Periodic Trends

Periodic table can be used to predict patterns of properties of the elements. Lets consider the periodic table of S, and P groups mainly. (Group number I.....0)

1. Number of electrons in the outermost shell

Group number is equal to the number of electrons in the outermost shell of any element.
e.g: Na Group number I - number of electrons in the outer most shell= 1
   
Group number 0 is also referred as group number VIII. All the elements have 8 electrons in the outer shell But He having only 2 electrons.

2. Valency of the element

Valency is the number of electrons given, taken or shared in making bonds by the elements. It is important in writing the correct formula of a compound as well. General rule that  can be used to calculate the valency is as follows.

Valency of Groups I -IV

Equal to the group number

e.g : Mg= Group number II= valency= 2;


Valency of Groups IV- VII

equals to 8- group number

e.g: Cl group number 7= valency=( 8-7)= 1


Valency of Group number 0

equals to zero

3. Number of electronic shells

number of electronic shells is equal to the period number (horizontal raws) 

e.g Li is in period 2 = has 2 shells with electrons

4. Reactivity

Some of the metals in group I and II and also some elements in group number III are metals. All the other elements are non metals. The usual trend of reactivity

for  metals  : 

increases down the group. Na is more reactive than Li. K is more reactive than Na. The reason for this can be simply described as follows. Metals releases electrons to achieve the nobel gas configuration when making bonds. Releasing of electrons become more easy when the outer most shell electrons to be released is more distant from the nucleus.

for non metals

Decreases down the group. E.g Fluorine is more reactive than chlorine. Chlorine is more reactive than iodine.

Next about Group properties

Chemical formulae

To write the correct chemical formula of a compound

1. Identify the elements that makes this compound

e.g: Calcium chloride is made with Calcium and Chlorine

2. Write the correct symbols of the elements in the order they appear in the chemical name

eg: Ca    Cl

3. Write the valency of the elements

Valency means the number of electrons given, taken or shared by an atom of the element when making a compound
Valency is equal to the group number for elements in group numbers I to IV
Valency is equal to ( 8 - group number) for elements in group numbers V to VII
Valency is equal to zero for group number 8 elements as they usually does not make compounds

Ca is in group number II . Its valency is 2
Cl is in group number VII .Its valency is (8-7)= 1

Ca  2   Cl  1

4. Exchange the valencies

Ca1 Cl2

5. Simplify and write the formula

further simplification is not possible in this example. If we take the formula of Magnesium oxide
and exchange their valencies it is

Mg 2    O2
2 and 2 can be simplified into 1 to 1

6. Write the correct formula
(ratio between atoms should be simplified in to whole numbers and number 1 is not written

CaCl2


This method is used only for compounds made between two elements. In addition to the simple elements there are complex cations and anions that makes compounds. (Cation: a positive ion
anion: a negative ion)
If the compound is made with simple elements they will end as :' ide' added to the end of the last element
e.g: sodium chloride : NaCl

mostly the compounds made with complex anions ends as : 'ate'
e.g: Sodium Sulfate Na2SO4

However the rules are same for writing the the formula but we have to keep in mind the name , formula and the charge of these complex anions and also cations

Complex anion name       formula         valency

                    

Nitrate                            NO₃^{-                           -1}

Hydroxide                      OH^{-                             -1}

Bicarbonate                    HCO₃^{-                     -1}

Carbonate                      CO₃^{2-                       -2}

Sulphate                         SO₄^{2-                       -2}

ThioSulphate                  S₂O₃^{2-                  -2}

Phosphate                       PO₄^{3-                   -3}

Formula of the  complex cation  Ammonium ion is      NH₄⁺

Now lets consider an example

1.  Aluminium Phosphate

Elements/ complex cation/anion

Al       and PO₄^3- 

^{Valancies Al =3  Phospahate= 3}

^{Exchange and simplify we get}

^AlPO4

2. Calcium hydroxide

Ca =2  OH^-   
exchange and simplify you get  Ca(OH)2



Questions:

Write the correct formula of

1. Sodium Carbonate
2. Aluminium Hydroxide
3. Magnesium Phosphate

Next on Mole concept

Ionic Bonding

Ionic bond is a bond between a metal and a non metal. Metals are elements with shiny surfaces, sonorous sounds, malleable and with good conductors of heat and electricity.

Metals usually have upto 3 electrons in their outer most shell (in non transitional elements). To achieve the noble gas configuration they would release these electrons and become positive ions.
Non metals in the other hand (mainly group V,VI,VII elements in the periodic table) has 5, 6 or 7 electrons in their outermost shell. In order to achieve a stable electronic configuration these elements can accept electrons and become negative ions. Electrostatic attraction between a positive ion and a negative ion makes ionic bonding.

compounds with ionic bonds are having properties such as high melting points, boiling pointsand readily soluable in water .

Next about covalent bonding