C2a Topic 1 - Atomic Structure and the Periodic Table

Structure of an atom:

• The nucleus is made up of protons and neutrons
• Electrons move in shells around the nucleus
• Protons equal the number of electrons and make up the nucleus

Particle      Relative Mass      Relative Charge

Proton                  1                           +1

Neutron                1                             0

Electron            1/2000                       -1

Electrons occupy shells which are always filled up first before there is another shell:

• 1st shell: 2     2nd shell: 8     3rd shell: 8     (electrons)

Elements consist of one type of atom only

Each element has a different number of protons

• Atomic numbers say how many protons there are in an atom

• Mass numbers say the total number of protons and neutrons in an atom

Isotopes are different atomic forms of the same element which have the same number of protons but different numbers of neutrons.

Relative atomic mass uses the average mass of the isotopes of an element

Relative abundance is the amount of each isotope compared to the total amount of the element in the world. This can be a ratio, fraction or percentage.

Example:

element     relative mass of isotope     relative abundance

chlorine                    35                                  3

                               37                                  1

(35 x 3) + (37 x 1)   = 35.5

         3 + 1

History: Dmitri Mendeleev arranged around 50 elements into groups in 1869 according to their properties and compounds. He placed them into a table of elements with similar chemical properties in the same vertical groups. He had to leave gaps to make this work and predict where new elements were.

The periodic table is arranged in periods and groups:

Periods are rows of the periodic table. They are arranged in increasing atomic number.

Groups are columns of the periodic table. These elements have similar properties because they have the same number of electrons in their outer shell. The group number is always equal to the number of electrons in the outer shell.