Chemistry encompasses the study of elements and the various species ( compounds/ions/complexes etc) which are formed from these elements. Thus, it is imperative to understand how atoms are held together in molecules , what forces hold these atoms together and what factors are responsible of specific orientations of molecules. Let us begin our discussion on this very fundamental topic in chemistry – THE CHEMICAL BOND.
Types of bonds.
In Chemistry,we primarily find two types of bonding –
sharing or transfer of electrons.
NO sharing or transfer of electrons.
Strong force of attraction
Weak force of attraction
e.g.- Ionic,Covalent ,Co-ordinate covalent,Metallic bonds
e.g.- Hydrogen bonds, vander Waal’s forces, dipole interactions.
In this post we start learning about ionic bonding in detail.
P1.THE IONIC BOND.
This bond is due to electrostatic (coulombic) force of attraction between opposite charged ions.
ELECTRO = electric charge i.e positive and negative charges
STATIC = stationary , which does not move.
electrostatic attraction = Coulombic attraction.
Electrostatic attraction was discovered by Charles Augustin de Coulomb (French Physicist) and thus this force is also called as Coulombic force of attraction.
Note that while writing we always write ‘C’ capital as it’s a scientist’s name.
So, when we say electrostatic/Coulombic force of attraction , we refer to +ve and -ve charges on ions. e.g.- Na+, Mg+,(SO4)2- , Cl– etc .
By observation , we know that like charges repel each other and unlike charges attract. This force of attraction between ions of opposite charge is the ionic bond.
How are ions formed?
Generally atoms prefer to have 8 electrons in their valence shell (except hydrogen and helium).This tendency of atoms is referred to as ‘octet stability‘. This stability comes from the fact that when the octet is complete, the orbitals (s and p orbitals) are completely filled with electrons. It’s like when hunger is satiated, a person becomes satisfied and is no more hungry.
The elements placed at the extreme left of the periodic i.e metals can lose valence electrons easily and attain an octet , thus showing a greater tendency to form positive ions.Similarly, the elements placed at the extreme right (except the zero group i.e. noble gases) , have a tendency to accept electrons and form negative ions. These opposite charges attract each other and form the ionic bond. Ionic bonding involves complete transfer of electrons. It is a chemical bond.
- Species which looses electron/s → Cation (+ve ion) → electropositive elements form cations e.g. – metals like Na+, Mg2+,Ca2+,K+ etc.
- Species which gains electron/s → Anion (-ve ion) → electronegative elements form anions. e.g.- non-metals like Cl–,Br–, CN– , S2- etc.
1 positive charge indicates loss of one electron.
∴Na+ → one electron lost from sodium atom.
Ca2+→ two electrons lost from calcium atom.
Al3+ → three electrons lost from aluminium atom.
Similarly, 1 negative charge indicates gain of one electron from another species.
∴Cl–→ one electron gained by chloride atom.
S2–→ two electrons gained by sulphur atom.
Let us study a few examples of molecules with ionic bonding –
Example 1 – Sodium chloride (NaCl molecule) – TABLE SALT.
Sodium Atom Na → electronic configuration is (2,8,1) i.e it has one electron in its valence/outer shell. If it looses this electron, it will have eight electrons in its outer shell and so will have octet stability.So, it readily gives up this electron to form a cation.
Na → Na+ + e–.
Chlorine Atom Cl → electronic configuration is (2,8,7) i.e it has seven electrons in its valence/outer shell. If it gains one electron, it will have eight electrons in its outer shell and so will have octet stability.So, it readily accepts this electron from a sodium atom to form chloride ion.
Cl+ e–→ Cl –.
The sodium and chloride ions have opposite charges on them and so they attract each other forming Na+Cl– molecule.
The energy changes that take place during the formation of this ionic bond can be studied with the BORN – HABER CYCLE.
THE BORN – HABER CYCLE.
We just saw that formation of ions and ionic bond between opposite charged ions is a favourable process – it is like a win-win situation for both ions. The energetics of this bond formation also shows us how this process is energetically favoured. To understand this we must first understand the folllowing –
Less energy ⇒ More stability.
So if energy of a system decreases in a reaction , the reaction is energetically favoured. To understand the energetics of any system, we need to know a few thermodynamic (relating to heat ; thermo = heat) terms like internal energy ,enthalpy, lattice energy etc. We shall study these parameters in detail ,when we begin our discussion on thermodynamics. For now, we shall try to get some basic understanding of these terms.
1)Internal Energy (U) – All the energy of a system is called it’s internal energy . Suppose, we consider a system ,enclosed in a box, then the internal energy is the sum of –
kinetic energy of all the particles in that system (Energy associated with movement of the particles) +
potential energy(Energy associated with position of the particle) +
bond energy (energy in various bonds present in the system) etc.
