T1 - The Periodic Table & Electronic Configurations

How was Mendeleev’s ‘Table of Elements’ organised?

How was Mendeleev’s ‘Table of Elements’ organised?

• he arranged 50 or so elements known at the time into the table

• he began sorting the elements into groups, based on their properties

• as he did this, he realised that if he put the elements in order of atomic mass, a pattern appeared → he could put elements with similar chemical properties in columns

• a few elements seemed to end up in the wrong columns, so wherever this happened he switched the order of the elements to keep those with the same properties in the same columns

• to keep those elements together, he would leave some gaps. he used the properties of the elements in the same column to predict the properties of undiscovered elements

The Periodic Table

shows elements in order of ascending atomic number

Groups

columns of elements with similar properties

Number of electrons an element has in its outer shell

corresponds to group number

Periods

rows in the periodic table ; each new period represents another full shell of electrons

Number of shells an element has

the period that the element belongs to

Electron Shell Rules

• electrons must occupy shells (sometimes called energy levels)

• the lowest energy levels are always filled first (they are closest to the nucleus)

• only a certain number of electrons are allowed in each shell:

  1st shell: 2 , 2nd shell: 8 , 3rd shell: 8

How to work out Electronic Configuration from period and group?

• the number of shells which contains electrons is the same as the period of the element

• the group number tells you how many electrons occupy the outer shell of the element

• the proton number tells you how many electrons there are because number of protons = number of electrons