PARTICLE NATURE OF MATTER

Matter is anything that has mass and occupies space. Matter may be in the form of solid, liquid and gas. For anything to occupy space, it must have volume. Reflecting about this, one may conclude that everything on earth is matter since everything on earth has mass and takes up space. It should be noted that different objects are made up of different matters. Matter is made up of molecules which are the smallest indivisible particles of a substance which can exist separately on their own and are capable of taking part in chemical reactions.

A molecule is made up of atoms. Just like molecules, an atom is the smallest indivisible particle of a substance which cannot exist alone and cannot take part in chemical reactions.

PARTICLE THEORY OF MATTER

This states that all matter is made of tiny particles which are called ‘Atoms’ and that these atoms are always in a rapid state of motion. The nature of this motion and its activity depend upon the temperature of matters and other factors.

EVIDENCE OF PARTICLE NATURE OF MATTER

There are four basic evidence that matter is composed of tiny particles. They are:

1. Brownian Motion
2. Diffusion
3. Osmosis
4. Law of Definite Proportion

1. Brownian Motion

This is the rapid and irregular motion of tiny particles in matter. This experimental evidence was first carried out by Lord Robert Brown in 1827. In his experiment, when he was trying to observe pollen grains in water through a microscope, he found that the pollen grains were in constant motion. The constant motion of the pollen grains was later attributed to repeated bombardment of the pollen rains by molecules of water as they move about in a zig-zag manner. The particles in Brownian motion are so large that they are not driven by their own energy but by the collisions with solvent particles. Though, the molecules are invisible to the naked eye, their effect could be seen in the random motion of the pollen grain.

                                                     Figure: Brownian Motion

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• Diffusion

This is the spreading of particles through random motion from regions of higher concentration to regions of lower concentration.

This may occur in solids, liquids and gas. Diffusion is fastest in gas and slowest in solids. For example, when an air freshener is placed in a room, after a while, the fragrance of the air freshener spreads or diffuses throughout the room. The particles have travelled randomly throughout the room and fills it completely. In liquids, it is easy to notice diffusion when one puts a cube of sugar in a cup of water. After a while, the whole water becomes sugary. This suggests that, the molecules that make up the sugar have spread throughout the water.

It should be noted that the major difference between diffusion and Brownian motion is that, diffusion depends on the concentration of the particles while Brownian motion is proportional to the temperature of the solvent.

Fig: Diffusion of particles from a higher concentration to lower concentration

Factors Affecting the Rate of Diffusion

• Size of the particle: The rate of diffusion is inversely proportional to the size of the particle, i.e., the smaller the size of the particle, the faster the rate of diffusion, and vice versa.
• Temperature: The rate of diffusion is directly proportional to the temperature, i.e., diffusion rate increases as the temperature increases.
• Concentration Difference: When a substance is diffusing between two compartments, the greater the concentration difference between the two compartments, the faster the rate of diffusion of the substance.

Other factors include; Density, Pressure, etc.

• Osmosis

This is the passage of a solvent through a semi-permeable membrane separating two solutions of different concentrations.

A semi-permeable membrane is one through which the molecule of the solvent can pass but the molecules of the most solutes will not. It should be noted that in osmosis however, the solvent flows from the solution of weaker concentration to that of higher concentration.

                                                            Figure: Osmosis

• Law of Definite Proportion

It states that when two or more elements combine to form a compound, they do so in the same proportion by weight. For instance, when water is being formed, the proportion by mass of the hydrogen to the oxygen will be 2:16, since the atomic mass of one oxygen atom is about sixteen times that of one hydrogen atom, i.e.,

               Hydrogen + Hydrogen + Oxygen = Water.

SIMPLE ATOMIC STRUCTURE

            An atom is the building block of matter. It is composed of a nucleus at the centre and electrons orbiting it. The nucleus is made up of neutrons and protons which are held together by strong nuclear force. The combination of both neutrons and protons in the nucleus of an atom is called Nucleons. Protons are positively charged, electrons are negatively charged, while neutrons have no charge. The number of protons in the nucleus of an atom is called The Atomic Number. In an atom, the number of protons is equal to the number of electrons. As a result of this, atom is said to be neutral.

Note: If the number of electrons and protons in an atom are not equal, the atom is called “An Ion”.

In terms of mass of the constituents of the atoms, the nucleons are more massive. Each of them is about 1840 times more massive than the electrons. Owing to this fact, the mass of electron is negligible.

The atomic number of any element does not change. When there are more or less neutrons in an atom, then the element formed from such an atom is called an isotope of the parent element. Isotopes are atoms of the same element which have the same number of protons but different number of neutrons.

MOLECULES

A molecule is a combination of at least two atoms (either of the same or different elements) in a definite arrangement held together by strong chemical bonds. The combination is such that it can have a separate existence. For example, a molecule of oxygen is a combination of two oxygen atoms:

                                                O + O → O₂

Size of Molecules

The size of a given molecule varies from molecule to molecule. It largely depends on the type and number of atoms that have combined to form the molecule. Generally, the size of a molecule is in the order of 10⁻⁹ to 10⁻¹⁰ m.

Measurement of size of molecules

Materials needed are water, talcum powder, oil, large flat sided shallow pan and pipette.

• Pour clean water into a large flat sided shallow pan.
• Sprinkle the talcum powder on the water surface.
• Use a pipette to add 1 cubic millimetre of oil to the water. Allow the oil to spread, forming a circular patch and pushing the powder away.
• Measure the diameter of the patch (possibly with the aid of magnifying glass).
• From the volume and the diameter measured, the thickness of the patch can be estimated. This thickness is approximately the diameter of the molecule.

Figure: Measurement of size of molecules

The patch of oil is approximately a very thin cylinder with volume πR²t.

The entire volume of the patch of oil will be;

Where r is the radius of the oil droplet and R is the radius of the patch of oil, t is the thickness of oil patch diameter of oil molecule.

By making t the subject of the formula,

Example:

An oil droplet of volume 5.0 mm³ spreads out to form a circular patch of diameter 700mm when placed in a clean water surface. Estimate the diameter of an oil molecule.

Solution

Given: volume of oil droplet = 5.0 mm³ = 5 x 10⁻⁹ m³, diameter of oil patch formed = 700mm = 0.7m

CLASS ACTIVITY:

• A small drop of oil with a diameter of 0.15 mm is dropped onto a dust covered water surface. The resulting circular patch of oil has a diameter of 20 cm.
  Calculate the maximum size of an oil molecule.