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Chemistry

Lewis Symbol of an Atom

Constructed by drawing the chemical symbol of atoms and valence electrons using dots around them as shown below. 

Chlorine atom is in group VIIA and has 7 valence electrons.

Octet Rule

  • The main group of atoms (group 1,2 (s-block) and 13-18 (p-block)) with fewer than eight electrons in their valence react with other atoms to form compounds. This allows them to have eight electrons in each atom’s valence shell that are forming the compound.
  • This rule applies to the atoms with an atomic number below 20

  • Only the electrons from s and p shells are involved in this rule.

Note: if you are taking basic level chemistry you might not be familiar with s and p shells. At this level just remember octet rule implies the atoms’ tendency to have a valence shell with eight electrons. This can be attained by either losing electrons, gaining electrons, or sharing electrons with other atoms.

Duet Rules

This rule states that Hydrogen and Helium can have only two electrons in their valence shells. 

Exceptions

There are fewer exceptions to the above duet and octet rules.

Examples

Boron – forms compounds with other atoms having less than eight valence electrons as the central atom.

Boron trifluoride (BF3)

In the above example, Boron as the center has only six electrons in its valence shells and Flourine has eight electrons satisfying the octet rule.

Beryllium: forms a compound with Hydrogen the fewer than eight electrons in its valence shell. The lewis diagram of such an example is given below. 

Beryllium Hydride (BeH2)

In this example, Beryllium has only four valence electrons in its valence shells and hydrogen has two electrons satisfying the duet rule. 

On the other hand, elements from third-row and beyond in the periodic table exceed the octet rule and are observed to have more than eight electrons surrounding the central atom.

Examples :

  • Tetrachloroiodide ion (ICl₄⁻)

  • Phosphorus Pentafluoride (PF₅)

Ionic Bonds

An ionic bond is formed when there is a complete transfer of valence electrons from one atom to the other in order to satisfy the octet rule where atoms tend to achieve noble gas configuration. The atom that gives up electrons is known as an electron donor (metal) and the atom that accepts electrons is the electron acceptor (nonmetals).  As a result of electron transfer, one atom (metal) becomes a positively charged cation and the other (nonmetal) becomes a negatively charged anion. The electrostatic attraction between these positively charged ions and negative charge ions forms the linkage known as an ionic bond

In the following example, the sodium atom is donating one electron to become a cation, and the chlorine atom is accepting that electron to become an anion.  

Na in the case forms Na+ ion is because it has one valence electron and would lose that to obey the octet rule. 

Cl has 7 valence electrons and accepts that one electron lost by Na to become Cl and satisfies the octet rule of having eight electrons in its valence shell. 

When Naand Cl to form NaCl their charges should cancel out to give zero charge on the final ionic compound in this case +1 charge from Naand -1 charge from Cladds up to give zero total charge. 

Covalent Bonds

Covalent bonds are formed when atoms share electrons among themselves. By sharing electrons atoms can satisfy the octet rule. 

Example: Chlorine gas (Cl2)

Depending on the number of electrons shared the bonds can be: 

  • Single bond: sharing one electron
  • Double bond: sharing two electrons
  • Triple bond: sharing three electrons

 

 

Polarity of Bonds

Depending on the electronegativity difference of the atoms in a compound, the bond formed can be one of the following

  • Ionic bond
  • Polar covalent bond
  • Nonpolar covalent bonds

Electronegativity is an atom's ability to attract electrons when they form compounds. The electronegativity difference among atoms of a compound determines the polarity of the bond. 

 

 

Examples:

  • Sodium Bromide (NaBr2

         ΔEN = ENBr – ENNa = 2.8-0.8 = 1.9 = Ionic bond

  • Water (H2O)

        ΔEN = ENO – ENH = 3.44 – 2.20 = 1.24 = Polar Covalent

  • Methane (CH4)

        ΔEN = ENC – ENH = 2.55 – 2.20 = 0.35 = Nonpolar Covalent

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