Lesson 6: Lewis Structure for Covalent Compounds

Who first suggested in 1916 that a molecule could be held together by a shared pair of electrons?

Gilbert Lewis.

hat type of chemical bonding occurs between two non-metallic atoms?

Covalent bonding.

What characterizes covalent bonding?

The sharing of pairs of electrons between atoms.

What are electron pairs shared in covalent bonding called?

Shared pairs or bonding pairs.

How is the covalent bond in a hydrogen molecule (H₂) typically represented?

By a single line, H – H.

In covalent bonding, what attracts the shared electrons?

The nuclei of both atoms.

What happens when two atoms are held together by one pair of electrons?

A single bond forms.

What is an example of a molecule with a double bond?

CO₂ (between O and C).

What is an example of a molecule with a triple bond?

N₂ (between the two nitrogen atoms).

How many pairs of electrons are shared in a triple bond?

Three pairs.

which elements does the octet rule work best?

Second-period elements.

What are the three main categories of exceptions to the octet rule?

(a) Incomplete octet, (b) Odd number of electrons, (c) Expanded octet.

resonance structure

The double sided arrow shows that the structures are resonance structures. A resonance structure is one of two or more Lewis structures for a molecule that cannot be represented accurately by only one Lewis structure.

What symbol indicates resonance between Lewis structures?

A double-sided arrow (↔).

Define resonance structure.

One of two or more Lewis structures for a molecule that cannot be represented accurately by only one Lewis structure.

Incomplete octet

An example of a molecule with incomplete octet is BeH2, beryllium hydride. Its structure is

Give an example of a molecule with an incomplete octet.

BeH₂ (beryllium hydride).

How many electrons surround Be in BeH₂?

Only 4 electrons (not 8).

Besides beryllium, which other elements form molecules with incomplete octets?

Boron and Aluminum.

Molecules with Odd Number of Electrons

Examples are nitric oxide, NO, and nitrogen dioxide, NO2. Their Lewis structures are shown below. Because they have an odd number of electrons, the octet rule cannot be followed. The odd numbered molecules are sometimes referred to as radicals. They are generally highly reactive.

Give two examples of molecules with an odd number of electrons.

Nitric oxide (NO) and nitrogen dioxide (NO₂).

Why can’t NO and NO₂ follow the octet rule?

Because they have an odd number of electrons.

What are molecules with odd numbers of electrons sometimes called?

Radicals

What is a typical property of radicals?

They are generally highly reactive.

Expanded Octets

Atoms belonging to the second period elements cannot have more than eight valence electrons around the central atom because they only have the 1s, 2s and 2p subshells which together limit the number of electrons to eight. This is different for atoms of elements in the 3rd period and beyond.

Why can second-period elements not have expanded octets?

They only have 1s, 2s, and 2p subshells, which limit them to 8 electrons.

Why can third-period elements and beyond have expanded octets?

They have 3d orbitals that can participate in bonding.

Give an example of a molecule with an expanded octet.

SF₆ (sulfur hexafluoride).

How many electrons surround the central sulfur atom in SF₆?

12 electrons.