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Chemistry is the study of the composition, structures, properties, and reactions of a substance. The structure and function of all living things is governed by the laws of chemistry.


Atoms:
An atom is the smallest particle of an element that retains the properties of the element. All atoms are made of the same particles. Protons and neutrons make up the nucleus of the atoms, and electrons circle around the nucleus in shells. Neutrons do not have a charge, protons are positively charged, and electrons are negatively charged. Because protons and electrons have opposite charges, they repel each other, which is why the electrons circle around the nucleus. This picture illustrates the helium atom.


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Another important term to know is isotope. An isotope is a form of an atom that has the same number of protons and electrons but numbers of neutrons. This changes the atomic mass of the atom. Atomic mass is simply the mass of the atom. In an atom, protons and neutrons are approximately 1 amu (atomic mass unit) each, whereas an electron is about 1/1800 amu. Because of this, we do not factor in the weight of the electron when calculating the atomic mass, and instead can add together the number of protons and neutrons. To find the number of neutrons given the mass of the atom of a particular element, simply subtract the number of protons from the given atomic mass, and the number remaining will be approximate number of neutrons.

Elements:

An element is a pure substance that cannot be broken down chemically into a simpler form of matter. The best way to explain elements is to take a look at the periodic table.

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Elements are very carefully organized. The columns represent groups/families, which are determined by the number of valence electrons. The rows represent periods. The entire table goes in order from left to right, top to bottom of the atomic number, which is the number of protons in one atom of the element. The strange shape of the periodic table actually has to do with sublevels within shells, something that will be explained further in chemistry. Generally speaking, as you go left to right across the periodic table, atomic mass increases. Exceptions to this are like with Te and I, because the number of valence electrons determines the family. In addition, as you go left to right across a period, atomic size decreases due to increased electronegativity holding the electrons and protons closer together. Finally, remember how the atomic mass is the number of protons plus the number of neutrons? How can you have a decimal then? Well, on the periodic table, the atomic mass is actually an average of the known possible atomic masses for the element with the abundance of each isotope factored in. Therefore, because it is an average, you can have decimals.

For an interactive look at the periodic table with more information on each element, please take a look at EnviromentalChemistry.com.

Or, if you wish to challenge yourself, you could use this song to just memorize them all!



Types of Bonds:
In a chemical reaction, chemical bonds are broken, atoms rearranged, and new chemical bonds are formed. There are two types of chemical bonds, covalent bonds and ionic bonds. Covalent bonds form when atoms share one or more pairs of electrons. A single bond is where one pair of electrons is shared, a double bond is where two pairs of electrons are shared, and so on. An ionic bond of formed by the electrical attraction between two oppositely charged ions. How does this happen? Take NaCl for example. Sodium has one electron in its outer shell, and chlorine has seven. Chlorine only needs one more electron to complete its outer shell, and sodium just needs to loose one. Therefore, sodium donates its electron to chlorine. However, although they are both filled and happy, sodium, because it has one more proton than electron, is positively charged, and chlorine, because it has one more electron than proton, is negatively charged. Therefore, the two ions are attracted to each other, forming an ionic bond.

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The number of valence electrons present govern all chemical bonds, for each element is searching for a full outer shell. The energy level order, or the number of electrons each shell can hold, is 2-8-8.

And here's a quick video describing covalent and ionic bonding!



Molecules:
Molecules are two or more atoms held together by covalent bonds. Water is one of the most unique molecules because of its different characteristics. There are four main characteristic of water that you must remember:
1) Water has strong cohesion (attractive forces between particles of the same kind) forces and adhesion (the attractive force between unlike substances) forces.
2) Water can absorb large amounts of thermal energy before its own temperature begins to rise. Because of this, water acts like a heat buffer for the globe.
3) Water is the universal solvent because it is very effective in dissolving many other substances. The only substances insoluble in water are nonpolar substances.
4) Water is polar—the electrons are unevenly shared between the hydrogen and oxygen atoms. This is because oxygen, having the stronger positive charge due to more protons, pulls the electrons away from the hydrogen atoms and towards its own nucleus, making the electrical charge uneven through the molecule. The oxygen side is more negative and the hydrogen side is more positive.
Water molecules are held to one another through hydrogen bonds, which is a result of water’s polarity (the negative ends are attracted to another molecules positive end). However, this is a weak bond that is easily broken.
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Click **HERE** for an animation on water's structure and polarity!

pH Scale:

pH stands for "percent hydrogen". pH measures the number of hydrogen ions present. An acid has more hydrogen (H) ions than hydroxide (OH) ions, giving it a pH of 0 to less than 7. A base has more hydroxide ions than hydrogen ions, giving it a pH greater than 7 to 14. A pH of 7 is neutral. The pH scale runs from 0-14. Therefore, the more hydrogen ions, the lower the pH and the less hydrogen ions, the higher the pH. In order to find the pH of a substance, it must be dissolved in a solution containing water. In this solution, water is the solvent, or the substance in which the solute is dissolved, and the substance is solute, or the substance dissolved in the solution. Equally important to know is what a buffer is. A buffer is a chemical substance that neutralizes a small amount of an acid or a base.
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