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IONIC BONDING

Effendy Jurusan Kimia, FMIPA Universitas Negeri Malang (UM) effendy_299@ymail.com


What is chemical bond? Chemical bond or chemical bonding may be defined as the force or, more accurately, the energy, that hold atoms together in chemical compounds. There are two principal classes of chemical bonding, namely ionic bonding and covalent bonding.


Valence electrons Valence electrons are those in the outermost shell of an atom. The remaining electrons are the core electrons.


Valence electron Na (2, 8, 1)

Core electron Core electron Valence electron


Valence electron Cl (2, 8, 7)

Core electron Core electron Valence electron


Lewis symbol Lewis symbol or Lewis dot symbol is the symbol of an element that includes dots represents the valence electrons of an atom of the element. Core electrons are not shown in the Lewis symbol.


For an atom with four or less valence electrons, these electrons are displayed singly around the four sides of the symbol of the element. If there more than four valence electrons, some of their dots are paired.

1

2

13

14

IA

IIA

IIIA

IVA

. .C. .

. :N. . 15 VA

. :O. : 16 VIA

:F .

:Ne :

17

18

VIIA

VIIIA

: :

or

. Be .

: :

Group

Li.

. .B .


The creation of ions is indicated by removing dots from or adding them to Lewis symbol and also by writing the net electric charge of the ion as a right superscript. _

:F:

_

:

:

:

:O :

2

:

_

:

:N:

3

:

:C:

_

:

:

Mg2+

Na+

4

or _

_

:F :

:

:

:

:O :

2

:

:N:

3

:

_

:

:C:

4

:

Mg

2+

:

Na

+

_


Combination of Lewis symbol of cation and anion in ionic compound is a Lewis structure of ionic compound.

: Cl :

_

NaCl

MgO

MgCl2

2 Na

+

:O : :

:

2 : Cl :

_

:

:

Mg

2+

:O :

2

:

:

Mg

2+

:

:

Na

+

Na2O

_ 2

_


The outermost shell of electron configurations of ions or atoms similar to those of noble gases, except helium, always contains eight electrons. This is usually said as an octet configuration. Ions and atoms with electron configuration similar to those of noble gases are said obeying octet configuration or octet rule. These atoms and ions are usually stable.


Classification of ion Ion

Cation

Anion

Simple cation

Polyatomic cation

Na+ Mg2+ Al3+

NH4+ PH4+

Simple anion ClO2N3-

Polyatomic anion NO3SO42PO43-


Classification of ionic compound (1) ionic compound consists of simple cation and simple anion. Examples: NaCl, MgCl2, MgO, K2O (2) ionic compound consists of simple cation and polyatomic anion. Examples: NaNO3, MgSO4, Na2SO4 (3) ionic compound consists of polyatomic cation and simple anion. Examples: NH4Cl, NH4Br, PH4I (4) ionic compound consists of polyatomic cation and polyatomic anion. Examples: NH4NO3, (NH4)2SO4


Ionic bonding An ionic bonding is a chemical bonding formed by the electrostatic attraction between positive and negative ions.


Formation of ionic compound (1) Formation of ionic compounds directly

from cation and anion such as given in the following examples Na2SO4(aq) + BaCl2(aq) → 2NaCl(aq) + BaSO4(s) Na2CO3(aq) + Ba(NO3)2(aq) → 2NaNO3(aq) + BaCO3(s)


(2) Formation of ionic compounds indirectly from elements which involves transfer electron from one element to other element such as given in the following example. Na(g) + Cl(g) → NaCl(g)

+

Na

: Cl : :

:

. Cl :

:

:

Na .

+

an ion pair

_


Formation of ionic compound is accompanied with the change of electron configuration of species involved in the reaction . . Cl : (2, 8, 7)

: Cl : :

:

(2, 8, 1)

Na

:

+

:

Na .

+

_

(2, 8) (2, 8, 8)


Na (2, 8, 1)

Cl (2, 8, 7)

Na+ (2, 8)

Cl- (2, 8, 8)


Formation of ionic compound is also accompanied with the change of the size of species involved in the reaction .

+ Na(g)

Cl(g)

Na+(g)

Cl-(g)


Structure of ionic compound In the gas phase, an ionic compound may exist as an ion pair. Sodium chloride, for example, exists as an ion pair.

Na

+

-

Cl


In the solid phase there are many Na + and Clˉ ions forming regular arrangement of periodic alternating positive and negative ions. The 2-dimension structure of NaCl is as follows. Na+

Na+ Cl-

Cl-

(a)

(b)


The 3-dimension unit cell of NaCl crystal is a face centered cubic (fcc) Face center Corner

Face center Corner

Side center

Side center

(a)

(b) = Cl-

= Na+


Energetic of ionic compound Gaseous ionic compound may be formed from its gaseous atoms if this formation is energetically favorable (accompanied with release of some energy). For the formation taking place at constant pressure, the energy involved is expressed in enthalpy (H) or the heat content of a system at constant pressure. Formation of ionic compound is energetically favorable if it is accompanied with decreasing of enthalpy (Î&#x201D;H < 0; Î&#x201D;H is the enthalpy changing).


