Chemistry Unit 3

C3.1 The Periodic Table

Newlands - Law of Octaves , arranged in order of mass.

Newlands Law of Octaves was not accepted at the time because

• He assumed that all elements had been found and at that time, elements were being discovered with some regularity.
• His pattern only worked until calcium and then it broke down.
• He grouped together some elements that were in no way similar.

Mendeleevs ideas were more widely accepted.

• He left gaps for undiscovered elements, and predicted their qualities.
• He arranged elements in order of atomic weight.
• When elements were discovered that matched his predictions, it was seen as a breakthrough.

After the discovery of protons, neutrons and electrons, chemists were now able to place elements on their correct order by the weight of the nucleus (which determines the atomic mass), the amount of electrons in the outer shell of the elements atom (which determines the group).

Group 1 Metals

Reactivity INCREASES as you travel down the group.

Because:

• the single outermost electron is less strongly attracted to the positively charged nucleus as the atom gets bigger
• this is because the charge from the nucleus is protected by more electron shells
• the outer electron therefore becomes easier to lose
• so the lower down elements re in group 1, the more reactive they are.

Group 7

Reactivity DECREASES as you travel down the group.

• the single electron…