<h2> Structure of Molecules</h2> <h3> Formation of Chemical Bonds</h3>

CHEMISTRY NOTES FOR CLASS IX:

4.1.1 Determine the number of valence electrons in an atom using periodic table

How to Find Valence Electrons

Understanding Electron Shells Finding Valence Electrons in All But Transition Metals Finding Valence Electrons in Transition Metals

Valence electrons lie in the outermost electron shell of an element. The number of valence electrons that an atom has determines the kinds of chemical bonds that it can form. The best way to find valence electrons is to use the periodic table of elements

Part 1 of 3: Understanding Electron Shells

1 Obtain a periodic table of elements. This is a color-coded table containing squares, each of which gives an element's 1- to 3-letter symbol and its atomic number.

Part 1 of 3: Understanding Electron Shells

Read the atomic number of an element. The atomic number appears above the element symbol in the square. For instance, boron (B) has an atomic number of 5, meaning that it has 5 protons and 5 electrons.

3Draw a simple diagram of an atom and place the electrons in orbits surrounding it. These orbits are called shells. The maximum number of electrons that can be in the same shell is fixed, and they are filled from the closest to farthest orbit.

K Shell (closest): 2 electrons maximum.

L Shell: 8 electrons maximum.

M Shell: 18 electrons maximum.

N Shell: 32 electrons maximum.

O Shell: 50 electrons maximum.

P Shell (farthest): 72 electrons maximum.

Find the number of electrons in the outermost shell. These are the valence electrons.

If the valence shell is full, then the element is inert.

If the valence shell isn't full, then the element is reactive. It means that it can form a bond with an atom of another element. Each atom shares its valence electrons in an attempt to complete its own valence shell.

Part 2 of 3: Finding Valence Electrons in All But Transition Metals

Number each column on the periodic table of elements from 1 to 18. Hydrogen (H) is at the top of column 1 and helium (He) is at the top of column 18. These are the element groups.

2Number each row from 1 to 7. These are the element periods, and correspond to the number of shells the atoms posses.

Hydrogen (H) and helium (He) both have 1 shell while francium (Fr) has 7 shells.

The lanthanides and actinides are grouped together under the main table. All lanthanides belong in Period 6, Group 3 and all actinides belong in Period 7, Group 3.

Locate an element that is not a transitional metal. Transitional metals are in Groups 3 to 12. The Group number of a non-transition metal indicates the number of valence electrons.

Group 1: 1 valence electron

Group 2: 2 valence electrons

Group 13: 3 valence electrons

Group 14: 4 valence electrons

Group 15: 5 valence electrons

Group 16: 6 valence electrons

Group 17: 7 valence electrons

Group 18: 8 valence electrons -- except for helium, which has 2

Part 3 of 3: Finding Valence Electrons in Transition Metals

Find an element from Groups 3 to 12, which are transition metals. 2 Determine the number of valence electrons based on the Group number. The Group number will correspond to a range of possible numbers of valence electrons.

Group 3: 3 valence electrons

Group 4: 2 to 4 valence electrons

Group 5: 2 to 5 valence electrons

Group 6: 2 to 6 valence electrons

Group 7: 2 to 7 valence electrons

Group 8: 2 or 3 valence electrons

Group 9: 2 or 3 valence electrons

Group 10: 2 or 3 valence electrons

Group 11: 1 or 2 valence electrons

Group 12: 2 valence electrons

4.1.3 State octet and duplet rules

Octet rule is the condition when outer most shell has 8 valence electrons or attaining eight electrons in the valence shell is called Octet rule.

Duplet rule

  This is the condition when outer most shell has two valence electrons. Duplet is the rule which shows the presence of the electrons in the 1st orbit. Hydrogen and Helium seem to be the only elements obeying it.

Hydrogen and Helium have the stable outer most shell of two electrons.

Noble gases have 2 or 8 electrons in their valence shell. It means all the noble gases have their valence shells completely filled. Their atoms do not have vacant space in their valence shell to accommodate extra electrons. Therefore noble gases neither gain, lose nor share electrons.

4.1.4 Describe the ways in which bonds are 

formed?

1. Ionic Bonding (done)

2. Covalent bonding (done)

3. Dative or co-ordinate covalent

4. Metallic Bonding

4.2.2 Characteristics of Ionic Bonds?

Some characteristics of ionic bonds include;

(1) They have high melting points,

(2) They conduct electricity,

(3) They dissolve easily in water,

(4) They have well-defined crystals and solids at room temperature.

(5) Ionic bonds are formed through an electrostatic attraction of two oppositely charges ions.

4.2.4, Characteristics of Ionic Compounds:

1. In Ionic bond, it is possible that any two ions are bonded to each other to produce molecule but in the crystals of ionic compounds the oppositely charged ions are surrounded by each other. Thus ionic compounds are solids at room temperature.

2. Ionic Compounds have high melting and boiling points because of ionic forces present between them.

3. Ionic compounds do not conduct electricity as the ions cannot move freely. If the solids melt the ions are free to move and conduct electricity. Solutions of ionic compounds can move freely and conduct electricity.

4. Ionic compounds are soluble in polar solvents such as water. But ionic compounds are insoluble in non polar solvents for example Alcohols, Spirit, etc.

4.4.1 Formation of Co-ordinate Covalent Bond

Definition:

This is a special type of covalent bond in which both electrons forming a bond are supplies the pair of electrons for bond formation is called as "Donor" and atom which accepts is known as acceptor. An arrow represents the bond formation. The pair of electron which is donated is called lone pair.

Example:

In the formation of Ammonium Chloride from ammonia(NH₃) and Hydrogen chloride molecules. The nitrogen of ammonia acts as donor of an electron pair  and hydrogen ion  of hydrogen  chloride  accepts it to form NH4cl as follows:

4.5.1 Metallic Bond:

This is defined as a bond formed between metal atoms due to mobile or free electrons.

In metals, the hold of nucleus over the outer most electrons becomes weak because of large sized atom and greater number of shells in between nucleus and valence shell. Metals have tendency to lose electrons from outer most shell, these electrons which are freely move in between atoms. Nucli of metal atoms appear submerged in sea of these mobile electrons. These mobile electrons are responsible to hold atoms of metals together forming a metallic bond.

fig. 4.2