Chapter 3The Periodic Table

The how and whyThe how and why

History Dmitri Mendeleev - Russian scientist

• taught chemistry in terms of properties.

Mid 1800’s - molar masses of elements were known.

Wrote down the elements in order of increasing mass.

Found a pattern of repeating properties.

Mendeleev’s Table Grouped elements in columns by similar

properties in order of increasing atomic mass.

Found some inconsistencies - felt that the properties were more important than the mass, so switched order.

Found some gaps. Must be undiscovered elements. Predicted their properties before they

were found.

The modern table Elements are still grouped by

properties. Similar properties are in the same

column. Order is in increasing atomic number. A column of elements Mendeleev didn’t

know about was added later. The noble gases weren’t found because

they didn’t react with anything.

Why? The part of the atom another atom

sees is the electron cloud. More importantly the outside orbitals. The orbitals fill up in a regular pattern. The outside orbital electron

configuration repeats. Outside electrons are called valence

electrons The properties of atoms repeat.

Electron Arrangements repeat The shape of the periodic table is a

representation of this repetition. When we get to the end of the row

the outermost energy level is full. Full energy level is the most stable

• Noble gases do not react because they are already stable

Determining Valence Electrons Valence electrons

determine how a chemical will react.

For group A elements, the group number is the number of valence electrons

Valence Electrons All elements would like to

have 8 electrons (except H and He)

8 electrons means full outside shell

8 electrons means more stable

Electron Dot diagrams A way of keeping track of

valence electrons. How to write them:

• Write the symbol.

• Put one dot for each valence electron

• Don’t pair up until they have to

X

The Electron Dot diagram for Nitrogen

Nitrogen has 5 valence electrons.

First we write the symbol. NThen add 1 electron at a time to each side.Until they are forced to pair up.

Write the electron dot diagram for

Na Mg C O F Ne He

Periodic Table Setup

Horizontal rows are called periods There are 7 periods

Vertical columns are called groups.

Elements are placed in columns by similar properties.

Also called families

1A

2A 3A 4A 5A 6A7A

8A0

The elements in the A groups are called the representative elements

The group B are called the transition elements

These are called the inner transition elements and they belong here

Group 1A are the alkali metals Group 2A are the alkaline earth metals

Group 7A is called the Halogens Group 8A are the noble gases

Transition Metals Chromium, gold, silver,

nickel, zinc, iron, etc. Transition metals do not

behave predictably Their atomic structure is

more complicated

Inner Transition Metals Two rows “under” main

periodic table First row is lanthanides – rare

earth metals Second row is actinides –

radioactive

Nonmetals Don’t conduct electricity Brittle as solids Low melting points

Metalloids Between metals and

nonmetals Semiconductors – conduct

some electricity

Semiconductors Electricity is the flow of

electrons Metals conduct electricity

because their electrons are free to move

Nonmetals do not conduct because their electrons are locked in place

Semiconductors Semiconductors work best

when a small amount of another element is added

Called doping Si (4 valence e-) doped with P

(5 valence e-) gives an extra electron

Called n-type for negative charge

Semiconductors P- type have one less

electron and are positive Can combine types to form pnp- or npn- type

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