Classwork Series (Physics – SS1): Particle Nature of Matter

Structure of matter

A. Evidence of the particle nature of matter

The idea that matter is made up of minute particles called atoms dates back to the ancient Greeks. According to the Greek philosopher Democritus, a given piece of substance, say a piece of yam, can be cut into smaller and smaller bits, until eventually a smallest piece of that substance would be obtained which not be further subdivided. This smallest, indivisible, piece was called an atom. The atomic theory of matter assumes that all matter is made up of tiny particles called atoms and that these are all at times in a rapid state of motion. The nature of this motion and its activity depend upon the temperature of matter and other factors.

The experimental evidence of this particle or atomic nature of matter is the Brownian movement, named after the Biologist Robert Brown who was credited with its discovery in 1827. While observing tiny pollen grains suspended in water under a microscope, he noticed that the tiny pollen grains moved about in zig-zag paths even though the water appeared to be perfectly still. The pollen grains were supposed to be jostled or knocked about here and there by the vigorously moving molecules of water.

Another evidence in favour of the particle nature of matter was obtained from analysis of chemical reactions. This crucial piece of evidence is known by chemists as the Law of Definite Proportions which states that when two or more elements combine to form a compound, they always do so in the same proportions by weight.

For example, the compound water (H₂O) is always formed by two parts of hydrogen (H₂) and sixteen parts of oxygen (O) and the common salt (NaCl) is always formed from 23 parts of sodium and 35 parts of chlorine by water.

A third evidence that matter is composed of extremely small is provided by the process of diffusion. If we place a few drops of liquid bromine at the bottom of a gas jar on top of which is placed a cover glass, after sometime the brown bromine vapour will be seen in the upper part of the gas jar in spite of the fact that bromine vapour is mush denser than air and should remain at the bottom of the jar. We can explain this diffusion or spreading out of the bromine by assuming that liquid bromine is composed of particles and that the particles can move about easily.

All the evidence we have given that matter is composed of particles are indirect evidence. We have no direct evidence for the existence of these particles since we cannot observe them with even the most powerful microscopes.

b. Simple atomic structure

An atom is the smallest indivisible particle of an element which can take place in chemical change.

Activity

1. Get a tuber of yam and cut out a piece of this tuber about the size of a cube of sugar.

2. Divide this tube into two halves.

3. Take one of these halves and cut it into another half. Continue this process of cutting each successive half into two until you can no longer cut the remaining portion into further bits.

If we suppose the piece of yam to represent an element (e.g. sodium) the smallest bit of the yam represents the atom of the element. The smallest particle of an element, in order to be an atom of that element must be capable of a separate existence.

There are many theories concerning the structure of the atom. It shall be discussed in higher classes, a little above this class. For now, we shall consider the atom as simply consisting of two parts held together by electric forces. The two parts of the atom are (1) the nucleus and (2) the electrons.

The nucleus is the heavy portion of the atom and its located at its centre. It consists of two parts – the protons and the neutrons. The protons carry a positive charge, the neutrons carry no charge.

The second part of the atom is the electron. The electrons are very light (about 1/840 of the mass of the proton.) They are negatively charged. The lightness of electrons make it easy to transfer them when two, materials are made to rub against each other. The electrons circle in orbits around the heavy nucleus and are held in place due to the electrostatic attraction between them and the protons of the nucleus.

In a neutral atom, the total charges due to the electrons must balance the total charges due to the protons.

Molecules

Most substances cannot exist by themselves as individual atoms. They combine their atoms with themselves or with other atoms to form molecules.

A molecule is the smallest particle of a substance which can have a separate existence and still retains the properties of that substance.

The molecule of anyone substance are identical. They have the same structure, the same mass and the same mechanical and electrical properties.

A molecule may be made up of similar atom of the same elements or different atoms or two or more elements. For example, a molecule of hydrogen is made up of two atoms of hydrogen but a molecule of water is a combination of two atoms of hydrogen and an atom of oxygen, and a molecule of sodium chloride is a combination of an atom of sodium and an atom of chlorine.

