Definitions and Important Concepts

Definitions and Important Concepts

Before we can talk about the trends in the periodic table, we first need to define some terms that are used:

• Atomic radius

  The atomic radius is a measure of the size of an atom.

• Ionisation energy

  The first ionisation energy is the energy needed to remove one electron from an atom in the gas phase. The ionisation energy is different for each element. We can also define second, third, fourth, etc. ionisation energies. These are the energies needed to remove the second, third, or fourth electron respectively.

• Electron affinity

  Electron affinity can be thought of as how much an element wants electrons.

• Electronegativity

  Electronegativity is the tendency of atoms to attract electrons. The electronegativity of the elements starts from about 0.7 (Francium (Fr)) and goes up to 4 (Fluorine (F))

• A group is a vertical column in the periodic table and is considered to be the most important way of classifying the elements. If you look at a periodic table, you will see the groups numbered at the top of each column. The groups are numbered from left to right starting with 1 and ending with 18. This is the convention that we will use in this book. On some periodic tables you may see that the groups are numbered from left to right as follows: 1, 2, then an open space which contains the transition elements, followed by groups 3 to 8. Another way to label the groups is using Roman numerals.

• A period is a horizontal row in the periodic table of the elements. The periods are labelled from top to bottom, starting with 1 and ending with 7.

For each element on the periodic table we can give its period number and its group number. For example, \(\text{B}\) is in period 2 and group 13. We can also determine the electronic structure of an element from its position on the periodic table. In Chemistry 101, you worked out the electronic configuration of various elements. Using the periodic table we can easily give the electronic configurations of any element. To see how this works look at the following:

We also note that the period number gives the energy level that is being filled. For example, phosphorus (\(\text{P}\)) is in the third period and group 15. Looking at the figure above, we see that the p-orbital is being filled. Also the third energy level is being filled. So its electron configuration is: \([\text{Ne}]3\text{s}^{2}3\text{p}^{3}\). (Phosphorus is in the third group in the p-block, so it must have 3 electrons in the p shell.)