# Get the Knowledge that sets you free...Science and Math for K8 to K12 students

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## Forces of Attraction

The world’s strongest glue Adhesive forces are the attractive forces between unlike molecules of surfaces. Adhesion occurs either due to a chemical bonding between two substances or due to a static electricity that hold the substances together or due to the van der Waals forces that develop between molecules of the two substances.
A glue adheres to the surfaces as the moisture-aided diffusion of the glue into the substrate, followed by hardening. The world’s strongest glue is derived from a common bacterium that clings to the inside of water pipes known as Caulobacter crescentus. Scientists determined that it can withstand a force equivalent to five tons per square inch (the pressure exerted by three or four cars balanced atop a quarter) before it is swept from its mooring.

## After completing the topic, the student will be able to:

• Define the phenomenon of Brownian motion and explain the reason for Brownian motion of suspended particles.
• Distinguish the intermolecular and intramolecular forces.
• Recall the properties of three intramolecular forces – ionic, covalent and metallic forces.
• Explain the reason for existence of matter in different states by describing various intermolecular forces.
• Define van der Waals forces and give the details of origin of van der Waals forces.
• Classify different types of dipole-dipole forces.
• Explain the origin of polarity in molecules and its effect on behavior of the molecule.
• Define hydrogen bonding and discuss the consequences of hydrogen bonding.
A plot of 2D Brownian motion The above plot shows the randomness in movement of particles. Particle changes its direction instantaneously giving rise to zigzag motion called as Brownian movement. Einstein, in his papers published in 1905 gave the explanation for this which states that matter is made of particles.
Brownian motion

The credit for indirectly observing the existence of molecules first goes to Robert Brown, a Scottish botanist. In 1827, Brown observed under a microscope that pollen grains, which were of the size 10−5 m, when suspended in water appeared to perform random motions. He initially thought that the pollens were alive! But more observations, with pollens of dead flowers, and pollens boiled in water, also showed similar motion. This type of random motion of particles is termed as Brownian motion. Zigzag motion of small particles can be seen in our everyday lives also. For example, small ash particles emitted from a coal fire can be seen to undergo zigzag motion in the air, dust particles can be seen to perform a similar motion in a strong beam of sunlight. Brownian motion increases or decreases with various parameters.

Einstein developed the theory of Brownian motion in 1905. He explained that random motion of pollen grains in water was due to the motion of water particles themselves. Thus he observed that Brownian motion is the result of the continuous impact of the molecules of the liquid on the suspended particles. Further, though it was known that the matter is made up of molecules, the discovery of Brownian motion and Einstein’s explanation of the same gave the first direct evidence of the presence of small physical particles, molecules, undergoing random motion.

Molecules Atoms combine to give rise to molecules . Different combinations will give a variety of molecules which differ from one another in their properties.
Atoms, Molecules and Compounds

Atoms combine to form molecules. Three atoms of hydrogen (H) combine with a single atom of nitrogen (N) to produce ammonia molecule (NH3). A molecule may be as simple as the two‐atom combination of oxygen (O2) or chlorine (Cl2) or as complex as the double helix of deoxyribonucleic acid (DNA), which consists of millions of atoms – the basic building blocks of life.

Smallest identity of an element is an atom. Similarly smallest identity of a compound is a molecule. The combination of atoms in a molecule of a compound is always in a definite proportion. The chalk is an example for a compound. In a compound, each of the constituent atoms loses its individuality and starts displaying a totally new property of the compound formed. The smallest basic unit of a compound that can maintain and display its properties is of course a molecule.

For a compound to form, a chemical reaction between the constituent atoms has to take place. Although there are less than 100 elements or pure substances in nature, there are millions and millions of compounds formed by various permutations and combinations of elements. Compounds generally decompose on heating and may either separate into the constituent elements or change into another compound.