Formation of gaseous NaCl from gaseous sodium and chlorine atoms involve three processes: (1) ionization of sodium atom to

sodium ion which is accompanied with absorption of some energy. The energy absorbed is ionization energy (IE). Na(g) â&#x2020;&#x2019; Na+(g) + e

IE = 495 kJ/mol


(2) ionization of chlorine atom to chloride ion

which is accompanied with release of some energy. The energy involved in this process is called electron affinity (EA). Cl(g) + e → Clˉ(g) EA = -348.5 kJ/mol

(3) formation of ionic bonding in an ion pair of

gaseous NaCl which is accompanied with release of some energy.

Na+(g) + Clˉ(g) → NaCl(g) ΔHip = -450.2 kJ/mol ΔHip is enthalpy changing for the formation of ion pair (ip).


The enthalpy changing for the formation of gaseous ionic pair of NaCl (ΔHf, f = formation) from its gaseous atoms can be calculated as follows: Na(g) Cl(g) + e

Na+(g) + e

IE = 495 kJ/mol

Cl-(g)

EA = -348.5 kJ/mol

Na+(g) + Cl-(g)

NaCl(g)

∆ Hip = -450.2 kJ/mol

Na(g) + Cl(g)

NaCl(g)

∆ Hf = -303.7 kJ/mol


Properties of ionic compounds (1) Ionic compounds tend to have high melting point because the ionic bonding in this compound is quite strong. In NaCl crystal, for example, there are many Na+ cations and Clˉ anions where each Na+ ion is attracted to many surrounding Clˉ anions and each Clˉ anion is attracted to many surrounding Na+ cations. For sodium chloride to melt every ionic attraction in the crystal must be overcome. This process requires large amount of energy so that NaCl has high melting point.


Melting point of some alkali halides

Alkali halide

m.p. (째C)

Alkali halide

m.p. (째C)

NaF NaCl NaBr NaI

993 801 747 651

KF KCl KBr KI

846 770 730 686


(2) In the solid state, ionic compounds do not conduct electricity because the attractive force between cations and anions prevent the movement of ions through the crystal. When the solid melted, however, the ions become free to move about and the liquid conducts electricity quite well.


(3) Ionic compounds are usually hard but brittle substances. When struck by a hammer the layer of ions in the crystal moves. The slight movement of a layer of ion within crystal suddenly places ions of the same charge next to each other, and for that instant there are large repulsive forces that split the solid and crystal of ionic compound shatters.


A blow is struck _ +

+

_

_

+

+

_

+

_

+

_

_

+

_

+

+

_

+

_

_

+

_

+

_

+

_

_

+

_

+

+

_

+

_

+

_

+

+

_

_

+

+

_

_

+

+

_

_ +

+

_

_

+

+

_

_

+

+

+

_

_

_

+

+

_

_

_

+

+

_

+

+

_

_

+

+

_

_

+

+

_

_

+

+

_

+

_

(a)

_

+

(b)

_

+

+ _ + _ + _

_

+ _

+ _

+ _

_

_

+ _

_

_ +

_

_

(c)

_

_

_ +

+

+

+

+

_

_

_

_

+

+

+

+

+

+

_

+

+

_

+

+ _

+ _ +

+ _


(4) Ionic compounds dissolves in polar solvent such as water. NaCl, for example, dissolves very easily in water. The solution of ionic compound in water conducts electricity because in the solution ions move freely. Ionic compound does not dissolve in most organic compounds such as acetone, alcohol, ether, and carbon tetrachloride.


Summary (1) Chemical bond or chemical bonding

may be defined as the force or, more accurately, the energy, that hold atoms together in chemical compounds. (2) There are two principal classes of chemical bonding, namely ionic bonding and covalent bonding. (3) Not all of electrons in an atom participate in chemical bonding. For main group elements, only valence electrons play a fundamental role in chemical bonding.


(4) In an ionic compound, the ions present tend to attain an electron configuration similar to those of noble gases or octet configuration. These ions with such electron configuration obey the octet rule. (5) Simple ionic compound consists of simple cation and simple anion. Simple ionic compounds form only between very active metallic elements and very active nonmetallic elements.


(6) An ionic bonding is a chemical bonding formed by the electrostatic attraction between positive and negative ions. (7) Ionic compounds tend to have high melting point, do not conduct electricity in the solid phase, usually hard but brittle substances. Ionic compounds dissolves in polar solvent such as water.

ionic bonding  

ionic bonding is the bonding of ions to form molecule.

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