1. Chemistry is the science dealing with the composition of matter and the transformations of matter.

Recall that matter is composed of different substances that are called elements. An element is something that cannot be broken down into other simpler substances, while retaining the unique chemical properties. The smallest indivisible unit of an element is called an atom. Atoms of each element have a characteristic structure: which gives each element its unique physical and chemical properties. Atoms are extremely small: 12 grams of carbon (about 1/2 ounce) contains 6.023 x 10²³ atoms, each of which is one to several angstroms (10^-10 m) long. A 100 million atoms lined up from end to end would equal about 1 inch.

2. Atoms in turn are composed of subatomic particles. The three particles of interest to us are protons, neutrons and electrons (these are in turn made of smaller components, called quarks, but we will not need to discuss them for this class). The proton, which has a positive electrical charge, and the neutron, which has no charge, are concentrated in an area at the center of an atom, called the nucleus. These two particles have essentially the same mass. Electrons, which have very little mass (about 1/1840th of a proton), bear a negative electrical charge and move around the nucleus in regular orbits, or energy levels (it's more complicated than that, but for the purposes of this class, we need not discuss it in more detail).

3. Scientists recognize various elements by their atomic numbers (the number of protons in the nucleus). Each element has a unique number of protons, and every atom of that element has the same number. For example, the atomic number of hydrogen is 1 (1 proton in the nucleus). Other elements and their atomic numbers: carbon (6), nitrogen (7), oxygen (8), uranium (92).

You may wonder what holds the protons and neutrons together in the nucleus (you may know that like charges repel each other). The nucleus is held together only by powerful counterbalancing forces, called nuclear forces, which hold the nucleus together, like a glue. However, as you will see later, these nuclear forces may be inadequate in holding the atom together, and then the nucleus can break up (fission) and radioactivity may occur. We'll come back to this later.

4. Atomic weight is another feature describing atoms and elements. The atomic weight is the number of protons and neutrons in the nucleus. The number of protons is all atoms of a particular element is constant, however, the number of neutrons may vary. For example, in carbon (the most important element for life), carbon ordinarily has 6 protons and 6 neutrons in the nucleus. However, some carbon atoms have 7 neutrons, and others may have 8 neutrons (this carbon atom, carbon-14, or ¹⁴C, by the way, is unstable and radioactive). Atoms of an element that have different atomic weights are called isotopes. They vary only in the number of neutrons. The atomic weights are often put above and prior to the chemical symbol: ¹⁴C and ¹²C for the radioactive and non radioactive isotopes of carbon, respectively).

5. Atoms of an element may vary in the number of electrons they possess. In a normal atom, the number of electrons equals the number of protons (the number of positive and negative charges are equal), and thus the atom is electrically neutral (the electron's negative charge is the same in magnitude as the proton's positive charge). The simplest (and most abundant) atom in the universe is the hydrogen atom (one proton in the nucleus, with one electron circling around it). Some atoms, however, may have more (or less) electrons than protons, and thus the entire atom has a net positive or negative charge. Atoms or groups of atoms (molecules, see below) may have one or more net positive or negative charges, and are thus called ions. Ions retain most of the properties of the uncharged atom of the element, but other properties may change. For example, a chlorine atom has an atomic number of 17 (17 protons and 17 electrons). However, most of the atoms found in nature (particularly in water) are charged ions (the chlorine atom receives one electron from another atom, thus becoming more stable in its outer electron shell (this topic of electron shells is more complex than needed here). It becomes a chlorine ion or chloride ion (Cl^-, 17 protons and 18 electrons). Similarly sodium (Na, atomic number 11), in order to be more stable, has only one electron in its outer shell. It readily donates its outer electron to another atom (such as chlorine), and thus normally in water, the sodium atom is an ion with a positive charge (Na⁺, 11 protons, 10 electrons). A molecule may have a net positive or negative charge, and thus be an ion (nitrate, NO₃, is composed of one nitrogen and three oxygen atoms covalently bound together as one molecule, and the whole molecule has a net negative charge of -1, or NO₃^-). Other common molecular and atomic ions are phosphate (PO₄^-3, 3 negative charges), calcium (Ca⁺², 2 positive charges), ammonium (NH₄⁺), sulfate (SO₄^-2) and hydrogen ions (H⁺, the simplest ion, consisting of one proton).

