Structure and Measurement of Matter

 All matter is made up of molecules. The smallest naturally occurring particle of any substance is called a molecule.    

Molecules are too small to be seen with the human eye, but their existence can be demonstrated by Brownian motion and diffusion.   

Matter can exist in three physical statuses – as a solid, a liquid and a gas. The kinetic theory explains the structure and behavior of substances in these states in terms of their molecules.   

Brownian Motion:

The molecules in liquids and gases are continually moving in a completely random fashion. This is known as Brownian motion, after the botanist Sir Robert Brown who first studied the nature of their movement. He demonstrated that pollen grains placed in water move erratically. This motion is due to the pollen grains unseen impact with water molecules. The tiny water molecules are able to move the much larger pollen grains because there is large number of water molecules and they are moving very fast.  

Diffusion:

Diffusion is the gradual mixing of two or more different gases or liquids. Diffusion happens when the molecule of the substances collides and intermingles. For example, the scent of flowers spreads through a room because its molecules diffuse through the air. The process of diffusion supports the idea of moving molecules, since the particle must be moving in order to mix.  

Atoms and Helium Atoms:

Molecules are made up of smaller particles called atoms. Atoms are formed of even smaller particles called electrons, protons and neutrons. The structure of an atom is shown here using the example of a Helium atom. The central nucleus of an atom is formed of protons and neutrons. Protons have a positive electrical charge and neutrons have no charge. Protons and neutrons are approximately 2000 times more massive than the electrons which orbit the nucleus. Electrons have a negative charge, equal in magnitude to the positive charge of the protons. The number of electrons in an atom is the same as the number of protons in the nucleus.    

The number of protons in a nucleus is called the proton number (Z). The total number of protons and neutrons in a nucleus is called the nucleon number (A).    

Measuring Volume:

The volume of an object is the measurement of the amount of space it occupies. It is measured in cubic meters (m^3) or cubic centimeters (cm^3). The volume of regular shaped solids is found using a ruler and mathematical formulae. For example, the volume of a rectangular block is found using the equation: length X breadth X height. The volume of liquid can be found by pouring it into a measuring cylinder. The volume of an irregular shaped solid is measured by displacement as in the diagram.    

Measuring Density:

Objects which are the same size and shape can vary greatly in mass. For example, one cubic centimeter of cork is much lighter than a cubic centimeter of lead. This is because the materials have a different density. Molecules of lead are heavier and more closely packed together than those of cork. This makes lead a more dense material than cork.    

To find the density of a solid or a liquid its mass and volume must be measured using the methods described above.  

These quantities are used in the equation: 

Density (D) = Mass (m)/Volume (v) 

Density is measured in kilograms per cubic meter (kg/m^3) or grams per cubic centimeter (g/cm^3)