A molecule in chemistry or biology is the smallest identifiable unit that formed by two or more atoms and it retains the chemical properties of formation substances. Different types of forces or chemical bonding are responsible for the formation of most types of molecules. In many crystalline solid molecules, the atoms are the structural units and these atoms are held together by ionic, covalent, and hydrogen bonding. Sodium chloride (NaCl) and potassium chloride (KCl) are well-known examples of molecules formed by ionic bonding. Zinc sulfide, silicon carbide, silver iodide, and water are examples of molecules formed by covalent bonding. In learning chemistry, the crystalline form of organic compounds like alcohol and carboxylic acid are supported by hydrogen bonding.
What is resonance in chemistry?
When the chemical properties of the molecules cannot be described by a single structural diagram, it forms different types of molecular structures, the true structure in chemical science meaning the resonance hybrid of the pure molecule. Resonance provides the different types of electronic structures of molecules. The resonance hybrid of acetic acid and ozone gas molecules are given below the picture,
Types of molecules
Covalent molecules are composed of positively charged nuclei and negatively charged electrons distributed in space. According to the structural arrangement and polarity, molecules are two types
A polar molecule is one in which there is a separation of the centers of gravity of positive and negative charges. The molecule developed a positive and a negative pole. Normally, heteronuclear diatomic molecules are polar due to the difference of electronegativity in bonding atoms. The polar character of a molecule is described by the term dipole moment. HF, HCl, HBr, HI are examples of heteronuclear diatomic polar molecules.
The non-polar molecule is one where the center of gravity of positive charge due to nuclei coincides with the center of gravity of the negative charge due to electron. H2, CO2, BCl3, CH4, PCl5, SF6, C6H6 are examples of non-polar molecules. Binary homonuclear molecules are non-polar since the bonding elements possess the same electronegativity.
Structure of molecules
After the development of quantum mechanics, two alternative facts explain the structure of different types of covalent bonding molecules. The theories are valence bond theory and molecular orbital model. The structure, properties, preparation, and reactions of some molecules are given below the topics,
In chemistry, hybridization is the mixing of pure atomic orbitals to give an equal number of hybridized orbitals. For example, the carbon atom in the methane molecule is sp3 hybridized.
Only orbitals of similar energies belonging to the same atom or ion can be hybridized together.
The number of hybridized orbitals produced equal to the number of orbitals undergoing hybridization.
Most of the orbitals are similar but they are not necessarily identical shapes.
From the type of hybridization, one can predict the geometry and bond angle of a molecule.
Types of hybridization
sp, sp2, and sp3 are the most common type of hybridization for the molecules. The type of hybridization predicts the shape of the molecules. Some common examples are given below the table,
Types of hybridization
A linear combination of one s and one px orbitals in suitable proportion gives two sp hybridized orbitals. This type of hybridization is called sp hybridization. The mixing of one s and one px are given below the picture,
The mixing of one s and two p (px and py) atomic orbitals gives three sp2 hybridized orbitals. The formation of sp2 hybridized orbitals is given above the picture. In sp2 hybridization, the hybridized orbitals formed trigonal planner structure.
One s and three p atomic orbitals mixing together to form four equivalent sp3 hybridized orbitals. This type of mixing of atomic orbitals is called sp3 hybridization. The formation of tetrahedral sp3 hybridized orbitals are given below the picture,
Examples of molecules
Structure of carbon dioxide molecule
A carbon dioxide molecule is an example, formed by the components like one carbon and two oxygen atoms having linear structural formulas. The valence shell electronic configuration of chemical element carbon in excited state 2s1 2px1 2py1 2pz1. Oxygen atoms (normal state) 2s2 2px1 2py1 2pz1. The liner shape of O=C=O suggests that the carbon atom is the central atom with sp-hydridized. Two sp-hybrid orbitals of carbon atoms overlap with two oxygen atoms to form two sigma bonds. Another two half-filled 2p-orbitals carbon atom binding by pi bonds with two oxygen atoms. The experimental bond length between carbon-oxygen in carbon dioxide molecule = 1.15Å. The resonating structure of the carbon dioxide molecule is equivalent with resonance bond energy 33 kcal/mole.
The boron atom is the central atom in the BCl3 molecule in chemistry. The valence shell electronic configuration of the boron atom in the excited state, 2s1 2px1 2pz1. On hybridization, these atomic orbitals form three sp2 hybrid orbitals. These hybridized orbitals overlap with three singly filled 3pz orbitals of the chlorine atom to form three B-Cl sigma bonds. Therefore, the BCl3 molecule is a trigonal planner shape. In this molecule central atom boron with an incomplete octet. From the definition of Lewis acids bases in chemistry, BCl3 molecule affinity to accept electron pair acts as a Lewis acid by forming a coordinate covalent bond with other molecules or Lewis bases.
Methane molecule model
The methane molecule is the simplest hydrocarbon of alkane or paraffin molecule having molecular formula CH4. In methane molecule, exited carbon atom look like 2px1 2py1 2pz1 electronic configuration. Mixing these four atomic orbitals give four equivalent sp3-hybrid orbitals. These hybrid orbitals are directed towards the corners of the regular tetrahedron. Therefore, each sp3-hybrid orbitals overlap with the 1s-atomic orbitals of four hydrogen atom by the formation of four sigma bonds in the methane molecule.
Structure of ammonia molecule
In the ammonia molecule nitrogen atom is the central atom, define valence shell electronic structure, 2s2 2px1 2py1 2pz1. Therefore, four atomic orbitals of nitrogen hybridized to form four equivalent sp3-hybrid orbitals. Three of these four orbitals in ammonia molecule overlap with 1s-orbitals of hydrogen to form three sigma-bond and the remaining hybrid orbitals contain a lone pair of electrons. The ammonia molecule looks like a distorted tetrahedral shape with a bond angle of 107.5º.
Structure of water molecule
The molecular formula of the water molecule is H2O. Water molecule is formed by sp3 hybridization. Water molecules contain two bond pairs and two lone pairs. Due to the presence of lone pairs on the central oxygen atom, the shape of the water molecule is not regular tetrahedral. The H2O molecule is V-shaped with an H-O-H bond angle of 105.5°.
Hydrogen fluoride molecule
The molecular formula of hydrogen fluoride is HF. Hydrogen fluoride molecule is formed by sp3-hybridization. The fluorine atom is the central atom in the HF molecule. Hydrogen fluoride molecules contain one bond pair and two lone pairs with linear structures.