Types of Chemical Bonds - Difference between ionic bond and covalent bond

Chemical Bonding

 

v The characteristics of molecules depend on the atoms it contains and the way the atoms are attached.

 

v In this article we discuss principles of structure of a molecule and chemical bonding.

 

v Each element possesses a unique atomic number Z, which is equal to the number of protons in the nucleus of element.

 

v A neutral atom has equal numbers of protons and electrons, protons are positively charged, while electrons, are negatively charged species.

 

v The attraction force between atoms in a molecule is called chemical bond.  Chemical bonds can be categorized into two main types cited as ionic and covalent bonds.

v Lets learn about chemical bond in the following notes

  

IONIC BONDS

 

v Ionic bond is the force of attraction between oppositely charged species (called ions).

 

v Cations are ions that are positively charged; while anions are those species that are negatively charged.

 

v During formation of ionic bond, elements in the left side of the periodic table (usually metals) typically lose electrons, to form a cation that has the same electronic configuration as the nearest noble gas.

 

v For example, loss of an electron from sodium atom, gives the positively sodium ion Na⁺, which has the same electronic configuration as neon (Ne).

v The ability of atom to loss electron and form a cation is determined by a property called ionization energy, which is defined as the energy that must be added to any atom in order to remove one of its electrons.

 

v Elements at the right of the periodic table (nonmetals) tend to gain electrons during chemical bonding to reach the electron configuration of the next higher noble gas. Adding an electron to chlorine atom, for example, gives the anion Cl^-, which has the same electronic configuration as the noble gas argon (Ar).

 

v The ability of atoms to gain electron and form an anion is determined by a property called electron affinity which is defined as the energy change for addition of an electron to an atom.

 

v Transfer of an electron from a sodium atom to a chlorine atom produces a sodium cation and a chloride anion, both of which have a noble gas electron configuration:

 

 

v These ions are attracted to each other by electrostatic attraction force forming an ionic bonds.

 

COVALENT BONDS

 

v The covalent bonds formation involves the sharing of valence electrons between atoms to achieve chemical bonding.

 

 

v The Lewis model limits second-row elements (Li, Be, B, C, N, O, F, Ne) to a total of 8 electrons (shared plus free electrons) in their valence shells. Hydrogen is limited to 2.

 

v Most of the elements that we’ll encounter in this text obey the octet rule

 

v Octet Rule states that in forming compounds atoms tend to gain, lose, or share electrons to give a stable electronic configuration containing eight electrons in their valence shell.

  

v When the octet rule is obeyed for carbon, nitrogen, oxygen, and fluorine atoms, they have an electronic configuration like the noble gas neon.

 

POLAR COVALENT BONDS AND ELECTRONEGATIVITY

 

v Electrons in covalent bonds are not necessarily shared equally by the two atoms that they bond.

 

v If one atom has a greater tendency to attract electrons toward itself than the other, we say the electron sharing is polarized and is not equal, and the bond is referred to as a polar covalent bond.

 

v Hydrogen fluorine HF, for example, has a polar covalent bond. Because chlorine atom attracts electrons more strongly than hydrogen atom, the electrons in the H-F bond are pulled toward fluorine, giving it a partial negative charge, and away from hydrogen giving it a partial positive charge as illustrated in the following picture.

 

 

  

v The tendency of an atom to attract the electrons in a covalent bond toward itself is known to as electronegativity.

 

v Electronegativity increases across a row in the periodic table. Electronegativity decreases in going down a column in periodic table.

 

The dipole moment (µ):

 

v Dipole moment (µ) is a method to express bond polarity or polarity of molecule.

 

v Centers of positive and negative charge that are separated from each other by a distance constitute a dipole. The dipole moment (µ) of a molecule is equal to the charge e (either the positive or the negative charge, they must be equal) multiplied by the distance between these charges.

 

How to determine type of bond in a molecule

 

v The type of the bond Ionic, pure covalent or polar covalent is determined according to the difference in electronegativity between the two atoms as follow:

a)     If the difference in electronegativity 0-0.3, then there will be equal sharing and the bond is pure covalent bond

b)     If the difference in electronegativity 0.3-2, then there will be unequal sharing and the bond is polar covalent bond

c)     If the difference in electronegativity is greater than 2, then the bond will be ionic bond.