Combinations of Atoms
• There are 92 naturally-occurring
elements in the universe.
• Like letters of the alphabet, different
atoms of different elements combine, or
bond, to form different types of matter.
• Compounds are made up of two or more
different elements bonded together.
• Compounds are different than the atoms
they are made of.
Chemical Properties
• Depending on the elements in a compound,
a compound may react with other
compounds.
• Iron will rust when exposed to oxygen and
water, but it won’t if it is melted and mixed
with chromium and nickel metals to make
stainless steel.
Bonds
Chemical Bonds
• Chemical bonds are forces that hold the
atoms together in compounds.
• Bonds typically form when atoms share or
transfer electrons.
Electron Energy Levels
a) Atomic number = number of protons in the
nucleus. Since protons are positive in charge,
the nucleus attracts an equal number of
negative electrons.
b) Electrons vary in the amount of energy they
possess, and they occur at certain energy levels.
c) Electron energy levels determine how an atom
behaves when it encounters other atoms.
Electrons are placed in energy levels
according to rules:
1) The 1st energy level can hold up to two electrons.
2) The second energy level can hold up to eight.
3) After that some more complicated rules apply.
You will learn these rules later.
4) The outermost energy level always attempts to
stabilize with eight electrons. It does this by
sharing or transferring electrons.
5) The electrons in the outermost energy level are
called “valence electrons.”
Octet Rule = atoms tend to gain, lose or share
electrons so as to have 8 electrons in the last level.
• C would like to gain 4 electrons
• N would like to gain 3 electrons
• O would like to gain 2 electrons
Why are electrons important?
• Elements have different electron
configurations.
– A configuration is just an arrangement.
– Different electron configurations mean
different levels of bonding.
– A carbon atom looks like this:
An Atom of Oxygen
• Chemical bonds happen when an atom
attempts to fill its outside electron energy
level.
• There are four main types of bonds:
1.
2.
3.
4.
Ionic bonds
Covalent bonds
Metallic bonds
Hydrogen bonds
Ionic Bonds
• An IONIC BOND is formed between two or more
atoms by the transfer of electrons.
• Ionic bonds happen between a metal and a
nonmetal.
• A good example is between sodium (Na) and
chlorine (Cl).
• Sodium is a metal, chlorine is a nonmetal.
• When sodium gives its
electron to chlorine, it
loses a negative and
has one more positive
proton than electrons.
• Chlorine gains the
electron and has an
extra negative charge.
• This makes sodium a
positive ion and
chlorine a negative
ion.
More on Ionic
Bonds
Covalent Bond
• A COVALENT BOND forms by the sharing of
electrons in the outermost energy level of two
or more atoms.
• Covalent bonds are between two or more
nonmetals.
Metallic Bond
• A METALLIC BOND is between atoms of
metals.
• A metallic bond holds metal atoms together
very strongly.
• Electrons flow around many nuclei of atoms,
acting like sheepdogs keeping sheep together.
Metallic Bond, A Sea of Electrons
Hydrogen Bonds
• Hydrogen bonds do not involve the sharing
or transfer (giving) of electrons.
• Hydrogen bonds occur because of unequal
sharing of electrons.
• Water molecules have “polar” bonds because
of this.
How water attracts other water
molecules because of polarity
Hydrogen Bonding in Water
http://www.wwnorton.com/college/chemistry/
gilbert2/tutorials/chapter_10/water_h_bond/
Hydrogen Bonding in Ice
• http://beyondpenguins.ehe.osu.edu/teaching
-about-snowflakes-a-flurry-of-ideas-forscience-and-math-integration
Mixtures
• A mixture is two or more elements or
compounds put together, but not
bonded.
– Heterogeneous
– Homogeneous
– Solution
Heterogeneous Mixtures
• Hetero means “different.”
• Heterogeneous mixtures are not mixed
evenly and can be fairly easily separated.
• The different parts can be seen.
• Examples
– Vegetable stew
– A bowl of Halloween candy
– A group of people
Homogeneous Mixtures
• Homo means “same.”
• Homogeneous mixtures are mixed
evenly throughout.
• The different parts cannot be easily told
apart from one another.
• Examples
– Salt water
– Air
Solutions
• Solutions are
homogenous mixtures.
• They are often liquids.
• Alloys of metals are
also considered
solutions, even though
they are solids.
• Alloys are different
metals mixed, not
bonded, together
when melted.
Separating Mixtures
• Mixtures can be separated by sifting,
evaporating, or by hand.
• Mixtures are between elements or
compounds.
• Examples
– Sugar and salt (C6H12O6 mixed with NaCl)
– Salt and pepper
• These methods are physical.
Separating Compounds
• Compounds must be separated by
breaking bonds between atoms.
• This method is chemical.
• Examples
– Fire
– Digestion
– Breathing/respiration
– Photosynthesis