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Chemical Reactions

When molecules or compounds are chemically changed it is called a chemical reaction.

Photosynthesis is an example of a chemical reaction. In photosynthesis (the chemical equation is shown below), the atoms in water and carbon dioxide are rearranged to form sugar and oxygen gas.


Molecules that participate in a reaction are reactants.

Molecules formed by a reaction are products.


Water's Importance to Life

Water is the single most important molecule of earth

All organisms are 70-90% water.

Water has unique properties that make it a life-supporting substance.

The Structure of Water

Atoms differ in their electronegativity, or their attraction for electrons in a covalent bond.

Oxygen has a very strong attraction for electrons, so when oxygen is sharing electrons with two hydrogen atoms, it gets the negative electrons slightly more than its fair share of the time. Since the negative electrons are near the oxygen end, more of the time, the oxygen is slightly negative. The hydrogen ends of water are slightly positive because the hydrogen atoms each have a positively charged proton that is left by itself when oxygen is sharing the electrons unfairly.

The unequal sharing of electrons in a molecule such as water makes the molecule polar.

Polar water molecules are attracted to one another and can form hydrogen bonds.

Properties of Water

Water is a solvent that can dissolve many substances.

NaCl is the chemical compound that we call table salt.

Molecules that are polar and which are attracted to water are hydrophilic (ex. sugar, salt).

Molecules that are non polar have no charges cannot attract water. These are called hydrophobic (ex. oil, grease, fat).

Water dissolves polar substances and ions.

Water molecules stick to each other and to other substances


Water also has a high surface tension.

The stronger the force between molecules in a liquid, the stronger the surface tension.



Frozen water (ice) is less dense than liquid water, so ice floats.

Unlike other substances, water expands as it freezes. This is because the hydrogen bonds of liquid water are continuously breaking and reforming. When water freezes, all of the water molecules are perfectly hydrogen bonded together. In order for them to be hydrogen bonded together, they must all be perfectly aligned with each other. the water molecules must spread out a little in order for them to all line up perfectly as water freezes. This makes them spread out as the water freezes. This spreading during freezing can burst water pipes and automobile radiators.

So far, we’ve studied atoms and compounds and how they react with each other. Now let’s take a look at how these atoms and molecules hold together. Bonds hold atoms and molecules of substances together. There are several different kinds of bonds; the type of bond seen in elements and compounds depends on the chemical properties as well as the attractive forces governing the atoms and molecules. The three types of chemical bonds are Ionic bonds, Covalent bonds, and Polar covalent bonds. Chemists also recognize hydrogen bonds as a fourth form of chemical bond, though their properties align closely with the other types of bonds.

In order to understand bonds, you must first be familiar with electron properties, including valence shell electrons. The valence shell of an atom is the outermost layer (shell) of an electron. Though today scientists generally agree that electrons do not rotate around the nucleus, it was thought throughout history that each electron orbited the nucleus of an atom in a separate layer (shell). Today, scientists have concluded that electrons hover in specific areas of the atom and do not form orbits; however, the valence shell is still used to describe electron availability.

One can determine how many electrons an atom will have by looking at its periodic properties. In order to determine an element’s periodic properties, you will need to locate a periodic table. After you’ve found your periodic table, look at the roman numerals above each column of the table. You should see that above Hydrogen, there’s a IA, above Beryllium there’s a IIA, above Boron there’s a IIIA, and so on all the way to Fluorine, which is VIIA. Also, note that the metals are all in group B—their roman numerals have the letter B afterwards instead of the letter A. For now, we are going to ignore the columns with a B, and focus on the columns with an A (the non-metals, generally speaking). Once you have located the group-A elements, we are going to count across, giving each column a number, like this:

The first A-column is I (1), then counting across, 2-8 (skipping the B group, which consists of metals). In the periodic table we labeled the 8th column as 0, however when counting electrons, we’ll count it as 8. Now, we can determine how many valence electrons each element has in its outermost shell. The elements in the IA column have 1 valence electron. The elements in the IIA column have 2 bonding electrons, and so on. By the time we get to the noble gases (the column labeled 0), we are up to 8 bonding electrons. This means that these gases can stand on their own, or donate electrons to another element, but they cannot accept any more electrons. This is because the electrons they have satisfy the octet rule.

The Octet and Duet Rules

When it comes to bonding, everything is based on how many electrons an element has or shares with its compound partner or partners. The octet rule is followed by most elements, and it says that to be stable, an atom needs to have eight electrons in its outermost shell. Elements that do not follow the octet rule are H, He, B, Li and Be (sometimes). Lithium gives up an electron whereas the other elements listed here gain one. These elements instead follow the duet rule which says that the atoms only need two valence electrons to be stable. When bonding, stability is always considered and preferred. Therefore, atoms bond in order to become more stable than they already are.

Not all atoms bond the same way, so we need to learn the different types of bonds that atoms can form. There are three (sometimes four) recognized chemical bonds; they are ionic, covalent, polar covalent, and (sometimes) hydrogen bonds.

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