Electron Configuration

And the Periodic Table

Periodic Table - History•How did the periodic table come about?

•Well scientists have always wanted to classify the ‘elements’ of the earth. The early Greek thinkers said there were 4 elements (earth, air, fire, water) and classified everything under these headings.

•Many years later John Dalton said that “the chemical elements are composed of….invisible particles of matter, called atoms…..atoms of the same element are identical in all aspects, particularly weight.”

•Many scientists attempted to arrange elements, some used physical properties and others used chemical properties.

Periodic Table - Mendeleev•Mendeleev (a Russian) used chemical and

physical properties, namely the atomic weights, of elements to arrange them in a table. In 1869 published his results.

•Mendeleev even predicted the position of elements not yet discovered, which was quite a feat considering he didn’t know what atoms were made of or why they behaved the way they did.

•His basic rule was that any element in any column or group behaved in a similar way.

Periodic Rules & Laws•The periodic table is a classification of the elements

based on a comparison of their physical and chemical properties. The fact that various properties occur at regular intervals when they are listed in order of atomic number is known as the PERIODIC LAW.

•The periodic table is laid out according to three rules:

•Firstly, elements are listed in order by atomic number

•Secondly, the row – or period – indicates the number of electron shells around a nucleus. That is, the first period has one shell, the second period has 2 shells and so on…

•Thirdly, the group the element is in indicates the number of valence electrons. The valence electrons are the number of electrons in the outer shell that the atom has to gain, lose or share to make a full shell. Hence, all the electrons in a particular group have the same number of electrons in the outer shell.

Periodic Table - Trends

1 H 2 He

3 Li 4 Be

5 B 6 C

7 N 8 O 9 F 10 Ne

11 Na

12 Mg

13 Al

14 Si 15 P 16 S 17 Cl

18 Ar

19 K

20 Ca

Group 1

Group 2 Group 3 Group 4 Group 5 Group 6 Group 7

Group 8

1st period

2nd period

3rd period

4th period

Groups•Group 1 are known as the alkali metals. They are highly reactive.

•Group 2 are known as the alkaline earth metals.

•Group 7 are the Halogens.

•Group 8 are the Noble gases.

•The transition elements between groups 2 and 3 form a kind of bridge between the metals on the left hand side of the periodic table, and the non-metals on the right hand side of the table. They are all metals and have a high boiling point.

•The change from metal to non-metal is gradual.

Metallic Properties of Atoms

H He

Li Be B C N O F Ne

Na Mg Al Si P S Cl Ar

K Ca As we move across the periodic table, the elements change from metals, to metalloids (some metallic qualities), to non-metals.

Metals Metalloids Non-metals

Electron Shells•Electrons are arranged in shells around the nucleus.

•As the shells move away from the nucleus they become larger and can fit in more electrons.

•Each shell can contain a maximum number of electrons. We can calculate this with the simple formula 2n2 where n is the shell number. For examples in the first shell 2 x (12) = 2. For the second shell 2 x (22) = 2 x 4 = 8.

•Can you calculate the maximum number of electrons in the 3rd & 4th shells?

•Your answers should be 18 and 32 respectively.

•The octet rule says that only 8 electrons can fit in any outer shell.

Electron Configurations

Electron Configurations

1 2

2,1 2,2 2,3 2,4 2,5 2,6 2,7 2,8

2,8,1

2,8,2

2,8,3

2,8,4

2,8,5

2,8,6

2,8,7

2,8,8

2,8,8,1 2,8,8,

2How do we write these electron configurations?

Predicting Charges•The metals on the left hand side of the periodic

table prefer to lose electrons. They usually lose the number it takes to have a complete outer shell.

•The non-metals on the right hand side of the periodic table prefer to gain electrons. They usually gain enough to fill their outer shell.

•Atoms that lose electrons gain a positive charge and are known as cations.

•Atoms that gain electrons gain a negative charge and are known as anions.

•We can predict the charges of the ions formed as we go across the groups on the periodic table…

Charges & Valency•Group 1 atoms have a +1 charge and a valency

of +1

•Group 2 atoms have a +2 charge and a valency of +2

•Groups 3 atoms have a +3 charge and a valency of +3

•Group 4 atoms don’t form ions easily as they can be +ve or -ve, but have a valency of 4

•Groups 5 atoms have a –3 charge and have a valency of -3

•Group 6 atoms have a –2 charge and have a valency of -2

•Group 7 atoms have a –1 charge and have a valency of -1

•Group 8 don’t form ions

In summary• The physical and chemical properties of

elements occur in trends/patterns/periods

• These trends can be shown using the layout – devised by Mendeleev – in the periodic table

• The position of an element on the periodic table allows us to know its qualities

• The position of an element on the periodic table can allow us to make predictions about how that element will behave

The End

Periodic Table