Molecular structures

Question 1

What information does a Lewis structure provide?

Answer 1

1)Arrangement of Electrons: It shows how valence electrons are arranged between atoms, indicating which electrons are involved in bonding and which remain as lone pairs.
2) Bonding Patterns: The structure highlights the types of bonds (single, double, or triple) formed between atoms.
3) Formal Charges: It allows for the calculation of formal charges on atoms, helping to determine the most stable arrangement of atoms within the molecule.
4) Molecular Shape Prediction: Although not directly showing molecular geometry, Lewis structures are the starting point for using the VSEPR model to predict the 3D shape of the molecule.

Question 2

What is the octet rule in Lewis structures?

Answer 2

The electrons in a covalent compound are distributed in such a way that each main-group element (except hydrogen) is surrounded by eight electrons (an octet of electrons). Hydrogen has two electrons in such a structure. When the octet rule
is satisfied, the atom attains the special stability of a noble-gas shell.

Question 3

How is the formal charge of an atom in a Lewis structure calculated, and why is it important?

Answer 3

formal charge =number of valence electrons - number of electrons in lone pairs - 1/2
(number of electrons in bonding pairs)
Formal charge is important because it helps determine the most stable Lewis structure, which typically has formal charges close to zero for all atoms. Minimizing formal charges and ensuring the charges are located on the most electronegative atoms can also indicate the most likely structure for a molecule or ion.

Question 4

How does the VSEPR theory help in predicting the molecular geometry of a molecule?

Answer 4

The Valence Shell Electron Pair Repulsion (VSEPR) theory helps predict the molecular geometry of a molecule by considering the repulsion between electron pairs around a central atom. According to VSEPR theory, electron pairs (both bonding and lone pairs) arrange themselves as far apart as possible to minimize repulsion. This arrangement determines the overall shape of the molecule. For example, in methane (CH₄), the central carbon atom has four bonding pairs of electrons, which repel each other and result in a tetrahedral geometry with bond angles of 109.5°.

Question 5

Determine the molecular geometries for the given molecules. Water (H₂O),
Carbon Dioxide (CO₂), Ammonia (NH₃).

Answer 5

Water (H₂O)
* Molecular Geometry: Bent
* Explanation: The oxygen atom has two bonding pairs (with hydrogen) and two lone pairs, resulting in a bent shape with a bond angle of approximately 104.5°.
Carbon Dioxide (CO₂)
* Molecular Geometry: Linear
* Explanation: The carbon atom is bonded to two oxygen atoms with double bonds, and there are no lone pairs on the carbon. The molecule has a linear shape with a bond angle of 180°.
Ammonia (NH₃)
* Molecular Geometry: Trigonal pyramidal
* Explanation: The nitrogen atom has three bonding pairs (with hydrogen) and one lone pair, resulting in a trigonal pyramidal shape with a bond angle of approximately 107°.