(N03) The Periodic Table

Things to note for (N03) The Periodic Table:

1. General points about the Periodic Table

• The elements in the Periodic Table are arranged in increasing order of proton number.
• The Group number indicates the number of valence electrons of an atom of the element.
• The Period number indicates the number of occupied shells of an atom of the element. 
• Elements are placed in different groups because of the number of valence electrons they possess.
• Elements in the same group will have the same chemical properties due to the same number of valence electrons they posses. Hence they react in the same way.
• No. of valence electrons = No. of electrons in the outermost shell
• Valency of an element = No. of electrons involve in a reaction regardless whether the electrons are gained or lost
• Charge is written as, for example '3+', '2-', with the quantity first, followed by the +/- sign.
• Reactivity of an element is related to how readily it forms an ion. Higher the tendency to form a stable ion, more reactive will be the element. The reaction of reactive elements will also appear to be more vigorous or violent. 
• Tendency to form a stable ion, depends on the distance between the nucleus and the valence electron, which will in turn affect the forces of attraction between the nucleus and the valence electrons. 
• (By convention) When naming or writing the chemical formula the compounds consisting of a metal and non-metal, the name of the metal will come first, followed by the non-metal. The name of the metal remains the same, while that of the non-metal will change.

Example: Sodium reacts with chlorine, the product is known as sodium chloride, NaCl, instead of sodium chlorine.


2. Group I elements (Alkali metals)

• The elements consist of only 1 valence electron.
• Melting and boiling point decreases down the group.
• Density increases down the group.
• In order to form a stable ion, the elements tend to lose 1 electron to form a positively charged ion. Therefore, valency will be 1.
• Reactivity increases down the group. (Fr is the most reactive metal.)
• Group I metal reacts chlorine to produce metal halide.
• Group I metal reacts with water to produce metal hydroxide and hydrogen. 

Note: In general, when a reactive metal reacts with water, metal oxide and hydrogen gas will be obtained. BUT for Group I elements, they form compounds which are soluble in water, the metal oxide which is formed will further react with water to produce metal hydroxide. (see example below.)

         Example:  When sodium reacts with water, the reaction should be: 
                            sodium + water → sodium oxide + hydrogen 
   BUT because sodium oxide is soluble in water, the reaction will then be:
           sodium + water → sodium hydroxide + hydrogen

• Most Group I metal compounds are white solids and when dissolved in water they form colourless solution. THERE IS NO WHITE SOLUTION. 

Example: Sodium chloride, the common table salt which we consume, is made by reacting sodium and chlorine. However, it will not take the colours of the elements which make it up. Being a Group I metal compound, it will be a white solid and when dissolved in water, a colourless solution is obtained. 

3. Group VII elements (Halogens)

• The elements consist of only 7 valence electrons.
• They exist as diatomic molecules, 2 atoms per molecule. 
• Melting and boiling point increases down the group.
• Density increases down the group.
• Colour of the element darkens down the group.
• In order to form a stable ion, the elements tend to gain 1 electron to form a negatively charged ion. Therefore, valency will be 1.
• Reactivity decreases down the group. (F is the most reactive non-metal.)
• Group VII elements can undergo displacement reactions, whereby the more reactive element displaces the less reactive element from its salt solution.

4. Group 0 elements (Noble gases)

• The elements consist of a stable electronic configuration also known noble gas configuration.
• They exist as monoatomic atoms, individual.
• Melting and boiling point increases down the group.
• They are unreactive due to their stable electronic configuration.

credit: 
some of the above points came from questions or comments contributed by students, from 3E4 and 3E5, who came for consultation on Saturday (27 March '10).

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------------------------------ Part 1: Elements and their reactions --------------------------------

Video 1: (Group I elements) Alkali metals and their reactions with water
Video 2: Alkali metals and their reaction with water (with Universal Indicator). 
Video 3: (Group VII elements) Halogens and their reactions

* For more videos related to reactions of the elements in the Periodic Table:
        http://www.periodicvideos.com/ (by the University of Norttingham)

In the presence of Universal Indicator, water will appear to be green in colour as it is neutral. However, when a Group I metal is placed in it, the Group I metal hydroxide will be produced. Thus, changing the colour from green to blue/purple. This is because the metal hydroxide which is formed is alkaline in nature. 

For Universal Indicator:
pH 1 (Acidic): Red
pH 7 (Neutral): Green
pH 14 (Alkaline): Purple