Chapter 1 – The Periodic Table

Section A: History of the Elements

  • Ancient Greeks – There are only four elements:Earth , water, fire and air
  • Robert Boyle – An element is a substance that cannot be broken down into a simpler substance by chemical means
  • Humprey Davy – Discovered elements by passing electricity through a molten solution of their compounds

Section B: History of the Periodic Table

  • Dobereiner

Triads:a group of 3 elements which had similar physical & chemical properties with the atomic weight of the middle element being halfway between the other two.

Example of a triad:lithium, sodium and potassium

  • Newlands

Law of octaves :When all of the known elements where listed in order of atomic weight ( mass) every 8th element had similar chemical and physical properties.

  • Mendeleev

When all elements listed in order of increasing atomic weight (mass) the properties repeated themselves at regular intervals

  1. Arranged elements in a table called the Periodic Table
  2. Left gaps for undiscovered elements
  3. Used it to predict properties of undiscovered elements
  4. Reversed the order of some elements so that the elements would fit into groups with other elements that shared similar chemical and physical properties

Differences between Mendeleev’s Table and the Modern Periodic table

Mendeleev’s table / Modern Periodic table
Had gaps for elements that were not discovered at the time / Has no gaps
Elements were placed in order of atomic weight / Elements placed in order of atomic number
There were no noble gases included / There are noble gases included
  • Moseley –

Arranged the elements according to their atomic number.

Section C: Case study of a group in the Periodic table

Group 1 (Alkali metals)

  • All have oneelectron in their outer shell which they lose readily so they are all very reactive.
  • Properties:Shiny , easy to cut with a knife, reactive with air and water, can float on water

Balanced equations showing reactions with water

  • The reactivity increases as you go down the group

Why?

  1. The outermost electron is further away from the nucleus ( the atomic radii increase) which makes it easier to remove
  1. Although the nuclear charge increases as you go down a group the number of shells also increase, this screening effect of the extra shells effectively cancel out the increasing nuclear charge

Self assessment of your learning of THE PERIODIC TABLE

Green = I have learnt this

Orange = I am not sure – have to revise this

Red = I don’t know this yet – have to learn

Can you.. / Green / Orange / Red
Define an Element?
Write the symbols of elements 1–36.
Tell briefly the contribution of the Greeks to the History of the idea of elements,
Tell briefly the contribution of the Boyle to the History of the idea of elements,
Tell briefly the contribution of the Davy to the History of the idea of elements,
HL Only:
Explain Dobereiner’s contribution to the development of the Periodic table
HL Only:
Explain Newland’s contribution to the development of the Periodic table
Explain Mendeleev’s contribution to the development of the Periodic table
Give three differences between Mendeleev's table with the modern periodic table.
HL Only:
Explain Moseley’s contribution to the development of the Periodic table
Define atomic number
Give a brief statement of the main similarities of elements within each main group, in particular :
alkali metals,
alkaline earth metals
halogens
noble gases
Explanations in terms of atomic radius, screening effect and nuclear charge for general trends in properties of elements
in group I.

St.Dominic’s CollegeChemistry NotesPage 1