School : SMA x
Subject : Chemistry
Grade/semester : XI / 1
Time Allotment : 25 minutes
II. Standard Competence
Understanding atomic structure to predict periodicity of elements, molecular structure, and properties of compounds.
III. Basic Competence
- Explaining theory of electron pair around atomic nucleus and theory of hybridization to predict molecular shapes
1.2.1 Predicting the shapes of molecules according to pair electron theory
V. Learning Outcomes
- Students are able to describe the VSEPR theory.
- Students are able to determine the VSEPR model of a given molecule.
- Students are predict molecular shape by using VSEPR theory.
VI. Teaching and Learning Topics
- VSEPR Theory
- Molecular shapes
The shape of a molecule deal with the position of atoms in the molecule. In this case shapes of molecules describe the position of atoms in the three dimensional space and the magnitude of the bond angle which occurs at the covalent bond in a molecule.
VSEPR (Valence Shell Electron Pair Repulsion) Theory
VSEPR (Valence Shell Electron Pair Repulsion) Theory is a concept used to predict geometric shape formed by atoms in molecules which are covalently bonded. This theory is based on the idea that all electron pairs which are directly attached to an atom, those are bonded electron pairs and non-bonded electron pairs (free electron pairs) at around the central atom, will arrange their position to be as far away from each other as possible. Bonded electron pairs are valence electron share by atoms in a molecule, while non-bonded electron pairs or free electron pairs are valence electrons in which a molecule at locations where those electrons are not involved in the bonding process.
According to VSEPR theory, non-bonded electron pairs or free electron pairs have a greater repulsion than bonded electron pairs, so these non-bonded electron pairs occupy the larger space than bonded electron pairs. They will adopt a shape so repulsions between those electron pair will be evenly distributed and at a minimum. As a result of these repulsions, atoms will form the certain geometries; those are linear, trigonal planar, tetrahedral, trigonal bipyramidal, and octahedral shapes.
The basic ideas of the valence shell electron pair repulsion (VSEPR) theory are:
Each set of valence shell electrons on a central atom is significant. The sets of valence shell electrons on the central atom repel one another. They are arranged about the central atom so that repulsions among them are as small as possible.
3. Predicting Shape of Molecule Using VSEPR Theory
The five basic molecular shapes.
- a. Linier
A linier arrangement of atoms occurs when they are all in a straight line. The angel formed between two bonds that go to the same central atom, which we call the bond angle is 180o.
- b. Planar triangular
A planar triangular arrangement of four atoms has them all in the same plane. The central atom is surrounded by three others located at the corners of a triangle. The bond angels are all 120o.
- c. Tetrahedral
A tetrahedron is a four-sided pyramid having equilateral triangles as faces. In a tetrahedral molecule, the central atom is located in the center of this tetrahedron and four other atoms are located at the corners. The bond angels are all equal and have values of 109.5o.
- d. Trigonal bipyramid
A trigonal bipyramid consist of two triangular pyramids (similar to tetrahedrons) that share a common face. In a trigonal bipyramid molecule, a central atom is surrounded by five other. The central atom is located at the five corners. In this kind of molecule, the bond angels are not the same. Between any two bonds that lie in the central triangular plane, the bond angel is 120o. The angle is only 90o between a bond in the central triangular plane and a bond that points to the top or bottom of the trigonal bipyramid.
- e. Octahedral
An octahedron is a geometrical figure that has eight faces. We can think of it as two square pyramids that share a common square base. Notice that octahedron has only six corners even thought it has eight faces. In an octahedral molecule, the central atom is surrounded by six others. The central atom is located in the center of the square plane that passes through the middle of the octahedron. The six atom bonded to it are at the six corners of the octahedron. The angel between any pair of adjacent bonds is the same, and has a value of 90o.
Molecular shape will as same as with the electron geometric in central atom if there were not non-bonding electron. The steps to predict molecular shape such as:
- Write down the electron configuration each atom in molecule
- Determine the valence electron all atoms in molecule
- Draw electron-dot structure (Lewis structure)
- Determine bonded electron pairs and lone pairs electron of central atom
- Determine molecular shape
The formula of electron pairs in a molecule is symbolized as follows.
With A = central atom
X = bonded electron pairs
E = lone pairs electron
n = the number of bonded electron pairs
m = the number of lone pairs electron
Example : H2O molecule
Electron configuration of 8O = 1s2 2s2 2p4 (valence electron 6)
Electron configuration of 1H = 1s1 (valence electron 1)
One atom O are bonded with 2 atoms H with lewis structure such as =
Bonded electron pairs : 2
Lone pairs electron : 2
Formula of electron pairs : AX2E2
Molecular shapes : bent or angular (V)
VII. Teaching-Learning Method
1. Class discussion (use visualization)
2. Ask and Question
VIII. Teaching and Learning Process
1. Preparation (± 2 minute)
a) Greetings and Introductions
b) Checking students attendance
c) Presenting the learning outcomes and the assessment procedure
d) Apperception and motivation (focusing) about lewis structure
2. Mains Activities (± 20 minutes)
|Teacher Activities||Student Activity||Time Allocation|
|Motivate the student with some questions related to the topics that will learn.||Students answer the teacher question.||5 minutes|
|– Presenting the topics, collecting information and problem that have students- Guiding the students to investigate understanding the shape of molecule according to VSEPR theory and hybridization theory
– Observing the student’s activity
|Paying a good attention to the teacher’s explanation,Give information answer the teacher’s question||15 minutes|
|– Giving an explanation if there is misconception- Ask the student to make a retention about the matter||Make a retention about the matter||5 minutes|
3. End Activities (± 3 minutes)
a. Students helped by the teacher conclude the topic
b. Giving homework
c. Giving information of the next topic
IX. TEACHING AND LEARNING RESOURCES
1. Chemistry book :
a. Brady, James E and Gerard E Humiston. 1982.General Chemistry Principles and Structure. New York: John Wiley and Sons.
b. Chang, Raymond. 2005. Kimia Dasar Konsep Konsep Inti Jilid II. Jakarta: Erlangga.
c. Parning, Horale, Tiopan. 2008. Kimia SMA XI Semester Pertama. Jakarta: Yudistira.
d. Sunardi. 2008. Kimia Bilingual Kelas XI SMA. Bandung: Yrama Widya.
2. Power Point
3. Video related with the topic
b. LCD Projector
c. Chemistry Books
1. Evaluation Method : Test, individual assignment.
2. Type of evaluation : Essay and multiple choice test
3. Instrument : cognitive and affective assessment rubric