A molecule is the smallest particle of a pure substance which can exist independently. It can be divided but retain its composition and chemical properties of that substance.
When atoms combine by forming covalent bonds then the collection of atoms are called molecule. It is also called as simplest unit of covalent compound .
It may contain more than one number of atoms. The number of atoms present in a molecule determine its atomicity. Thus molecules can be monoatomic, diatomic and triatomic, if they contain one, two and three atoms respectively. Molecules of element may contain one, two or more same type of atoms like He, Cl2, O3, P4, S8. On the other hand, molecules of compounds consist of different types of atoms. For example, HCl, NH3, H2SO4, C6,H12O6.
The size of molecule is greater than atoms. The size is depend upon number of atoms present and their shapes. Some molecules are so large so they are called macromolecules like Haemoglobin present in blood. It helps to carry oxygen from lungs to all parts of our body. Each molecule is made up of approximately 10,000 atoms and it is 68,000 times heavier than hydrogen.
Types of molecules
On the structural point of view a molecule is consist of atoms which are held together through valence force. There are different types of molecules:
The molecules which contain two atoms and chemically bonded with each other. If the two atoms are same or identical like oxygen O2 they known as homonuclear diatomic molecule, but if the atoms are not identical like carbon monoxide CO then they called as heteronuclear diatomic molecule.
The molecules which contain more than two atoms are called as polyatomic molecules. For example carbon dioxide CO2.
It is large molecule like protein, made by polymerization of smaller subunits. They are consists of thousands of atoms or more.
Physical properties of molecules are depend upon intermolecular forces of attraction. These are the following properties:
1. Melting and boiling points
Molecular compounds like liquid and gases have low melting and boiling point because their atoms are bonded by covalent bond which can separate from each other when small amount of energy is applied to them. Melting and boiling point is depend upon the strength of intermolecular forces. Large molecules have more van der Waals force of attraction so they required more energy to break .
Molecular compounds are insoluble or sometime sparingly soluble in water, they usually react and form hydrogen bonds with them. Methane is soluble in water because it is a gas and weaker van der Waals forces so small amount of energy is evolved as resultant. Ammonia has also ability to make hydrogen bonds. When ammonia reacts with water it form ammonium ions and hydroxide ions. The substances which are readily dissolve in water are ethanol and sucrose. Some molecular compounds are soluble in organic solvent. Both solute and solvent are attracted each other by van der Waals forces.
3. Electrical conductivity
Molecular compounds are not dissociate into ions but dissolve into molecules so they don’t conduct electricity . Electrons are delocalized within molecule, there is not sufficient connection between molecules are electron which allow them to move freely in the whole solid or liquid.
Structure of Molecules
The structural chemistry of the molecules is related with the valence shell of electrons, it determines how atoms combine together in proper ratios. It is also related to the bond type, direction and its length. The properties are related with the structure like water has hydrogen bonding and has bent structure, however carbon dioxide has no dipole moment that’s why it has linear structure. But some molecules like ethane (H3CCH3) have free rotation around carbon carbon single bond, they may not be rigid in structure .
In a molecule the position of nucleus is determined through microwave vibration rotation spectra or by neutron diffraction. Molecular structure is find theoretically through solving the quantum mechanical equation for the motion of electrons. Numerically it can be find out through Schrȍdinger equation, which is highly developed process use of computers and supercomputers.
VSPER (Valence shell electron pair repulsion) theory determined the geometry of molecules and also electron group geometry. This theory defines that electron pairs repel each other either they are in bond pair or lone pair, that’s why electron pairs spread themselves to repulsion. It also focus on the electron group which can be a double bond or triple bond on the center atom, an electron pair, a single pair. Through bond pair VSPER theory predict the shape of a molecule.
The shape of molecule is predict by the location of nuclei and its electron. They minimize repulsion and increase attraction. So molecule’s shape show its equilibrium state. It also predict the distribution of electron. It is also depend upon spatial orientation of covalent bonds to atoms and having two or more bonding partners. Configuration of molecule is three dimensional. The bonding of carbon is very easy to remember O=C=O.
A molecule which has one or more covalent bonds which are polar so they have dipole moment due to accumulation of bond dipoles. In O-H covalent bond is polar due to difference in electronegativity of oxygen and hydrogen.
The molecular weight is sum of atomic weights of its component, molecular weight is termed as mole. It can be determined by mass spectrometry and some techniques which are based on thermodynamics or kinetic transport phenomena.
Some individual examples
It is dark grey crystalline solid. Its melting point is 114oC and boiling point is 184oC. Iodine is very slightly soluble in water but easily dissolve in organic solvent. Its structure is defined as face centered cubic, at the center of each face another iodine molecule is attached. The orientation with the structure is very difficult to draw. The distance between the centers of atom in the crystal show two different values, the atoms which are at shorter distance have covalent bond and those which are at larger distance have van der Waals force of attraction, much weaker as compare to other force of attraction.
Ice molecules can be arranged in different ways, it is good example of hydrogen bonded solid. Huge number of molecules are arranged in three dimension. In the diagram hydrogen bonding is represented by straight lines and lone pairs are attached at the left side. Ice in cubic form stable at temperature below -80o C. The ice in hexagonal structure called “ice lh”. There is large amount of empty space present in between molecules in the structure, so when ice melts molecules fit in the empty spaces. Most of the solid will increase in volume on melting but ice has very unusual behavior in this respect. The opposite situation happened when water freezes hydrogen bonded structure is expanded but liquid contract on freezing.
Density of water increases from 0oC to 4oC molecules are free themselves from open structure and occupy less space. But after 4oC density fall because of the motion of molecules .
Bonding in polymers
Polymers consist of very long chain molecules like poly(ethene) which is also called polythene. These are made up of long chain of ethane molecules in which hydrogen and carbon atoms are covalently bounded. Along with main chain short branches are also present, carbon chain with attached hydrogen are also present.
It also contain two more distinct branches by controlling the conditions in which ethane is polymerized.
High Density polythene
It has very few branches, that’s why molecules lie very close to each other just like crystal. Lack of branches make plastic very strong because dispersion force is strong. It has also high melting point as compare to low density polythene.
It is also used for household chemicals like washing up liquid, for bowls and bucket preparation.
Low density polythene
It has many branches along the chain. That’s why molecules are not lie close together. It makes very low quality plastic. Molecules are not in a tidy manner also make dispersion force less effective. It also has low melting point and density.
It is used for the preparation of plastic bags.