Science 9: Unit B – Matter and Chemical Change Topic 5: The Periodic Table

Elements Elements are pure substances made of only atoms of the same size and mass. They are organized according to their atomic number in the periodic table. Elements are given symbols in the periodic table. For example the symbol C stands for carbon. The symbols are based on their old names, some of which come from Latin. For example sodium, a metal has Na as its symbol in the periodic table. This is because its old name is natrium.

Elements which have the same chemical properties are grouped into families. These are represented by columns in the periodic table. These elements have the same chemical properties because they have the same number of valence electrons. All elements try to be stable, either by gaining or losing electrons to fill their valence/outer orbital. For example: here is a picture of sodium with all of its orbitals and the electrons in them. Because it has only one valence electron

For example: here is a picture of sodium with all of its orbitals and the electrons in them. Because it has only one valence electron it tries to donate it to another atom.

Info provided on an element In any periodic table there are three bits of information provided for each element: element symbol, atomic number, and atomic mass. For the above example of Lithium, the atomic number is given at the top, and the atomic mass is listed on the bottom. 3Li6.94

Atomic Number The atomic number tells us how many protons an element’s atoms have. When an atom is neutral the number of protons = number of electrons, but like you’ve learned, most element’s atoms are trying to become stable by adding or losing electrons, so the number of electrons changes, but the number of protons is constant.

Atomic Mass The atomic mass of an element is more complicated. The atomic mass counts the mass of one proton as 1 atomic mass unit (amu), and the mass of one neutron as one atomic mass unit. So if an atom has two protons, and two neutrons, it has an atomic number of 2, and an atomic mass of 4 amu.

Atomic Mass Cont’d Why are most elements’ atomic masses in decimal form? It is impossible to have 0.9 of a proton or neutron so what’s the explanation? An element is determined only by the number of protons it has. If you have an atom with 1 proton in its nucleus, it will always be 1 amu, however, different hydrogen atoms have different numbers of neutrons causing the atomic mass to be different between them. Element atoms with different numbers of neutrons are called isotopes.

Atomic Mass Cont’d The solution chemists have come up with is to average out the atomic masses by figuring out the different isotopes of an element and measuring how often they occur. Then they calculate an average atomic mass which goes on the periodic table. For Example 99.2% of Hydrogen atoms in universe have an atomic mass of 1 (one proton, no neutron), 0.8% of hydrogen atoms have atomic mass of 2 (1 proton and 1 neutron). So when you average all of this out you get an atomic mass of amu.

Rows and Periods The columns in a periodic table are called groups, so there are a total of 18 groups. The most important groups are the families: alkali metals, alkali earth metals, halogens, and noble gases. The rows on a periodic table are known as periods. The elements in the same period have the same number of orbitals, but different numbers of electrons. The electrons increase by one, left to right in a period. Rows are numbered from 1 to 7.