Lesson 1: Electrons and Bonding
Elements in the periodic table are arranged in order of increasing atomic number. The number of
valence electrons also increases as you go across the rows. All elements in a column have the same
number of valence electrons. You already know when the lowest energy level becomes full; a new level
of higher energy begins to fill. The electrons in the full shells are known as inner electrons. Electrons in
the partially filled outer shell are known as valence electrons. It is the valence electrons that are
involved in bonding with atoms because they are the outermost regions of an atom.
A Lewis structure consists of the atomic symbol of an element surrounded by its valence electrons,
arranged in pairs. Inner electrons, protons, and neutrons are not shown.
Lesson 2: Ionic and Covalent Bonds
An atom can form bonds in different ways: it can lose electrons, gain electrons, or share electrons with
another atom. It is very rare for an atom to exist individually. Most substances you can think of are
made up of combinations of atoms that are held together by chemical bonds. During the formation of a
chemical bond, valence electrons rearrange themselves into a more stable configuration.
Most chemical bonds can be described as either ionic bonds or covalent bonds.
Lesson 3: Chemical Formulas
Chemical formulas have two parts:
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The symbol of each element in a molecule of the substance.
A number indicating how many atoms of each element are in each molecule of the substance.
The same principle applies to molecules of compounds, which contain atoms of more than one element.
For example, the compound carbon dioxide has the chemical formula CO2. We breathe out CO2 when we
exhale. The chemical formula for carbon dioxide shows that carbon dioxide contains atoms of carbon (C)
and oxygen (O).
The subscript 2 on the O shows that there are two atoms of oxygen in each molecule of carbon dioxide.
There is only one atom of carbon in each molecule of carbon dioxide. We do not write the number 1 as a
subscript in chemical formulas. If an element has no subscript, assume there is one atom of the element
represented.
The name "carbon dioxide" also gives you information about the compound. In covalently bonded
compounds made up of two elements, we use prefixes to indicate the number of atoms of each element
in a molecule of the compound. The di– prefix in carbon dioxide tells you that there are two atoms of
oxygen per carbon dioxide molecule. Compounds made up of atoms that are bonded ionically do not use
prefixes. For example, the compound calcium chloride (CaCl 2) contains ionic bonds between calcium and
chlorine atoms. You do not need to use the prefix di– to indicate that there are two atoms of chlorine in a
molecule of calcium chloride.
Lesson 4: Hydrogen Hydroxide
One way to write the formula for Hydrogen Hydroxide is HOH. You can also write it as H2O. A molecule
made of two atoms of hydrogen bonded to one atom of oxygen can be called hydrogen hydroxide,
dihydrogen monoxide, hydronium hydroxide, or even hydric acid.
A water molecule contains covalent bonds between the two hydrogen atoms and the oxygen atom. Recall
that in a covalent bond the valence electrons are shared by both atoms in the bond.
Some atoms that form covalent bonds share the valence electrons equally. In the case of a water
molecule, this is not true.
The valence electrons spend more time near the oxygen atom than they do near the hydrogen atoms
because of this attraction.
Since electrons are negatively charged particles, the shared pairs of electrons in a water molecule give
the oxygen portion of the molecule a slight, or partial, negative charge. The oxygen atom does not have a
full negative charge of −1. It has a negative charge, but that negative charge is very small — a partial
negative charge. Each hydrogen atom in a water molecule has a partial positive charge. The overall
molecule has no total charge.
The bonds between oxygen and hydrogen in water are called polar covalent bonds. In a polar covalent
bond, the electrons spend more time near one of the atoms than the other.
Polar covalent bonds most commonly form between atoms of elements that are far apart on the periodic
table.
Notice that all of the positive partial charge is concentrated at one "side" of the molecule.
All of the positive charge is concentrated on one side of the water molecule, and the negative charge is
concentrated at the other. Therefore, the water molecule is said to be a polar molecule. Each water
molecule acts like a tiny, electrically charged particle.
The positive "side" of the water molecule can attract negatively charged particles. The negative "side" can
attract positively charged particles.
A covalent bond in which the electrons are shared equally between the atoms is called a nonpolar
covalent bond. Nonpolar covalent bonds typically form between atoms of the same element.
It is important to realize that most bonds are not completely polar or completely nonpolar. Some bonds lie
somewhere in between.
