THE PERIODIC TABLE

Dmitri Mendeleev

• A Russian scientist who began to classify the 60 known elements of his time

• Worked with the indivisible model of an atom• Atomic masses were known in comparison to

hydrogen, which was set as "1"

“I began to look about and write down the elements with their atomic weights and typical properties, analogous elements and like atomic weights on separate cards, and this soon convinced me that the properties of elements are in periodic dependence upon their atomic weights.” --Mendeleev, Principles of Chemistry, 1905, Vol. II

• Mendeleev discovered that the properties of the elements repeated at regular intervals when the elements are arranged in order of increasing atomic mass

periodicity: the occurrence of similar physical and chemical properties of elements at regular intervals

• He left blank spaces for where he predicted (based on properties) an element should be

– He predicted (accurately!) the properties of yet-undiscovered elements!!!

Observed properties of Eka-Silicon and observed properties of Germanium:

Property Observed for Si

Predicted for Eka-Silicon

Observed for Sn

Found for Ge

Atomic Mass

Melting Point (oC)

Density (g/cm3)

Boiling point of chloride

(oC)

28

1410

2.33

57.6

72

High

5.5

100

118

232

7.28

114

73

947

5.35

84

In the meantime…

• Thomson and Rutherford discovered the proton and the electron

• Chadwick, in Rutherford’s lab made another discovery: the neutron!

• SO… an atom is made of

– Positive protons, mass of 1, symbol p+

– Uncharged neutrons, mass of 1, symbol no

– Negative electrons, mass of 0, symbol e-

Atomic mass = number of protons + neutrons

Atomic number = number of protons

Number of protons = number of electrons (in a neutral atom)

• Moseley working together with Rutherford discovered that the element's properties could be even better predicted using the number of charges the atom had: atomic number

• elements were re-arranged by atomic number (as in the modern Periodic Table)

• elements arranged in order of increasing atomic number show a periodic repetition of properties

• each column of the periodic table (called a group) has similar properties (periodic trends)– The vertical columns are called "groups“ or “families”.– The horizontal rows are called "periods".

The Periodic Law- chemical and physical properties of element vary in a

periodic way with their atomic number

The modern Periodic Table

• Scientists divide elements into 3 categories

1. Metals• Alkali metals

• Alkaline earth metals

• Transition elements

• Inner transition elements

2. Non-metals• Halogens

• Noble gases

3. Metalloids

representative elements

transition elements

inner transition elements

alkali metalsalkaline earth metals

lanthanoids

actinoids

transition metals

halogens

noble gases

Metals• left side and centre of periodic table

• Solid at room T (except mercury, Hg)

• Silver (except Cu and Au)

• Shiny, conduct electricity and

heat, malleable and ductile

malleable:capable of being shaped, the degree to which it can be shaped by pounding with a hammer

ductile:physical property of being capable of sustaining large deformations without breaking (for example being drawn into a wire)

Non-metals

• Right side of periodic table• Found in all three states• Variety of colours• Poor conductors of heat/electricity• Usually brittle (having little elasticity: easily cracked

or fractured or snapped )

Metalloids• Along zig-zag line dividing metals

and non-metals

• Metalloids are: Boron, Silicon, Germanium, Arsenic, Antimony, Tellurium, Polonium

• Have some metallic and non-metallic properties

• Ex. Si - silicon: shiny, nonmalleable, a semi-conductor