of each atom is
involved in a molecule or compound.
 For example MgCl2 has only 1 Mg
(magnesium) and 2 Cl (chlorine). This is
known because the Mg has no number
after it which means there’s only 1 and Cl
has a #2 after it which tells us there’s 2
Chlorine atoms.
Chemical Formulas
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So how many carbon atoms are in C6H12O6?
How many oxygen atoms are in H2O?
When you hear the name of an ionic
compound such as sodium chloride (table
salt) you can know that the sodium gave up
electrons because the name of the positive
ion comes first and the negative ion (the one
that gained electrons) comes second.
What would you call MgCl2 that we talked
about earlier?
Properties of Compounds
 Ionic
compounds usually form crystals like
salt and they usually have a very high
melting points.
 When ionic compounds dissolve in water
it causes the water to conduct electricity.
Because water itself is a stable molecule,
without ions water does not conduct
electricity.
Covalent Bonds
 These
bonds are formed when atoms
share electrons and neither become
positive or negative atoms.
 These bonds are usually between non
metals like oxygen, nitrogen, phosphorus
etc.
 In covalent bonds no electrons are given
away or gained so the atoms stay neutral.
Covalent Bonds
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How are covalent bonds different from ionic
bonds?
For every electron atoms share there is one
bond.
For example hydrogen’s outside ring can only
hold two but actually only has one. This
means for hydrogen’s outside ring to be full it
needs to share one electron with another
atom. This means hydrogen can only form
one bond.
Covalent Bonds
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While oxygen has 6 valence electrons on its
outside ring it needs 2 more electrons to be
stable.
Why do you think water is H2O instead of HO?
Some atoms can share so many electrons
that they form double or triple bonds with the
element it is sharing with. The only atoms that
can form quadruple bonds are the atoms in
the same column as Carbon. This doesn’t
happen that often.
Molecular Compounds
 Molecular
Compounds form when
molecules are bonded together with
covalent bonds instead of ionic bonds.
 Molecular compounds usually have lower
melting and boiling points and won’t
conduct electricity if dissolved in water.
 What are two differences between Ionic
Compounds and Molecular Compounds?
Unequal Sharing
 Sometimes
in a molecular compound one
atom pulls harder on the electrons than
the other. This give the atoms a SLIGHT
electrical charge, but not like the larger
charge of an ion.
 When one atom pulls harder on the
shared electrons it makes a polar
molecule. This is then called a polar
bond.
Unequal Sharing
 Polar
molecules often have an attraction
to the other polar molecules near it. So
the oxygen end of one water molecule is
attracted to the hydrogen end of the
water molecule next to it even though
each molecule has its own hydrogen or
oxygen atoms.
 What unique property of water do you
think this attraction causes? Think back to
the penny lab and the soap.