General Chemistry Index
Where are we going with this? Getting to the current models of atomic theory didn't happen over night. This page will give the ability to explain how and why models of atomic structure have changed over time.
The Periodic Table
That thing has a lot of information!
Of all the tools in modern chemistry, the periodic table is the goto guy for information about the elements. It is a fantastic way to organize and manage many, many details related to elements.
Further… Well, it's the 21st Century. This is no ditto sheet with letters and numbers. Take a look at this!
I'm now wondering if anyone knows what a ditto sheet is, anyway. But that's not important.
The periodic table was, once upon a time, arranged based on the masses of the samples, but in time, that was changed. The elements of the periodic table are arranged in ascending order based on the number of protons (i.e. the atomic number) in each element.
Further, it is arranged into 18 columns and 7 rows. How the columns and rows are organized is the basis for the "periodic" part of the name.
Time for a not-chemistry break: Things that are periodic occur from time to time. Some things occur in regular periods. Full moons are periodic, occurring roughly every 28 days.
The periodic nature of the periodic table relates to the way electrons are located in the orbitals.
Orbitals? Yeah, the places in the atom where the electrons can be located.
Let's start with the rows…
The rows on the periodic table represent the available sets of orbitals for the elements on that row. For example, the first row has only one available set of orbitals (an n1 orbital with only a s-type orbital). The second row has two sets of orbitals available: 1 n1 and 1 n2 set. The third row has three sets of orbitals.
Rows are also called periods.
Okay, what about the columns?
So, the atomic number (number of protons) determines the sequence of the elements. However, the columns also partially indicate something very important.
The columns indicate how the available sets of orbitals are being filled. For instance, in column 1, all of the elements there have only one electron in the orbitals with the highest energy level. In column 17, all of the elements have every orbital full, except one which is missing 1 electron.
All of the elements in each column are similar with regard to where the electrons are and which ones are missing. Columns 1, 2, 13, 14, 15, 16, 17, and 18 are VERY similar in that their last n2 set of orbitals are uniformly filled.
The elements in columns 3 through 12 are less uniform, but the location of present and missing electrons in the orbitals is still influential in where they are located, coming in after atomic number to determine how the elements are arranged.
Columns are also called families or groups.
The Ptable.com site offers a great way to dig in and look at what's going on with the electrons. For instance, the image to the right shows where each of the electrons of scandium is located within the orbitals.
As you move from element to element, the configuration is shown.
Many periodic tables will include something called a valence number. This will be an integer with a + or - sign.
The valence number represents the tendency of the atom to either give up its electron in a reaction or take on the electrons from another atom. The plus valences tend to give up the electrons. The negative valences tend to take on electrons.
But That's Not All!
Another super important part of the periodic table is the atomic weight/mass. The atomic mass is a number usually presented at the bottom of the listing and it will not be a whole number. The atomic mass is the sum of the masses of the atom's sub-atomic particles.
Now, for the magic. Okay, not really magic. But still awesome and important.
The atomic mass is the weight in grams of one mole of the element.
The importance of this fact is vast. When actually "doing chemistry" this relationship is massive!
This relationship will be used whenever "weighing out" reactants or mixing solutions. If you want to produce eighteen grams of water, you need to start with sixteen grams of oxygen and two grams of hydrogen. Okay, that's getting ahead. So, trust me. This is really, really important.