Lewis DOT Structure AND Vsepr Theory PDF

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LAB PRACTICAL 3 LEWIS DOT STRUCTURE AND VSEPR THEORY BSCH 1222 DATE PRACTICAL: MARCH 15, 2021

1.0 INTRODUCTION OBJECTIVES 1. To understand theory behind Lewis structure and Valence Shell Electron Pair Repulsion Theory. 2. To experience molecule’s geometrical shapes by the model that has been built. The chemical and physical properties of a chemical substance are often influenced by the shape of the molecules that make up the substance. The molecular form influences solubility, density, boiling and melting points, dipole moment, and reactivity. Lewis structures show molecules in two dimensions, but since molecules are three-dimensional, the Lewis views are fundamentally deceptive. The Valence Shell Electron Pair Repulsion Theory (VSEPR) is a method for predicting a molecule's shape based on its chemical formula and electron distribution as depicted in the Lewis structure. The theory of reducing electron repulsion between valence electrons by organising each group of valence electrons around the central atom as far away from the other groups underpins the distribution of atoms in three-dimensional space. Single bonds, double bonds, triple bonds, and even lone electrons are all forms of electron groups. These arrangements produce distinct molecular shapes that are solely determined by the relative locations of the atomic nuclei. The aim of this experiment is to build a model of a molecule to experience its geometrical shapes. There will be three parts of the experiment. The first part will be about Lewis structure of atom while the second part is about Lewis structure of molecules. The last part is about VSEPR. All part required to build the atoms or molecules for each parts model using molecules model kit. From the model, we got to know the shape for the molecules that required for the last part of the lab. The model of the molecules will be evaluating and observe after each part.

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1.2 APPARATUS AND CHEMICALS NO 1

APPARATUS Ball and spring connector

Table 1.0. Data table of apparatus and chemicals used in this lab.

2.0 PROCEDURE OF THE EXPERIMENT The first part of the lab was about the Lewis structure of atom. Atoms in table 2.0 was built using molecular model kit. The total number of electrons in the atom and the number of valence electrons of each atoms in table 2.0 was identified and recorded. Then, dots around the element symbol were drawn in the first column on table 2.0 that represent the valence electrons as a Lewis structure. The number of lone pairs of electrons on each atom and the number of unpaired electron sides were counted and recorded in table 2.0. The use model was set to identify the colour representing each element, then the number of holes in the atom was counted and recorded in table 2.0. The second part of the lab was about the Lewis structure of molecules. Each diagram in table 3.0 was built using the modelling set. The structural formula of the molecules was drawn on table 3.0. The bonds in the same angles and spacing was arranged following the model built. The chemical formula of each molecule was written in the same table. The last part of the lab was about Valence Shell Electron Pair Repulsion (VSEPR). Lewis structure for each chemical formula was drawn in table 4.0. Model set was used to create the molecule model for each chemical formula in the table. Then, the bonding and non-bonding electron domains around the central atom was identified and recorded in table 4.0. The molecular shape of the central atom was also recorded in the table 4.0.

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3.0 RESULTS AND OBSERVATIONS i. Atom symbol

Total number of electrons in atom

Number of valence electrons

Number of bonds that can form

Model colour for atom

Number of holes on model atom

1

1

1

White

1

6

4

4

Black

4

7

7

3

Blue

4

8

5

2

Red

2

15

5

3

Purple

3

17

6

1

Green

1

Table 2.0. Data table from Lewis structure of the atoms. ii. Lewis structure

Structural formula

Chemical formula

CH4 Cl2

N3

Table 3.0. Data table from Lewis structure of the molecules.

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CH2 O

O2

iii. Chemical formula

Lewis structure

Number of bonding electron domains 2

Number of nonbonding electron domains 2

Molecular shape of central atom

3

1

Trigonal pyramidal

2

0

Linear

4

0

Tetrahedral

3

0

Trigonal planar

Bent

H2 O

PCl3

CO2

CH4

CH2 O

Table 4.0. Data table from Lewis structure and VSEPR of the molecules.

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4.0 DISCUSSION OF DATA AND CONCLUSION 4.1 DISCUSSION OF DATA Lewis electron dot structures, also known as Lewis structures, are a useful tool for keeping track of valence electrons in representative components. Valence electrons are expressed by dots circling an element's atomic symbol in this notation. Several examples are shown below in first column of table 2.0. The urge for atoms to achieve the noble gas configuration drives the creation of molecular bonds and the exchange of electrons. This is interpreted in quantum mechanics as having filled orbitals or a s 2 p6 configuration like that of noble gases. This is known as the octet law, which states that all atoms (except H and He) want 8 electrons in the outermost orbital. Cations lose electrons and anions gain electrons in ionic compounds, but molecular compounds are required to share electrons to obtain octets. The octet rule is just a guideline, and it fails when dealing with d-orbitals and a variety of other scenarios. The most notable exception is hydrogen, which can reach a noble gas configuration with just two electrons. Molecules' 3D structure is also difficult to image using a 2D Lewis structure. Molecular model kits can be used to build 3D models to consider the real 3D structure of molecules. This will make the typical geometric patterns that Lewis theory predicts molecules will shape more visible like data recorded in table 4.0. Atoms in molecules and polyatomic ions are structured in geometric shapes that allow electron pairs to remain as far apart as possible, reducing repulsive forces between them. The underlying principle is known as VSEPR theory. The angle formed between two end atoms with respect to a central atom is often referred to as a bond angle. There is no bond angle if there is no central atom. The repulsive forces between electron pairs around the central atom determine the size of the angle. The atoms and electrons around the central atom strive to stay as far apart as possible, according to VSEPR 5

theory. The bond angles calculated are just estimates, and the actual bond angles will vary by several degrees depending on the molecule. From table 4.0, we can also conclude that the existence of lone pairs effects bond angles of a molecule. The presence of a lone pair of electrons at the central atom affects the bond angle. The central atom's lone pair of electrons still attempts to repel the mutual pair of electrons. As a result, the bonds are slightly displaced within, resulting in a decrease in bond angle like H2 O and PCl3 . The bond angle increases as the repulsion between electron pairs increases as the electronegativity of the central atom increases.

