Chemistry - Atomic Structure

Atoms are made of protons, neutrons and electrons.

The nucleus of the atom contains protons and neutrons.

The electrons are found at large distances from the nucleus.

Virtually all the mass of the atom is concentrated in the nucleus because the electrons hardly weigh anything.

The masses and charges are measured relative to each other because the actual values are incredibly small. For example, it would take about 600,000,000,000,000,000,000,000 protons to weigh just 1 gram.

The number of protons in an atom is called its atomic number or proton number. Each of the one hundred or so different elements has a different number of protons. For example, if an atom has 8 protons it must be an oxygen atom.

Atomic number =  Number of protons

The mass number is the number of protons and neutrons there are in the nucleus of the atom.

Mass number = Number of protons + number of neutrons

          59 -------- Mass number

                 Co

        27 -------- Atomic number

The number of neutrons in an atom can vary slightly. For example, there are three kinds of Carbon atom, Carbon - 12, Carbon - 13 and Carbon - 14. They all have the same number of protons, 6, but the number of neutrons varies. These different atoms of Carbon are called isotopes. 

The fact that they have varying numbers of neutrons makes no difference whatsoever to their chemical reactions. This is because the chemical properties are controlled by the number and arrangement of electrons in the atom and the electrons are not changed with isotopes.

The electrons are the negative part of the atom and in an atom it follows that:

Number of electrons = Number of protons

Atoms are arranged in the Periodic Table in order of increasing atomic number. You use the Periodic Table to find out the Atomic number so you can work out how many protons and electrons there are in the atom.

The electrons are arranged in levels called energy levels/shells. Each energy level can only hold a certain amount of electrons. Low energy levels are always filled before higher ones.

1st Shell = 2 electrons

2nd Shell = 8 electrons

3rd Shell = 8 electrons (Sometimes can expand to 18 electrons)

The number of electrons in the outer shell is the same as the group number for groups 1 to 7. For example if Barium is in Group 2 then it will have 2 electrons in its outer shell. However this pattern does not work beyond Calcium.

The elements in Group 0 have 8 electrons in the outer shell (apart from helium which has 2 electrons. These are thought as being 'full' shells which is true for helium and neon, but not the elements from Argon downwards because the third shell must contain 18 electrons to be 'full'.

Group 0 is known as the Noble gases because they are almost completely unreactive. The lack of reactivity is associated with their electronic structures.