Atoms and The Periodic Table WJEC Additional Science Chemistry

Scientists express the mass of an atom relative to the mass of the carbon atom, 12C. The mass of the 12C is given as 12, its mass number. The mass of another atom is compared with 1/12 od the mass of a carbon-12 atom. This is called relative atomic mass, symbol Ar. 

When atoms form molecules, the relative mass of the molecule can be calculated by adding together the relative atomic masses of the atoms that make it up. This is called relative molecular mass, symbol Mr.

Relative molecular mass is known as relative formula mass when talking about ionic compounds. Eg: 

Lead Sulphate has the fomula 'PhSO4'. It contains 1 atom of lead, 1 atom of sulfur and 4 atoms of oxyfen. The Ar of these atoms are: 

Lead (Pb) = 207 Sulfur (S) = 32 Oxygen (O) = 16

So the Mr of lead sulphate would be: 

207 + 32 + 4 * 16 = 303 

The electrons in an atom are at different energy levels around the nucleus. The energy levels are sometimes called shells or orbits. Each shell can contain up to a maximum number of electrons: 

1st shell: up to 2 electrons

2nd, 3rd and so on: up to 8 electrons 

Electrons don't occupy an outer shell unless the inner shells are full. Nitrogen has an atomic number of 7. From this we can work out its electronic strucure. 

The electronic structure is written as the number of electrons in each shell, seperated by a comma. Nitrogen would therefore be 2,5. 

Electronic figueration is an alternative term for elecronic structure. 

Over many years, the idea of atomic structure has changed through observations and the gathering of experimental evidence. 

Atoms consist of a central nucleus surrounded by one or more shells of electrons. 

The nucleus consists of two types of particles, protons and nuetrons. 

The proton and nuetrons are very small. They have a relative mass of 1. The relative mass of an electron is so small its negligible. 

A proton has a positive charge, an electron has a negative charge, and nuetrons have no charge. 

Overall, an atom has no electrical charge, because the number of protons are equal to the number of electrons, therefore cancelling out the charges. 

PARTICLE                      RELATIVE MASS                 CHARGE

Proton                                      1                                  +1 

Nuetron                                    1                                   0

Electron                              Negligible                           -1 

The number of protons in an atom is its proton number or atomic number. 

The number of nucleons (protons + nuetrons) is the mass number. 

Therefore, the number of nuetrons equals mass number - proton number. 

Eg; Carbon has a mass number of 12 and a proton number of 6, we can calculate the numebr of nuetrons : 12 - 6 = 6 Carbon has 6 nuetrons.

Different elements have different atomic numbers: no two elements have the same numebr of protons. However, the number of nuetrons in the nucleus is not fixed. Some atoms of the same element have different numbers of nuetrons therefore a different mass number. These different forms of the same element are called ISOTOPES. 

The most common form of Carbon has 6 protons and 6 nuetrons, and so has a mass number of 12 ( 6 + 6 = 12 ). There is another type of Carbon which has 6 protons and 8 nuetrons, therefore this carbon isotope has a  mass number of 14.

The electronic structure of an element can be worked out from its position in the Periodic Table: 

The group number gives the number of electrons in the outer shell. The exception of this is in Group 0, which indicated that the outer shell is full. 

The period number indicates how many shells of electrons the atom has.