Introduction to bonding

Group 1

Li

Na

K

Rb

Cs

Fr

Group 2

Be

Mg

Ca

Sr

Ba

Ra

Group 7

F

Cl

Br

I

At

All elements in the same group have the same number of electrons in their outer shell.

Group 1 = 1

Group 2 = 2

Group 3 = 3

Group 4 = 4

Group 5 = 5

Group 6 = 6

Group 7= 7

Group 8 = 8

Group 3

B

Al

Ga

In

Tl

Group 4

C

Si

Ge

Sn

Pb

Group 5

N

P

As

Sb

Bi

Group 6

O

S

Se

Te

Po

Group 8

He

Ne

Ar

Kr

Xe

Rn

.Group 1

Li 2s1

Na 3s1

K 4s1

Rb 5s1

Cs 6s1

Group 7

F 2s2 2p5 = 7

Cl 3s2 3p5 = 7

Br 4s2 4p5 = 7(ignoring d

orbitals)

I 5s2 5p5 = 7(ignoring d

orbitals)

Group 8 all have a complete outer shell, this makes them very stable and unreactive, hence they are called “inert” or “noble” gases.

He2

Helium has two electrons, 1s2, so the first shell is full.

Ne2 + 8 =10

Neon has 8 electrons, 2s2 2p6, so the second shell is also full.

Ar2 + 8 + 8 = 18

Argon has 18 electrons, only the first part of the third shell is full, 3s2 3p6, but this is still very stable.

Kr2 + 8 + 8 + 10 + 8 = 36

Krypton has 36 electrons, the third shell is full, as is the first part of the fourth shell, 4s2 4p6.

Inert gases and bonding

• Most elements are not as stable as the inert gases.

• They react, forming chemical bonds, so that they will have a complete outer shell and hence be as stable as an inert gas.

• Two main types of bonds are formed in the process;

1) Ionic 2) Covalent

Ionic bonding

• In ionic bonding one atom loses an electron(s), so that it has a complete outer shell (ie has the same number of electrons as an inert gas) , forming a positive ion.

M → M+ + e

The other element accepts an electron(s), to complete its outer shell, forming a negative ion.

X + e → X-

Metals generally form positive ions.

Non metals generally form negative ions.

Group 1

Li

Na

K

Rb

Cs

Fr

Group 2

Be

Mg

Ca

Sr

Ba

Ra

Group 3

Al

To obtain a complete outer shell metals loose electrons to form positive ions.

Group 1 lose 1 electron

Group 2 lose 2

Aluminium lose three.

Group 8

He2

Ne10

Ar18

Kr36

M+ ion

Li+2

Na+10

K+18

Rb+36

M atom

Li3

Na11

K19

Rb37

They will then have the same number of electrons (isoelectronic) as an atom of a stable inert gas in group 8. Eg Group 1.

Group 8

He2

Ne10

Ar18

Kr36

M2+ ion

Mg2+10

Ca2+18

Sr2+36

M atom

Mg12

Ca20

Sr38

Likewise for Group 2…

Group 8

He2

Ne10

Ar18

Kr36

Xe54

X Atom

F9

Cl17

Br35

I53

X- ion

F-10

Cl-18

Br-36

I-54

To get a complete outer shell non metals gain electrons to form negative ions. Eg Group 7

Group 8

He2

Ne10

Ar18

X atom

O8

S16

X2- ion

O2-10

S2-18

N7

P15

X3- ion

N3-10

P3-18

Ne10

Ar18

• Generally opposite charges attract.

• This means that the positive ion and the negative ion will attract each other.

• This attraction is an ionic bond.• The ions attract each other

strongly, so the ionic bond is a very strong bond.

Dot and cross diagrams;

• The outermost electrons of one atom are represented by a dot, those of the other by a cross.

• Eg; Lithium Hydride Li H• Li = 1s2 2s1 = Li.

• H = 1s1 = HX

• Li H = (Li)+ (.xH)-

Covalent bonding

• In covalent bonding atoms share electrons to get a complete outer shell of electrons (ie the electronic configuration of an inert gas.)

•Eg; H2

•H = 1s1 = H.

•H2 = H:H

Group 8

He2

Ne10

Ar18

Kr36

Xe54

X Atom

F9

Cl17

Br35

I53

X2 molecule

F : F8-2-8 = 10 each

Cl : Cl17-2-17 = 18 each

Br : Br35-2-35 = 36 each

I : I52-2-52 = 54 each

To get a complete outer shell non metals share electrons to form covalent molecules. Eg Group 7

Group 6 elements need a share of two additional electrons to achieve a complete outer shell.

O atom

O6

Group 8

He2

Ne8

O2 molecule

::O :: O::4-2-2-4

Group 5 elements need a share of three additional electrons to achieve a complete outer shell.

N5

:N .::. N:2-3-3-2

Ne8

Group 4 elements need a share of four additional electrons to achieve a complete outer shell.

C4

11 C 1

1

H1

H1

H1

H1

Ne8

Valency and covalent bonding• The number of extra electrons that an

atom needs to share is also known as its valency or combining power.

• Hydrogen needs 1 electron and has a valency of 1.

• Chlorine also has a valency of 1.• Oxygen needs 2 and has a valency of 2.• Nitrogen has a valency of 3.• Carbon has a valency of 4

Multiple bonds

• Each covalent bond involves sharing a pair of electrons.

• So in a double bond two pairs of electrons are shared.

• Eg O2

• O = 1s2 2s2 2p4

• To complete its shell each oxygen must gain two electrons.

• O::O

Electron deficient compounds

• Beryllium, aluminium and boron, all form compounds in which they do not have a full outer shell.

• Eg Beryllium chloride• Be = 1s2 2s2 • Cl = 1s2 2s2 2p6 3s2 3p5• Be uses its two outer electrons to make

two covalent bonds with Cl.• Giving it 2 + 2 = 4, not 8, electrons in

its outer shell.

Electron rich compounds

• Some elements, such as sulphur, can form covalent compounds in which they have a share of more than 8 electrons in their outer shell.

• Eg; Sulphur hexaflouride (SF6)

• S = 1s2 2s2 2p6 3s2 3p4• F = 1s2 2s2 2p5• Sulphur uses each of its six outer electrons

to form a covalent bond with chlorine, giving it a share of 12, not 8, electrons.

Dative bonds

A dative bond is a covalent bond in which one atom provides both electrons to be shared.

:NH3 H+Ammonia has a lone pair on N that is not involved in bonding as N has a full shell.

Protons have no electrons.

:NH3HAn ammonium ion, NH4

+

forms by N sharing both the lone pair electrons with the proton.

+