Elements, Compounds, Mixtures, and the Periodic Table
In the last chapter, we introduced the concept of atoms, those tiny particles which make up all matter. More specifically, each atom is the smallest differentiable unit of matter--the smallest unit where two separate particles of the same type can be distinguished. At sub-atomic scales, there is no way to tell two different particles (say, two electrons) apart, because all subatomic particles in a particular group are identical.
Well, you may ask, what makes two atoms different? The answer lies in the atom's atomic number, or the number of protons in the nucleus. This quantity determines which element the atom belongs to: an atom with one proton has an atomic number of one, and so belongs to element one, hydrogen. An atomic number of two results in a helium atom, and so on, until with 92 protons, we arrive at uranium. Elements are defined as substances which cannot be broken into simpler substances with distinctive properties. For instance, water can be broken into hydrogen and oxygen atoms. While these atoms can be broken down still further into protons, neutrons, and electrons, these particles are indistinguishable from members of that same group and have no unique qualities. Therefore, hydrogen and oxygen are classified as elements.
Most elements can be found as single atoms, but some always form molecules with other atoms of that element. For example, hydrogen is only found in a diatomic (two-atom) molecule, H2. The same is true for nitrogen, oxygen, fluorine, and a few other elements. Thus, when a chemist refers to "elemental hydrogen," he or she means the H2 molecule, not the hydrogen atom by itself.
Example Problem 1
Using the above text and your own experience, explain why each substance below is not an element.
A. Wood can be burned, reducing it to simpler substances
B. Like wood, plastic can also be combusted. You may also know that plastic must be made from other substances (petroleum products, to be specific).
C. This one is trickier! Steel is an alloy of iron (it has other substances mixed in), meaning it is not an element.
D. Marshmallows can be burned or digested, producing simpler substances such as carbon dioxide.
In the examples above, the substances listed are compounds. Compounds are substances formed from two or more elements, such as water (composed of two hydrogen and one oxygen atoms) and sugar (made up of 6 carbon, 12 hydrogen atoms, and 6 oxygen atoms). Most substances you encounter in the world are compounds; pure elements are relatively uncommon.
A third type of substance is a mixture, in which mulitple compounds or elements are simply mixed together, as the term indicates. There are no chemical bonds between the substances in a mixture, and they can usually be separated mechanically. For instance, air is a mixture of nitrogen, oxygen, argon, carbon dioxide, water vapor, and other gases. However, we can separate air into its component elements and compounds fairly easily. Another commonly-used example of a mixture is a jar of iron filings and sand mixed together. How does one separate the substances? A magnet can be used to pull out the iron filings.
There are 92 elements in nature, as well as about twenty more that have been generated artificially. Each element has specific and unique properties: some are gases, some liquids, and most are solids; they vary in color from black to yellow to green; some are so light they can lift aircraft, while others are very dense; some elements can be cut with a knife (or not cut at all, if they are gases or liquids), but others are so hard that they must be cut with diamond saws or lasers. Some elements even decompose over time into other elements!
The Periodic Table of the Elements is a chart that gives a wealth of information about each element. This web site includes a complete Periodic Table, which you can access by clicking here or by going to our "Reference" section and clicking on the Periodic Table link. In our Table, clicking on an element gives information about that element, including a brief description of the element's physical properties. Other datas about the elements can be found in various pages in the "Reference" section as well.
However, you are likely to find Periodic Tables everywhere: in other textbooks, wall charts, AP tests, worksheets, handouts, and even on advertisements for the Marine Corps! It is essential to know how to use these resources. The Periodic Table has a form generally similar to the one below.
The columns of the Table are called groups; each group is a family of elements with similar chemical properties. Several groups have proper names. The first group, with lithium, sodium, etc., is called the Alkali Metals group. The next group, including beryllium and magnesium, is the Alkaline Earth Metals group. The groups of elements indented vertically, from scandium's to zinc's, are commonly referred to as the Transition Elements. The next-to-last group, including fluorine and chlorine, is called the Halogen group. Finally, the last group of elements, such as helium and neon, are the Noble Gases.
A row in the Table is called a period; as one moves across a period, certain trends are apparent (these will be discussed later). The two rows of elements below the main table are called the Lanthanides and the Actinides, because they can be placed immediately after Lanthanum (element 57) and Actinium (element 89), respectively, but are usually placed below to save space. No other periods have names.
Each element is commonly listed in box such as the one below. Note that the location of each item in the box may vary from table to table.
The top item in this image is the full name of the element. The next item is the number of the element--in this case, chromium is number 24. This number corresponds to the number of protons in each atom of the element, meaning that each chromium atom has 24 protons in its nucleus. The largest item is the element's symbol, which is unique to that element. Most symbols are two letters, with the first capitalized and the second lowercase, but some elements have a one-letter symbol. The capitalization system makes it easy to distinguish elements in a long molecule--since the second letter of the symbol is always lowercase (if the second letter exists), there can be no confusion between PB (phosphorus and bromine) and Pb (lead).