Atoms combine in simple proportion to form molecules. The simplest model of a molecule is that of a rigid sphere, like a small billiard ball, capable of moving and colliding with other molecules or with a wall, and of exerting attractive or repulsive forces on neighbouring molecules.

The size of a molecule

The size of a molecule is extremely small. It is of the order of 10^-9 – 10^-10 m (10^-7 – 10^-8 cm). As a result of this small size, one gram of an element contains several millions of molecules. For example a gram of hydrogen contains about 10²³ molecules.

Brownian motion

Brownian motion, also called Brownian movement,  any of various physical phenomena in which some quantity is constantly undergoing small, random fluctuations. It was named for the Scottish botanist Robert Brown, the first to study such fluctuations (1827).

Brownian motion is the rapid, constant and irregular motion of tiny particles

If a number of particles subject to Brownian motion are present in a given medium and there is no preferred direction for the random oscillations, then over a period of time the particles will tend to be spread evenly throughout the medium. Thus, if A and B are two adjacent regions and, at time t, A contains twice as many particles as B, at that instant the probability of a particle’s leaving A to enter B is twice as great as the probability that a particle will leave B to enter A. The physical process in which a substance tends to spread steadily from regions of high concentration to regions of lower concentration is called diffusion. Diffusion can therefore be considered a macroscopic manifestation of Brownian motion on the microscopic level. Thus, it is possible to study diffusion by simulating the motion of a Brownian particle and computing its average behaviour. A few examples of the countless diffusion processes that are studied in terms of Brownian motion include the diffusion of pollutants through the atmosphere, the diffusion of “holes” (minute regions in which the electrical charge potential is positive) through a semiconductor, and the diffusion of calcium through bone tissue in living organisms.

Diffusion

Diffusion, process resulting from random motion of molecules by which there is a net flow of matter from a region of high concentration to a region of low concentration. A familiar example is the perfume of a flower that quickly permeates the still air of a room.

Heat conduction in fluids involves thermal energy transported, or diffused, from higher to lower temperature. Operation of a nuclear reactor involves the diffusion of neutrons through a medium that causes frequent scattering but only rare absorption of neutrons.

The rate of flow of the diffusing substance is found to be proportional to the concentration gradient. If j is the amount of substance passing through a reference surface of unit area per unit time, if the coordinate x is perpendicular to this reference area, if c is the concentration of the substance, and if the constant of proportionality is D, then j = –D(dc/dx); dc/dx is the rate of change of concentration in the direction x, and the minus sign indicates the flow is from higher to lower concentration. D is called the diffusivity and governs the rate of diffusion.

Osmosis

Osmosis is the spontaneous net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration, in the direction that tends to equalize the solute concentrations on the two sides. It may also be used to describe a physical process in which any solvent moves across a semi permeable membrane (permeable to the solvent, but not the solute) separating two solutions of different concentrations. Osmosis can be made to do work.

 

The osmotic pressure is defined to be the pressure required to maintain an equilibrium, with no net movement of solvent. Osmotic pressure is a colligative property (properties of solutions that depend upon the ratio of the number of solute particles to the number of solvent molecules in a solution, and not on the type of chemical species present), meaning that the osmotic pressure depends on the molar concentration of the solute but not on its identity.

Osmosis is a vital process in biological systems, as biological membranes are semipermeable. In general, these membranes are impermeable to large and polar molecules, such as ions, proteins, and polysaccharides, while being permeable to non-polar and/or hydrophobic molecules like lipids as well as to small molecules like oxygen, carbon dioxide, nitrogen, and nitric oxide. Permeability depends on solubility, charge, or chemistry, as well as solute size. Osmosis provides the primary means by which water is transported into and out of cells. The turgor pressure of a cell is largely maintained by osmosis across the cell membrane between the cell interior and its relatively hypotonic environment.

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