Introduction to Elements and Compounds
Chemistry is quite possibly one of the most relevant subjects you will ever study. It explains matter, reactions, and why the world around you works the way it does. It explains volcanoes, rain, laundry, Kool-Aid, and much more! In this section, you will find explanations about some of the most discussed topics in general chemistry.
But first, here are some basic things you need to know as you study chemistry. All elements are made up of atoms, the “building blocks of life.” Atoms are the smallest functional units of matter that make up our universe. Atoms consist of three parts: protons, neutrons, and electrons. Think of it like the human body—your body is functional, and made up of your organs (heart, lungs, stomach, liver, and so on). You can separate your organs, but if you just have a heart, your body is not functional, because you need all the parts working together. The same is true for an atom—you need protons, electrons, and (usually) neutrons. These three parts are often referred to as subatomic particles. This term makes sense—particles means they are part of the whole (which is an atom). Atomic means they come from an atom, and sub- means that they do not make up the whole atom, but they work together to make it functional. These particles each have a charge—protons are positive (+), electrons are negative (-), and neutrons are neutral (n) meaning they do not carry a positive or negative charge.
Atoms, and their subatomic particles, are measured in a.m.u., or atomic mass units. An atomic mass unit is the equivalent of 1/12 of the mass of a carbon-12 atom. Elements are most often measured in a.m.u., grams, or moles. Each of these measurements will be further explained in the following pages, as needed.
The Periodic Table
If you look up an element on the periodic table, you will see the following:
Or, it might simply be displayed this way:
The following useful information is provided on most periodic tables:
Symbol: This is often a one or two letter abbreviation for the element; for example, carbon’s symbol is C. When writing the symbol, the first letter is always capitalized, and if it has a second letter, it is lower-case.
Element name: This is what the element is called. Some periodic tables have both the symbol and the name, while others just have the symbol, so it is important to know both.
Atomic Number: This is the number of protons the element contains. For example, carbon’s atomic number is 6, because there are 6 protons in a carbon atom.
Atomic Weight: This is often the decimal number given, and it is usually in units of grams/mol. This number will be used greatly in gram to mol conversions later on. For example, carbon’s atomic weight is 12.011 g/mol.
Atomic Mass Number: This is a whole number that is usually very close to the atomic weight. The atomic mass number is the number of protons plus the number of neutrons in an atom. For example, carbon’s atomic mass number is 12, which means it has 6 protons and 6 neutrons. This is helpful in calculating mass ratios.
Ions and Isotopes
All naturally occurring atoms (of the elements listed on the periodic table) are made up of neutral atoms, meaning they have the same number of protons and electrons. An imbalance of protons and electrons creates an ion, an atom that contains more protons than electrons, or vice versa. Ions are created by the gain or loss of electrons. Gaining an electron would give an ion a negative charge, because electrons make the atom more negative. Losing an electron would give the ion a positive charge, because taking away the electron (which is negative) would make the atom more positive. A positive ion is called a cation, and it can have a +1, +2, or +3 charge. A negative ion is called an anion, and it can have a -1, -2, or -3 charge. Sometimes the charges may be higher than positive or negative 3, but generally they are either + or – 1, 2, or 3.
Isotopes are also important when determining mass and charge. Some elements have isotopes, which means that they have different types of atoms—some of which have more neutrons than others. Now remember, neutrons do not have a charge, so an isotope does not have a different charge than that of the original elemental atom, it simply has a different number of neutrons, which changes the atomic mass slightly. For example, the most often occurring form of carbon is carbon-12, which means that that form of carbon has 6 protons, 6 electrons, and 6 neutrons. However, some common isotopes of carbon are carbon-13 and carbon-14, which can be interpreted as follows: carbon-13 would have 6 protons and 6 electrons (as usual) but this isotope would have 7 neutrons, as 13 – 6 = 7. Carbon-14 would have 6 protons and 6 electrons, as usual, but this isotope would have 8 neutrons, as 14 – 6 = 8.
Sometimes, compounds can be ions as well. Remember, we said ions were elements with a positive or negative charge. Well, ions can be made up of compounds as well. For example, NO3- is the negative nitrate ion that bonds with hydrogen to form HNO3. Ions that are compounds are called polyatomic ions, because they are made up of more than one atom of elemental material. See our General Chemistry Reference Guide for a complete monatomic and polyatomic ion list.