Chemistry Unit Notes
Almost everything you can think of is “matter” -- meaning it’s made of something that has a mass
and volume that can be measured. Fire and light are not matter, they’re energy. All matter is
made of atoms of elements. Elements are pure substances that are made of only one kind of atom.
Atoms have 2 basic regions: the nucleus and the electron cloud.
The nucleus is made up of protons and neutrons while
electrons constantly orbit around the nucleus in different
energy levels of the electron cloud.
Subatomic particles:
A proton carries a positive charge.
A neutron carries no charge.
An electron carries a negative charge.
A proton and a neutron have about the same mass (1 amu) but an electron’s mass is so small that it
contributes very little to the overall mass of the atom. Therefore, we generalize and say that the
mass of the atom is in its nucleus.
Every proton in the world is exactly like every other proton; every neutron is exactly like every
other neutron; every electron is exactly like every other electron. What makes the elements
different is not the KIND of particles that make them up but the NUMBER of them. An element
is identified by the number of protons its atoms contain, and they are arranged in order on the
Periodic Table of Elements:
There is a lot of information about an atom located on the period table:
The atomic number is also the number of protons.
Since we’re dealing with neutral atoms (with no charge),
the number of protons and electrons will be equal.
The atomic mass is the mass of the nucleus which contains
both protons and neutrons (each with a mass of 1.0) so to
calculate the number of neutrons you simply subtract the
atomic number from the atomic mass.
Example:
Atomic number: 5
Atomic mass (rounded): 11
Number of protons: 5 (the same as the atomic number)
Number of electrons: 5 (the same as the number of protons)
Number of neutrons: 6 (the atomic mass minus the atomic number)
So one atom of Boron looks like this:
Elements are identified based on the number of protons…..every atom of boron has 5 protons. If
it didn’t, it wouldn’t be boron. The number of neutrons, however, can vary, and this does not affect
the identity of the element but it does affect its mass. Perhaps some boron atoms contain 4
neutrons, some 5, some 6, some 8. Different atoms of the same element but with different
numbers of neutrons are called isotopes. Because the number of neutrons can vary, the atomic
mass on the periodic table is an average mass.
While the protons and neutrons are located together in the nucleus, the electrons orbit the
nucleus in specific energy levels or “shells.” Only 2 electrons can be in the first level, up to 8
electrons can be in the second and third level.
Some elements are more reactive than others, meaning they combine with other elements easily to
form new substances. This is mostly due to the number of electrons in the outermost shell of the
atom.
The elements are also arranged in the Periodic Table so that the left side is almost completely
metals, the few on the right are non-metals, and those along the diagonal line from Boron are
metalloids.
Properties of Metals
They tend to be shiny
They are ductile: they can be stretch and pulled into thin wires
They are malleable: they can be pounded into thin sheets
They’re usually good conductors: heat and energy can easily flow through them
They tend to react with oxygen and water to corrode (break down)
[Rust is how iron corrodes; the green color of the Statue of Liberty is due to the
corrosion of copper]
Properties of Nonmetals
They are dull
They are brittle (break easily)
The are insulators (heat and energy do not flow easily through them)
Metalloids have varying properties, sometimes like a metal and sometimes like a non-metal.
REALLY, REALLY SIMPLIFIED CHEMISTRY
Each element has its own characteristics. If you simply mix two elements together, they will
usually keep their own characteristics. In order for a new substance to form, a chemical reaction
has to take place where the atoms are actually bonded together.
For example, sodium is a soft, shiny metal that will sizzle and flame in water. Chlorine is a green,
poisonous gas. If you simply put some sodium (Na) in a bottle of chlorine (Cl), you will still have a
soft, shiny metal in some green, poisonous gas. However, if you chemically combine them in a
chemical reaction, they will form NaCl (table salt) which is white, solid, not explosive in water, and
necessary for your survival.
When two or more atoms combine, they make a molecule. Molecules are shown with a chemical
formula. If the atoms of the molecule are different, they form a compound.
Example:
When two oxygen atoms combine, they make oxygen gas, a molecule.
Chemical formula: O2
When three oxygen atoms combine, they make ozone, a molecule.
Chemical formula: O3
If two oxygen atoms combine with one carbon atom, they make a
molecule of carbon dioxide, which is also a compound
because the atoms are different.
CO2
Subscripts are small numbers written below and after an element to tell us how many atoms of
that element are present in the molecule.
Example:
There are two hydrogen atoms and one oxygen atom
in a molecule of water, so the “2” goes after the hydrogen.
When only one atom is present, no number is written.
H2O
The difference between carbon monoxide (CO) and carbon dioxide (CO2) is the number of oxygen
atoms in the molecule. And this makes a completely different compound with very different
properties.
carbon monoxide
carbon dioxide
= hydrogen peroxide H2O2 (very different from water)
There are seven elements that exist as diatomic molecules, meaning two atoms are bonded
together: H2, Br2, I2, N2, O2, Cl2, F2
When you have more than one molecule, a coefficient (large number) is placed in front of the
entire molecule.
For example, three molecules of water would be written: 3 H2O
In 3 molecules of water, there are 6 hydrogen atoms and 3 oxygen atoms.
New substances are formed in a chemical reaction and it is shown by writing a chemical equation.
In a chemical equation, the reactants are located on the left of the arrow and the products are
located on the right of the arrow. During a chemical reaction, bonds are broken between existing
molecules and the atoms are rearranged to form new substances.
For example, hydrogen and oxygen can combine to form water. But it is important to realize that
in order to make a new product you can only use the atoms you start with in the reactants.
Hydrogen and oxygen are both diatomic elements so they can only be written as H2 and O2. But if
we complete the reaction correctly using the same amount of atoms on both sides, we don’t end up
with water as the product. (Hydrogen peroxide is a different product.)
H2
+
O2

H2O2
To make water with the correct ratio of two hydrogen atoms to every one oxygen atom, we would
have to balance the chemical equation so that the same number of each atom is on both sides of
the arrow.
2 H2
4 hydrogen
atoms
+
O2
2 oxygen
atoms

2 H2O
4 hydrogen atoms
2 oxygen atoms
The Law of Conservation of Mass states that the atoms that go into a reaction must be the same
as those that come out, even if they are rearranged differently. You cannot create atoms or
destroy atoms in a chemical reaction. This is why to show the proper reaction, the equation must
be balanced.
Substances that are combined physically but not chemically are called mixtures.
Types of Mixtures
Homogenous mixture -- the substances are mixed so well that it looks the same throughout (you
can’t see the individual substances in it). This is also called a solution. In a solution, the solute
(usually a solid) is dissolved in the solvent (usually a liquid).
Examples: saltwater, blood, coffee, wine, orange juice (without pulp), Kool-Aid, shampoo
When making a solution, solutes dissolve faster with
1) higher temperature
2) stirring
Heterogeneous mixture -- the substances are NOT mixed well and you can often see the
individual substances that make it up. These also tend to be easier to separate.
Examples: soil, Italian salad dressing, orange juice (with pulp), cider, chicken soup
Physical vs. Chemical changes
Physical changes are those that may change the way a substance looks but the substance is still
the same (same chemical formula).
Examples of physical changes:
Breaking or sharpening a pencil
Cracking an egg
Freezing water*
Pounding bread dough
Bending steel
*It’s important to note that phase changes are physical changes: if you boil water, you are
simply changing liquid to gas but it’s still H2O
Chemical changes are those that change the actual chemical makeup of the substance (a new
chemical formula) and can only occur if a chemical reaction takes place.
Examples of chemical changes:
Burning a pencil
Cooking an egg
Baking bread dough
Milk souring
Steel rusting