Chap 8 Covalent and Metallic Bonding

I) Covalent Bonds

The bond formed between atoms that share electrons is called a covalent bond After bonding each electron attains the electronic configuration of a noble gas or attains stability This bond is formed between a non-metal and a non-metal

A molecule is a group of two or more atoms held together by covalent bonds Covalent compounds do not have chemical formulae instead they have molecular formulae

Formation and Arrangement

Many non-metallic elements exist as diatomic molecules This means that they always exist in the molecular state with two identical atoms joined together through covalent bonding For example hydrogen gas always exists as H2 which is its diatomic form

Fig 81 Covalent bond in Hydrogen molecule

Note always show the electrons of different members with different signs ie one with a dot and the other with a cross

There are other ways of representing a hydrogen molecule as well

Molecular Formula H2

Dot amp cross diagram HtimesHStructural Formula H_H (The single line shows a single covalent bond)

The sharing of two electrons or a lsquopair of electronsrsquo forms a single covalent bond

Between each molecule of a covalent compound there are weak van der Waalrsquos forces holding the molecules together

Other Examples

Fig 83 Cl2 molecule Cl_Cl

Fig 82 O2 molecule O=O

Covalent Compounds

Molecules made from two or more different types of atoms linked together by covalent bonding are called molecular compounds or covalent compounds

A common example is H2O water It is made up of two single covalent bonds

Fig 83 H2O molecule

Another example is CH4 methane It has four single bonds in it

Fig 84 CH4 molecule

Carbon dioxide CO2 has two double bonds

Fig 85 CO2 molecule O=C=O

Physical Properties

They have low melting and boiling points They are mostly gases or volatile liquids at room temperature Most are insoluble in water and soluble in organic compounds Most do not conduct electricity in either solid liquid or gaseous state

II) Metallic BondingMetal atoms are held strongly to each other by metallic bonding In the metal lattice the atoms lose their valence electrons and become positively charged The valence electrons no longer belong to any metal atom and are said to be lsquodelocalisedrsquo They move freely between the metal ions like a cloud of negative charge Hence this lattice structure is described as a lattice of positive ions surrounded by a lsquosea of mobile electronsrsquo Therefore a metallic bond can be defined as lsquothe force of attraction between positive metal ions and the sea of delocalised electronsrsquoPhysical Properties of Metals

They are good conductors of electricity They are good conductors of heat They are malleable (can be hammered into different shapes) They are ductile (can be drawn into wires without breaking)

Fig 83 Cl2 molecule Cl_Cl

Fig 82 O2 molecule O=O

Covalent Compounds

Molecules made from two or more different types of atoms linked together by covalent bonding are called molecular compounds or covalent compounds

A common example is H2O water It is made up of two single covalent bonds

Fig 83 H2O molecule

Another example is CH4 methane It has four single bonds in it

Fig 84 CH4 molecule

Carbon dioxide CO2 has two double bonds

Fig 85 CO2 molecule O=C=O

Physical Properties

They have low melting and boiling points They are mostly gases or volatile liquids at room temperature Most are insoluble in water and soluble in organic compounds Most do not conduct electricity in either solid liquid or gaseous state

II) Metallic BondingMetal atoms are held strongly to each other by metallic bonding In the metal lattice the atoms lose their valence electrons and become positively charged The valence electrons no longer belong to any metal atom and are said to be lsquodelocalisedrsquo They move freely between the metal ions like a cloud of negative charge Hence this lattice structure is described as a lattice of positive ions surrounded by a lsquosea of mobile electronsrsquo Therefore a metallic bond can be defined as lsquothe force of attraction between positive metal ions and the sea of delocalised electronsrsquoPhysical Properties of Metals

They are good conductors of electricity They are good conductors of heat They are malleable (can be hammered into different shapes) They are ductile (can be drawn into wires without breaking)

Fig 84 CH4 molecule

Carbon dioxide CO2 has two double bonds

Fig 85 CO2 molecule O=C=O

Physical Properties

They have low melting and boiling points They are mostly gases or volatile liquids at room temperature Most are insoluble in water and soluble in organic compounds Most do not conduct electricity in either solid liquid or gaseous state

II) Metallic BondingMetal atoms are held strongly to each other by metallic bonding In the metal lattice the atoms lose their valence electrons and become positively charged The valence electrons no longer belong to any metal atom and are said to be lsquodelocalisedrsquo They move freely between the metal ions like a cloud of negative charge Hence this lattice structure is described as a lattice of positive ions surrounded by a lsquosea of mobile electronsrsquo Therefore a metallic bond can be defined as lsquothe force of attraction between positive metal ions and the sea of delocalised electronsrsquoPhysical Properties of Metals

They are good conductors of electricity They are good conductors of heat They are malleable (can be hammered into different shapes) They are ductile (can be drawn into wires without breaking)