These are notes and a review of 11th grade or high school chemistry. 11th grade chemistry covers all the material listed here, but this is a concise review of what you need to know to pass a cumulative final exam. There are several ways to organize the concepts. Here is the categorization I've chosen for these notes:
Chemical Properties: properties that describe how one substance reacts with another substance. Chemical properties may only be observed by reacting one chemical with another.
Examples of Chemical Properties:
Physical Properties: properties used to identify and characterize a substance. Physical properties tend to be ones you can observe using your senses or measure with a machine.
Examples of Physical Properties:
Chemical Changes result from a chemical reaction and make a new substance.
Examples of Chemical Changes:
Physical Changes involve a change of phase or state and do not produce any new substance.
Examples of Physical Changes:
The building blocks of matter are atoms, which join together to form molecules or compounds. It's important to know the parts of an atom, what ions and isotopes are, and how atoms join together.
Atoms are made up of three components:
Protons and neutrons form the nucleus or center of each atom. Electrons orbit the nucleus. So, the nucleus of each atom has a net positive charge, while the outer portion of the atom has a net negative charge. In chemical reactions, atoms lose, gain, or share electrons. The nucleus does not participate in ordinary chemical reactions, although nuclear decay and nuclear reactions can cause changes in the atomic nucleus.
The number of protons in an atom determines which element it is. Each element has a one- or two-letter symbol that is used to identify it in chemical formulas and reactions. The symbol for helium is He. An atom with two protons is a helium atom regardless of how many neutrons or electrons it has. An atom may have the same number of protons, neutrons, and electrons or the number of neutrons and/or electron may differ from the number of protons.
Atoms that carry a net positive or negative electric charge are ions. For example, if a helium atom loses two electrons, it would have a net charge of +2, which would be written He2+.
Varying the number of neutrons in an atom determines which isotope of an element it is. Atoms may be written with nuclear symbols to identify their isotope, where the number of nucleons (protons plus neutrons) is listed above and to the left of an element symbol, with the number of protons listed below and to the left of the symbol. For example, three isotopes of hydrogen are:
11H, 21H, 31H
Since you know the number of protons never changes for an atom of an element, isotopes more commonly are written using the element symbol and the number of nucleons. For example, you could write H-1, H-2, and H-3 for the three isotopes of hydrogen or U-236 and U-238 for two common isotopes of uranium.
The atomic number of an atom identifies its element and its number of protons. The atomic weight is the number of protons plus the number of neutrons in an element (because the mass of electrons is so small compared with that of protons and neutrons that it essentially doesn't count). The atomic weight sometimes is called atomic mass or the atomic mass number. The atomic number of helium is 2. The atomic weight of helium is 4. Note that the atomic mass of an element on the periodic table isn't a whole number. For example, the atomic mass of helium is given as 4.003 rather than 4. This is because the periodic table reflects the natural abundance of isotopes of an element. In chemistry calculations, you use the atomic mass given on the periodic table, assuming a sample of an element reflects the natural range of isotopes for that element.
Atoms interact with each other, often forming chemical bonds with each other. When two or more atoms bond to each other, they form a molecule. A molecule can be simple, such as H2, or more complex, such as C6H12O6. The subscripts indicate the number of each type of atom in a molecule. The first example describes a molecule formed by two atoms of hydrogen. The second example describes a molecule formed by 6 atoms of carbon, 12 atoms of hydrogen, and 6 atoms of oxygen. While you could write the atoms in any order, the convention is to write the positively charged past of a molecule first, followed by the negatively charged part of the molecule. So, sodium chloride is written NaCl and not ClNa.
The periodic table is an important tool in chemistry. These notes review the periodic table, how it is organized, and periodic table trends.
In 1869, Dmitri Mendeleev organized the chemical elements into a periodic table much like the one we use today, except his elements were ordered according to increasing atomic weight, while the modern table is organized by increasing atomic number. The way the elements are organized makes it possible to see trends in element properties and to predict the behavior of elements in chemical reactions.
Rows (moving left to right) are called periods. Elements in a period share the same highest energy level for an unexcited electron. There are more sub levels per energy level as atom size increases, so there are more elements in periods further down the table.
Columns (moving top to bottom) form the basis for element groups. Elements in groups share the same number of valence electrons or outer electron shell arrangement, which gives elements in a group several common properties. Examples of element groups are alkali metals and noble gases.
The organization of the periodic table makes it possible to see trends in properties of elements at a glance. The important trends relate to an atomic radius, ionization energy, electronegativity, and electron affinity.
Chemical bonds are easy to understand if you keep in mind the following properties of atoms and electrons:
The two main types of chemical bonds are ionic and covalent bonds, but you should be aware of several forms of bonding:
You may be wondering how you can tell whether a bond is ionic or covalent. You can look at the placement of elements on the periodic table or a table of element electronegativities to predict the type of bond that will form. If the electronegativity values are very different from each other, an ionic bond will form. Usually, the cation is a metal and the anion is a nonmetal. If the elements both are metals, expect a metallic bond to form. If the electronegativity values are similar, expect a covalent bond to form. Bonds between two nonmetals are covalent bonds. Polar covalent bonds form between elements that have intermediate differences between the electronegativity values.
In order for chemists and other scientists to communicate with each other, a system of nomenclature or naming was agreed upon by the International Union of Pure and Applied Chemistry or IUPAC. You'll hear chemicals called their common names (e.g., salt, sugar, and baking soda), but in the lab you would use systematic names (e.g., sodium chloride, sucrose, and sodium bicarbonate). Here's a review of some key points about nomenclature.
Compounds may be made up of only two elements (binary compounds) or more than two elements. Certain rules apply when naming binary compounds:
In addition to the rules for naming binary compounds, there are additional naming conventions for ionic compounds: