Notes

An element is a type of atom, and there are over 100 different types. The periodic table, first organized by Dmitri Mendeleev in the mid-1800s, displays all the elements according to increasing atomic number (number of protons in the nucleus). Due to this arrangement, Mendeleev noticed that groups of elements had similar characteristics. For example, elements classified as nonmetals were all poor conductors of electricity. Mendeleev also noticed other trends. For example, the number of valence electrons in an atom increases from left to right across a row of elements. Mendeleev was able to use the patterns he saw to predict the properties of other elements that had yet to be discovered. When those elements finally were discovered later on, most of his predictions were proven true.

The periodic table gives specific information about each element, including the name of the element, its atomic number, its atomic mass, and chemical symbol. The element’s chemical symbol is an abbreviation of its name. For example, the chemical symbol for carbon is C. Some elements were originally given non-English names; their chemical symbols are a reflection of this. Pb, the chemical symbol for lead, is an abbreviation of its Latin name, plumbum. Recall that isotopes are atoms that have the same number of protons, but different numbers of neutrons. There can be several isotopes of the same element. Carbon, for example, has 15 known isotopes.

The vertical columns of the periodic table are called groups or families. The horizontal rows are called periods. There are 18 groups and seven periods. Elements within the same group have very similar chemical properties. Recall from a previous lesson that the number of valence electrons determines the reactivity of an atom, and that all atoms want to have full outer shells. For the first 18 elements, the optimal number of valence electrons is eight, except for hydrogen and helium, which only need two. If an atom has less than the optimal number of valence electrons, it will be reactive. For example, all the elements in Group 1 have only one electron in their outermost energy level, which makes these elements reactive. However, the elements of Group 18 have the optimal number of valence electrons. These elements are not reactive. The chart below demonstrates how to use the periodic table to predict the reactivity of elements.

Group*

Number of Valence Electrons

Reactive?

1

1

yes

2

2

yes

13

3

yes

14

4

yes

15

5

yes

16

6

yes

17

7

yes

18

8

no