The elements in the periodic table are organised in order of increasing atomic number.
1.18 Organising the Elements
Students should:
1.18 understand how elements are arranged in the Periodic Table:
- in order of atomic number
- in groups and periods
1.19 understand how to deduce the electronic configurations of the first 20 elements from their positions in the Periodic Table
The modern periodic table provides a list of all the known elements categorized in increasing order of increasing atomic number.
Each row across the periodic table is known as a period.
Each column is known as a group.
1.19 Deducing electron configurations
A carbon atom has 6 protons and therefore 6 electrons. The electrons are arranged in two shells; 2 electrons in the first shell and 4 electrons in the second shell. The electron configuration of a carbon atom can be therefore represented as: 2, 4
The electrons in an atom arrange themselves in shells ( or levels) around the nucleus. The electrons fill the innermost shells first . Each shell can contain a maximum number of electrons.
This model works well for explaining the properties of the first 20 elements in the periodic table - a more complex model is needed to explain the structure of atoms above atomic number 20.
1.19 Activity 1. Looking for patterns
Chlorine atoms have 7 electrons in their outer shell.
Filling up nicely.
Chlorine has an atomic number of 17. This means that the atoms have 17 protons. Because atoms have no overall charge the number of electrons must also be 17. These are arranged as follows:
Starting with the innermost shell .
2 electrons ( maximum for shell 1)
8 electrons ( maximum for shell 2)
7 electrons left over = (17 - (8 + 2))
Question.
The atomic number of Potassium is 19 and that of Calcium is 20. What is the electron configuration for each atom?
Answers: Potassium K, Calcium Ca
1.20 - 1.22 Activity 2. Periodic variations
Students should:
1.20 understand how to use electrical conductivity and the acid-base character of oxides to classify elements as metals or non-metals
1.21 identify an element as a metal or a non-metal according to its position in the Periodic Table
1.22 understand how the electronic configuration of a main group element is related to its position in the Periodic Table
Good conductors?
Acid or base?
This video animation shows how the acid/base properties of the oxides varies across the periodic table. The pattern is not perfect partly because many non-metals can form more than one oxide and also because their solubility varies. However a general trend can be seen.
Stop the video after 21 seconds and try to predict what the trend will be. Test your prediction by watching the rest of the video.
1.23 Family resemblances
Hydrogen is often listed above group 1 the Alkali metals. Although like group 1 atoms, hydrogen atoms have a single electron in their outer shell, Hydrogen is not a metal.
Students should:
1.23 understand why elements in the same group of the Periodic Table have similar chemical properties
1.24 understand why the noble gases (Group 0) do not readily react
Looking at the electron arrangements of the atoms in groups 1, 2 and 3 you can see that the atoms of group 1 elements have 1 electron in their outermost shell, group 2 elements all have 2 electrons and group 3 elements 3. This carries on in groups 4,5,6,7.
This similarity of electron configuration within a given group of elements explains why the elements in a group behave in similar ways; they have similar physical and chemical properties.
For example: lithium Li , sodium Na, and potassium K :
- all react violently with water.
- are all soft low density metals
- combine with chlorine atoms in a 1:1 ratio
- form oxides which dissolve to give an alkaline solution
1.24. Activity 4. A Noble family
The chemical behaviour of a element is partly determined by the number of electrons in the outer shell.
In the noble gases or group 0 the outermost shell of the atoms contains 8 electrons (2 in case of Helium) which is a stable state configuration.
Hence, the noble gases are chemically unreactive or inert.
noble, inert, 8 or 0?
In the shortened periodic table, the group number is the same as the number of outer shell electrons in the atoms of that group. This is not true for the noble gases. Helium only has 2 electrons in its outer shell, while the others all have 8 . Because of this, the group number was changed to 0 .
Originally the group was known as the inert gases as it was thought that they would not react with anything. However in 1962 a chemist called Bartlett made a compound from Xenon. The preferred name for the group is now the noble gases.