Chemistry Lesson 1: Dot Structures and VSEPR

Questions and Answers

In the second period of the periodic table, it is possible to exceed 8 electrons in the valence shell, but an atom may not have less than 8.

False

Any molecule with an odd number of valence electrons cannot give a full octet to each of its component atoms.

True

An atom X is neutral and has 3 valence electrons. Which of the following atoms could X be?

Ga (correct)

How many valence electrons does Silicon have?

4 Signup and view all the answers

How many valence electrons does Fluorine have?

7 Signup and view all the answers

How many total valence electrons does SiF4 have?

32 Signup and view all the answers

Which should be the central atom for SiF4 and why?

Si Signup and view all the answers

How do you assign formal charges to atoms in a molecule?

Formal Charge = (# of valence electrons in free atom) - [Lone pair electrons + 1/2 shared electrons] Signup and view all the answers

In a hypothetical molecule, a Nitrogen has two sets of lone pair electrons and two covalent bonds. What would its formal charge be?

-1 Signup and view all the answers

In calculating formal charges, a double bond is treated exactly the same as a single bond when doing calculations.

False Signup and view all the answers

Nitrate has several resonance structures. What does this mean?

The extra electrons are shared between all three bonds all at the same time Signup and view all the answers

Which of the following is NOT one of the main tenets of VSEPR theory?

The electron and molecular geometries will always be the same, since they are both predicted by electron placement. Signup and view all the answers

What is the name of the number of atoms that will bond to the central atoms, and is often used when describing lattices?

Coordination number Signup and view all the answers

What is the purpose of VSEPR?

To help represent and predict the shape of a molecule. Signup and view all the answers

In VSEPR theory, a double bond is treated exactly the same as a single bond when determining electron and molecular shapes.

True Signup and view all the answers

What is the difference between electron geometry and molecular geometry?

Electron geometry is the shape of the electron clouds about the central atom. Molecular geometry is the shape of the atoms about the central atom. Signup and view all the answers

How many 'electron clouds' are surrounding the central atom of BeCl2?

2 Signup and view all the answers

What is the electron geometry of BeCl2? Molecular geometry?

Both are linear. Signup and view all the answers

Why doesn't Chlorine share its lone pair of electrons with Beryllium in BeCl2?

Because Beryllium has a neutral formal charge with only one bond to each Chlorine and would be less stable if it received more electrons. Signup and view all the answers

What is the electron geometry of BF3? Molecular geometry?

Both are trigonal planar. Signup and view all the answers

What is the electron geometry of CH4? Molecular geometry?

Both are tetrahedral. Signup and view all the answers

For a tetrahedral compound, which of the following is the ideal bond angle (in degrees)?

109.5 Signup and view all the answers

What is the electron geometry of H2O? Molecular geometry?

Electron geometry is tetrahedral, molecular geometry is bent. Signup and view all the answers

What is the electron geometry of NH3? Molecular geometry?

Electron geometry is tetrahedral, molecular geometry is trigonal pyramidal. Signup and view all the answers

What is the electron geometry of CO2? Molecular geometry?

Both are linear. Signup and view all the answers

What is the electron geometry of O3? Molecular geometry?

Electron geometry is trigonal planar, molecular geometry is bent. Signup and view all the answers

What is the electron geometry of Carbonate? Molecular geometry?

Both are trigonal planar. Signup and view all the answers

What is the electron geometry of Sulfate? Molecular geometry?

Both are tetrahedral. Signup and view all the answers

Study Notes

Octet Rule and Valence Electrons

In the second period, atoms can have fewer than 8 valence electrons but not more than 8, following the octet rule.
Odd number of valence electrons in a molecule prevents giving a full octet to each atom.

Identifying Atoms and Valence Electrons

Gallium (Ga) is a neutral atom with 3 valence electrons, being part of group 3A.
Silicon possesses 4 valence electrons, while Fluorine has 7 valence electrons.

Calculating Total Valence Electrons

For SiF4, total valence electrons calculated as (7x4) + 4 = 32.

Central Atoms and Dot Structures

In SiF4, Silicon is the central atom as it is less electronegative than Fluorine.
Carbon typically forms double bonds to satisfy the octet rule when required.

Formal Charges and Stability

Formal charge is calculated as: Valence electrons - (Lone pair electrons + 1/2 shared electrons).
Resonance structures indicate that extra electrons are shared among bonds concurrently.

VSEPR Theory Fundamentals

VSEPR theory predicts molecular shapes based on electron pair repulsion.
Electron clouds around a central atom determine the geometry; lone pairs exert greater repulsion than bonding pairs.

Electron and Molecular Geometry

Electron geometry refers to the arrangement of electron clouds; molecular geometry describes the arrangement of bonded atoms.
Common geometries include linear, trigonal planar, tetrahedral, bent/angular, and trigonal pyramidal.

Bond Angles and Electron Clouds

Ideal bond angles: tetrahedral (109.5 degrees), trigonal planar (120 degrees), with each geometry having distinctive arrangements.
BeCl2 has linear electron and molecular geometries, while NH3 is tetrahedral in electron geometry but trigonal pyramidal in molecular geometry due to lone pairs.

Resonance Structures

Nitrate exhibits resonance, representing multiple possible structures while actually sharing electrons throughout.

Key Differentiations

Coordination number describes how many atoms bond to a central atom, critical in lattice illustrations.
Not all molecules adhere strictly to ideal bond angles due to varying repulsion from lone pairs.

Summary of Molecular Geometries

Carbonate and sulfate both exhibit trigonal planar and tetrahedral geometries, respectively.
Understanding these shapes and the underlying principles is crucial for predicting molecular behavior and reactivity.

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Description

Test your knowledge on dot structures and the VSEPR theory in this flashcard quiz. Learn about the concepts of the octet rule and electron configurations in the periodic table. This lesson covers essential foundational topics in chemistry for better understanding.