4.2 QUESTIONS 1.

Holes on the atom indicates that the atom can form a bond with another atom. The holes

shows that there is electron that are not shared with another atom in a covalent bond on the atom that can form bond with another electron. 2.

Two different atoms can have the same number of valence electrons. Shown in the table

2.0, nitrogen and phosphorus have the same number of valence electrons. The number of valence electrons each element in that group has is the same as the group number. As a result, the number of valence electrons in nitrogen and phosphorus would be equal. Since the valence shell of electrons is involved in both bonding and ionization processes, atoms with the same number of valence electrons behave similarly. Both nitrogen and phosphorus have 5 valence electrons and belong to group V.

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3.

Carbon can form a stable, robust backbone for a large molecule because carbon-carbon

bonds are exceptionally heavy. The number and configuration of electrons in carbon allow it to form bonds with four other atoms. Since carbon has an atomic number of six, six protons and six electrons in a neutral atom, the first two electrons fill the inner shell, while the remaining four are left in the valence outermost shell. Carbon must find four more electrons to fill its outer shell, bringing the total number of electrons to eight and satisfying the octet law. As a result, carbon atoms can form bonds with up to four other atoms. 4.

The absence of holes on atom always indicates the presence of lone pairs. It is because

the holes on atom only present when there is electron that does not in pairs or not bonding or shared with another atom. The absence of holes because the pairs of electrons does not need to make another bond because their orbitals already full of electrons. 5.

The most information about valence electrons can be found in the Lewis structure. The

valence electrons of atoms inside a molecule are described by Lewis structures, which are diagrams. The valence electrons of atoms and molecules, whether they exist as lone pairs or inside bonds, can be visualised using these Lewis symbols and Lewis structures. 6.

The molecular model contains the most information about the molecule's 3D structure.

The number and types of atoms, the nature of the bonds, bond lengths, angles and dihedral angles, molecular energy, geometry optimization, enthalpy, and vibrational frequency of molecular systems can all be analysed using molecular modelling. 7.

H2 O and CO2 are the molecules with two bonding electron domains on the central atom.

H2 O is in bent shape meanwhile, CO2 is linear shape.

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8.

PCl3 and CH2 O are the molecules with three bonding electrons domains on the central

atom. The shape of PCl3 is trigonal pyramidal and CH2 O is trigonal planar. 9.

The cause is lone pairs on the central atom of H2 O and PCl3 . A bent molecule is the

water molecule. With six valence electrons, oxygen needs two more electrons from two hydrogen atoms to complete its octet. This leaves two lone electron pairs with no other atoms to bond with. At approximately 109° bond angle, the two hydrogen atoms and two lone electron pairs are as far apart as possible. This is the geometry of tetrahedral electron pairs. The two lone electron pairs generate a small compression to a 104° bond angle by exerting extra repulsion on the two bonding hydrogen atoms. Same case with PCl3 , it has a trigonal pyramidal shape. One lone pair of electrons and three bond pairs of electrons make up the central P atom. It is sp3 hybridised, resulting in tetrahedral electron pair geometry and trigonal pyramidal molecular geometry. The lone electron pairs generate a small compression to between 90° to 109.5° bond angle by exerting extra repulsion on the two bonding hydrogen atoms.

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4.3 CONCLUSION All in all, based on the experiment conducted, we got to understand theory behind Lewis structure and Valence Shell Electron Pair Repulsion Theory. The valence electrons of atoms and molecules, whether they exist as lone pairs or inside bonds, can be visualised using these Lewis symbols and Lewis structures and in most cases, the VSEPR model helps one to predict which of the possible structures is observed. We also got experience molecule’s geometrical shapes by the model that has been built that helps to understand the theory of Lewis structure and VSEPR. 5.0 REFERENCES

1. Silvi, B. (2002). Chemical Bonding and Molecular Geometry: from Lewis to Electron Densities. Journal of Molecular Structure, 610(1–3), 277. https://doi.org/10.1016/s0022-2860(02)00050-9 2. Hargittai, I. (2009). Ronald J. Gillespie; the VSEPR model; and molecular symmetry. Structural Chemistry, 20(2), 155–159. https://doi.org/10.1007/s11224-009-9439-7 3. Gillespie, R. J., & Hargittai, I. (1991). The Vsepr Model of Molecular Geometry. Allyn & Bacon. 4. Libretexts. (2020, December 23). 9.2: The VSEPR Model. Chemistry LibreTexts. https://chem.libretexts.org/Bookshelves/General_Chemistry/Map%3A_Chemistry__The_Central_Science_(Brown_et_al.)/09._Molecular_Geometry_and_Bonding_The ories/9.2%3A_The_VSEPR_Model#:%7E:text=of%20a%20molecule.-,The%20valen ce%2Dshell%20electron%2Dpair%20repulsion%20(VSEPR)%20model,electron%20 pair%E2%80%93electron%20pair%20repulsions.

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