Water has some unusual properties compared to many other substances. Many of these properties are
vital to life as we know it. Many of water’s properties are the result of the partial charges on the water
molecule and the hydrogen bonds that form between the positive end of one water molecule and the
negative end of another water molecule. When most liquids freeze, the molecules pack together and
move closer. Water is an exception. When water molecules freeze, they actually move farther apart.
Some substances require more heat energy to raise their temperature than do other substances. The
amount of heat needed to raise the temperature of 1 gram of a substance 1°C is the specific heat of that
substance. A substance with a high specific heat requires more heat energy to raise its temperature than
a substance with a low specific heat. A substance with a low specific heat will heat up and cool down
more quickly than a substance with a high specific heat.
Water has a very high specific heat. The water has to absorb a lot more heat to warm up than the sand at
the beach does, so the water generally feels cooler than the sand during the day. At night, the sand loses
heat to the air much more easily than the water does. That is why the sand cools down faster than the
water at night. Now let’s look at why water has such a high specific heat.
Remember the hydrogen bonds that form between water molecules? When heat is applied to water,
much of the heat energy is used to break the hydrogen bonds. That leaves less energy available to heat
up the water. As water cools down, energy is used to reform the hydrogen bonds between the water
molecules. Less energy is released as heat, so the water cools down slowly.
Lesson 5: Acids and Bases
Chemically speaking, an acid is a compound that increases the concentration of H + (hydrogen) ions
when dissolved in water. A hydrogen atom is a proton and an electron. When it loses the electron, it is
just a proton. Hydrogen ions are protons, but they do not remain alone in water — they chemically bond
to water to form H3O+, or hydronium ions. For all intents and purposes, H+ and H3O+ mean the same
thing. They both refer to positively charged hydrogen.
Most acids contain hydrogen ions, which break off from the compound when it dissolves in water.
However, some acids do not contain hydrogen ions. These compounds, called acid anhydrides, increase
the H+ concentration by reacting with water. Examples include CO 2, which creates carbonic acid in water,
and SO3, which creates sulfuric acid in water. Let’s look at how CO 2 reacts with water to form carbonic
acid, which is the acid present in carbonated soft drinks. CO 2 and H2O react to form the compound
H2CO3, some of which then separates into ions.
By tradition, acids that contain hydrogen usually have H as the first element in the chemical formula. Here
are some examples:
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H3PO4: phosphoric acid (found in colas)
HNO3: nitric acid (used in fertilizers)
HF: hydrofluoric acid (used to clean metal)
A base is a compound that increases the concentration of hydroxide ions when dissolved in water.
Many bases contain hydroxide ions. Their chemical names reflect this.
pH is a measure of the concentration of H+ ions in a solution of an acid or base. The pH scale plots the
concentration of solutions in a range from 0–14.
Pure water is a neutral substance and has a pH of 7. Substances with pH values below 7 are acids. The
closer to 0 a solution is on the scale, the higher the concentration of H + ions in the solution.
Substances with pH values above 7 are bases. The closer to 14 a solution is on the scale, the lower the
concentration of H+ ions.
Molarity is one way of measuring the concentration of a solution. We use the abbreviation "M" to record
"molarity." A solution with a H+ concentration of 1 M (read "one molar") contains one mole of H+ ions per
liter of solution. One mole of H+ ions is equal to 6.02 × 1023 H+ ions.
A solution with a H+ concentration of 0.5 M contains half a mole (3.01 × 10^23) of H+ ions per liter of
solution. When working with acids and bases it is important to know the pH of the solution. When you
calculate the pH of a solution, you must always start with the H+ concentration in molarity.
Lesson 6: Lab time: Acid and Base Indicators.
Many of the substances found in homes are acids or bases. These items include soaps, cleaning
solutions, medicines, and foods. In this activity we will test some of these items using an acid–base
indicator. An acid-base indicator is a substance that changes color in response to the concentration of
hydronium ion. An acid–base indicator’s color is related to hydronium ion concentration. Therefore, it can
be used to determine the pH of a substance.
Acidic substances contain high concentrations of hydronium ions and low concentrations of hydroxide
ions. In contrast, basic substances contain low concentrations of hydronium ions and high concentrations
of hydroxide ions.
You can use the color change of the indicator to learn what pH is represented in the substance. A
reference color chart is supplied that allows you to determine the pH range that corresponds to a specific
color change. This reference chart was created using solutions of known pH and testing them to see the
corresponding color of the indicator at that pH.