However, internal energy changes with change in temperature and pressure. e.g.- As the temperature increases, the molecules in the system start to move around more rapidly (as they get more heat energy) .Thus, their kinetic energy increases and so does the internal energy of the system.
So when we study energy changes in a reaction, this parameter(internal energy) is not useful as in a reaction there are temperature changes(endothermic/exothermic reactions) and pressure changes involved. Thus, we use another parameter – ENTHALPY- while talking about energy changes in a reaction.
2)Enthalpy (H) – This term gives you the total heat content of the system.It takes into account the pressure and volume changes that ocuur during the reaction.
H = U + PV , where,
H ⇒ Enthalpy.
U ⇒ Internal energy.
P ⇒ Pressure.
V ⇒ Volume.
3) Dissociation energy(ΔH dissociation)- The energy required to dissociate a compound is called as dissociation energy. Dissociation of a compound is always an endothermic process and requires an input of energy.
4) Sublimation energy(ΔHsublimation)– The energy required by a substance to change phase from solid to gas, is called as sublimation energy.
5)Heat of formation( ΔHformation)-The energy change during the formation of a compound from its elements is known as heat of formation.
The stages of formation of an ionic compound MX are as follows –
- Msolid + ΔHsublimation → Mvapor (1) The electropositive element M is converted from solid to vapor phase by providing heat of sublimation to it.
- Mvapor + I.E (M) → M+ +e– (2) In vapor phase, the valence electron/s is/are lost to form positive ion.Energy associated with this process is ionisation energy(we have already studied this parameter)
- 1/2 X2(g) +Hdissociation → 1/2 X2(g) → X(g) (3) The electronegative element dissociates.
- X(g) + e– → X– (g) +EA(X) (4) Negative ion is formed with release of energy equivalent to electron affinity (EA) of X.
- M+ + X–(g) → MX(s) + ΔHformation (5) The two ions combine to form the ionic solid MX. Heat of formation is released in this process.As heat is given out, the process is exothermic and this shows that the energy of the system decreases(as heat is given out), making it more stable.
- M+(g) + X–(g) → MX (s) (6) Lattice enthalpy (U): The enthalpy change during the formation of one mole of sodium chloride from its constituent ions is called as lattice energy of lattice enthalpy. It is an exothermic step.
ΔHsublimation , I.E ,ΔHdissociation and EA , can be calculated experimentally.
WE HAVE TO CALCULATE ΔHformation .
How do we do that from the experimental data? We use Hess’s Law of Constant Heat Summation /Hess’s Law –
Regardless of the multiple stages or steps of a reaction, the total enthalpy(energy) change for the reaction is the sum of all changes.
ΔHsublimation , I.E ,ΔHdissociation and EA quantities give us energy changes at respective stages of the reaction and their values can be determined experimentally.Thus, we can calculate the total energy change by just adding them up.
∴ΔHformation = ΔHsublimation + I.E + ΔHdissociation + EA
e.g.– formation of NaCl molecule –
Na(s) → Na(g) +107kJ mol -1
Na(g) → Na+(g) + e– +495 kJ mol -1
1⁄2Cl2(g) → Cl(g) +122 kJ mol -1
Cl(g) + e– → Cl -(g) –349 kJ mol -1
Na+(g) + Cl–(g) → NaCl (s) U = -786 kj/mol
Na+(g) + Cl-(g) → NaCl(s) ΔHformation to be calculated
According to Hess’s law –
ΔHformation= 107.3 +495.8+122 – 348.6 -786 = – 411 Kj/mol.
∴Na(s) + 1⁄2Cl2(g) → NaCl(s) – 411 kJ mol-1.
Ionic compounds are generally solids at room temperature and have high melting points.This is due to the strong electrostatic force which binds the positive and negative ions together in a crystal lattice structure.
The melting point of NaCl is higher than potassium nitrate(KNO3) 304oC, as the anion (NO3–)is large compared to Cl– ion and the negative charge dispersed over a larger volume. This weakens the attractive forces and lowers the melting point. Ammonium sulfate (NH4SO4), where both ions –(NH4+) and (SO4–)are large, has the lowest melting point (235oC).
∴ As size of ions ↑ M.P ↓
Ionic solids are insulators but they conduct electricity when dissolved in water as they dissociate in water.Molten salts also conduct electricity.
In our next post we shall study covalent bonds and their properties. Till then ,
Be a perpetual student of life and keep learning..
Good Day !
References and further reading –
5.Modern aspects of solid state Chemistry by C.N.R.Rao
Image Sources –
1.By http://leradiofil.com/COULOMB.htm, Public Domain, https://commons.wikimedia.org/w/index.php?curid=21081