6. Each element has its own unique chemical symbol, which consists of one or two letters, the first is often the first letter of the name, and is always capitalized. Hydrogen is H, carbon (C), nitrogen (N), oxygen (O), sulfur (S), phosphorus (P), calcium (Ca), chlorine (Cl), uranium (U). Some elements, such as sodium (Na), iron (Fe), silver (Ag), lead (Pb) and gold (Au), are named after their Latin names (such as natrium for sodium, for example).

7. Atoms react with and join with each other, by sharing or transferring electrons, to form molecules. Molecules can be made of one element, for example, oxygen gas is O₂ (the 2 to the left of the symbol means that two oxygen atoms are bound together in the molecule). A compound is a molecule that is made of two or more atoms of different elements. Scientists use a 'chemical shorthand' (or chemical formula) to indicate these molecules. Water, for example, is a compound written as H₂O; this chemical formula symbolizes both the elemental composition of the molecule and their relative proportions. Water is composed of 2 atoms of hydrogen and one of oxygen, covalently bound together. NaCl represents sodium chloride, or table salt, consisting of one atom of sodium and one of chlorine.

8. There are four important types of chemical bonds. The first is ionic bonding, the bonding that occurs between sodium and chlorine in table salt. This bonding occurs between two ions, often forming an orderly array of positive and negative ions into a crystal, called a salt. This link is due to sodium giving up its one outer electron to chlorine, which needs one electron to fill up its outer shell. Both atoms become electrically charged (ions), and are thus attracted to each other by the electrical charges.

The bonds may be weak; this is why interactions with water molecules causes table salt crystals to break apart, or dissolve, into the water. Table sugar, or sucrose (C₁₂H₂₂O₁₁), although polar, does dissolve in water, but not as readily as table salt.

A second type of bonding involves the sharing of a pair (or multiple pairs) of electrons. A shared pair of electrons is called a covalent bond. A carbon atom, for example, needs four electrons to fill its outer shell, it thus can form a total of four covalent bonds with other atoms, including other carbon atoms (this is the crucial important aspect to the importance of carbon for life: because of its versatility in forming covalent bonds, it forms the large variety of carbon containing molecules found in living things. A second important aspect of carbon is that it is relatively abundant). After a covalent bond forms, each pair of electrons orbits both of the atoms. In methane (CH₄, the simplest hydrocarbon), carbon shares a pair of electrons with each of the hydrogen atoms. Each atom has its outer shell filled with electrons, and thus methane is a relatively stable molecule. Covalent bonds are the strongest of the bonds, and represent energy (when you 'burn' glucose in your cells during respiration, or 'burn' natural gas in heating, the covalent bonds holding glucose and natural gas molecules are broken, releasing heat and light energy).

The last two types of bonds are the weakest, compared to ionic and covalent bonds. The third type of bond is hydrogen bonding. This is a weak bond that occurs between two molecules. Some molecules are what we call polar (they have slight separation of electrical charges at different parts of the molecule). Water for example, is a polar molecule. These slight charges can be attracted to opposites charges (of other polar molecules, and to ions). The slightly negative region of a water molecule (around the oxygen nucleus, which due to the larger size and greater number of protons in the oxygen nucleus, attracts the shared pair of electrons more strongly than the smaller hydrogen nucleus) can be attracted to the slightly positive regions around the hydrogen nucleus of another water molecule, thus a weak bond forms between the two molecules. These bonds are quite weak, however, by their sheer numbers, can be important. This is why water molecules can be carried to the tops of trees, and small insects can scurry around on the surface of the water. More about this property of water later). Polar molecules are hydrophilic ('water-loving'), i.e. , they readily react with polar water molecules.

The fourth bond is hydrophobic bonding. This occurs between a polar molecule (such as water) and a nonpolar molecule (such as oil). A nonpolar molecule has no slight separation of charges in its structure, so it does not interact with water ('water-hating' or hydrophobic). Nonpolar molecules will react often with each other, but not with polar molecules, on the whole, and this bond is quite weak. Hydrophobic bonding is a very important aspect of cell membrane formation, molecular folding and shape, and environmental concerns (such as oil slicks, where the oils and other hydrocarbons do not dissolve in water. This makes it possible to remove the oil in some cases, because it may 'float' on the surface, if it is of the right type).

There will be more chemistry that will be presented when needed in the class later in the semester.