The final item is the atomic weight of this atom. These numbers are not integers for three reasons. First, the small mass of the electrons in the atom is included, slightly increasing the atomic mass if included in the table. Second, neutrons are very slightly heavier than protons, which is sometimes included in very precise tables (don't worry about this difference when doing calculations, however). The third factor accounts for most of the variation. Recall the definition of an isotope from the previous section: atoms with the same number of protons that have a different number of neutrons. Therefore, isotopes of the same element will have different masses. In determining the atomic mass of an element, lighter or heavier isotopes must be accounted for. Therefore, an isotope's mass is multiplied by its frequency in nature; this calculation is repeated for all isotopes, and the totals are summed.
For example, let's say chlorine is found in two main isotopes: Chlorine-35, with 18 neutrons, and Chlorine-36, with 19 neutrons. If Cl-35 makes up 60% of all chlorine, then Cl-36 composes the remaining 40%. To find the average atomic weight, multiply each weight by its percentage and add the two numbers: (35)(.6) + (36)(.4) = 21 + 14.4 = 35.4. This is just an approximation, as the actual percentages are slightly different, but it demonstrates the method involved.
Example Problem 2
You have just discovered a new element, X, that has 112 protons. You find that X has three main isotopes, X-267 (27% of total), X-270 (43%), and X-272 (30%). In order to put the element on the Table, you must find the atomic weight using the method above.
First, multiply the mass of each isotope by its percentage of occurrence.
Isotope 1: (267)(.27) = 72.09
Isotope 2: (270)(.43) = 116.1
Isotope 3: (272)(.3) = 81.6
Then, add up all the results to get the atomic weight.
72.09 + 116.1 + 81.6 = 269.79.
Your final atomic weight is 269.79. Don't worry about adding the extra mass of electrons or neutrons for these types of problems.
As you can see, the atomic mass represents the average mass of the atoms of an element. Note that the atomic mass doesn't mean that most or even any of the atoms of that element have the mass given. For instance, if two isotopes of element X were equally common and had masses of 268 and 270, the mass given on the table would be 269, even though no atoms would ever have that mass!
Now that you know how to find atomic weight with the Periodic Table, calculating the weight of a molecule should be easy. Simply add up the weights of all the component atoms in the molecule to find the molecular weight. For example, CH4 has a molecular weight of 12 for carbon + 4 * (1 for hydrogen) = 16. Conversely, knowing the molecular weight of a molecule can tell us about it. Many times, a scientist will know the empirical formula of an unknown substance. An empirical formula a formula that expresses the ratios of each atom in a molecule; to find the real formula, the empirical formula must be multiplied by some factor. For instance, a substance with an empirical formula of CH4 could have an actual formula of CH4, C2H8, C3H12, etc. If a scientist knows the molecular weight and empirical formula of a molecule, he or she can find the actual formula by using the following equation: Molecular Weight = X * (Empirical Weight), where X is the multiplication factor. In the example above, let's assume the molecular weight is 48. By using the equation 45 = X * (12 + 1*4). By simplifying, we have 48 = 16X. Obviously, X = 3, so the real formula is C3H12.
Example Problem 3
Find the molecular weight of the following molecules.
A. Sum the component atoms (N=14, H=1): M.W. = 14 + 3*1 = 17
B. Pb=207, Cl=35.5, so M.W. = 207 + 2*35.5 = 207 + 71 = 272
C. Note that the "2" subscript on the NO3 group indicates that two of this group are present. Mg=24.3, N=14, O=16, so M.W. = 24.3 + (14 + 3*16)*2 = 24.3 + 2*(62) = 148.3
D. Fe=55.8, S=28, C=12, N=14, so M.W. = 55.8 + 28 + 12 + 14 = 109.8
Example Problem 4
Find the formula of a molecule if its molecular weight is 989.6 and its empirical formula is SrBr2 (Sr=87.6, Br=79.9).
First, find the weight of the empirical formula:
87.6 + 2(79.9) = 247.4
Next, divide the molecular weight by the empirical weight to find the multiplier:
989.6 / 247.4 = 4
Therefore, the molecular formula is four times the empirical formula, or Sr4Br8.
The final concept covered in this section returns to elements that naturally form molecules. Remember that hydrogen, oxygen, and other elements form diatomic molecules of H2, O2, etc. Some elements can form several kinds of molecules: carbon, for instance, commonly forms graphite, a slick black solid used in pencils and as a lubricant. It also forms diamond under intense temperature and pressure. Carbon can even form third type of molecule, a hollow sphere of carbon atoms shaped like a soccer ball. These groups of 40, 60, 120, 600, or other amounts of carbon atoms are named after Buckminster Fuller, the architect who invented geodesic domes, structures that closely resemble these molecules. This type of molecule's scientific name is buckminsterfullerene, but it is often shortened to "buckyball."
As you can see, carbon exists naturally in a variety of forms. Each of these forms is called an allotrope; for instance, diamonds and buckyballs are allotropes of carbon. Elements that form different molecules are said to be allotropic. Some allotrophic elements are carbon, sulphur, oxygen (which also occurs as O3, ozone), and phosphorus.