Every single entity in this universe is made of atoms. That is the key to understanding most of the properties of matter. There are 92 naturally occurring types of atoms and they form molecules, compounds, and other types of aggregations to create the complex world around us. Though the name 'atom' was derived from the Greek word 'átomos', meaning 'indivisible', modern physics has revealed that it is not the ultimate building block of matter and is indeed 'divisible' into subatomic particles. These particles are the real fundamental entities that create this world.
The Subatomic Particles: Electrons, Protons, and Neutrons
The atom has a systematic and orderly underlying structure, which provides stability and is responsible for the various properties of matter. The search for these subatomic particles began more than a hundred years ago and by now, we know a lot about them. Towards the end of the 19th century, scientists had advanced instruments to probe the interior of the atom. What they saw inside, as they investigated, surprised them beyond measure. Things at the subatomic level, behave like nothing on the macroscopic level. Let us have a look at what makes up an atom.
It was revealed that most of the volume of the atom was empty and sparsely populated by 'electrons'. They are negatively charged light particles that orbit around a central heavy mass. This mass, known as the nucleus, contains 99.99% of all the mass of the atom, though it occupies a very fractional volume. Imagine that if you magnify the atom to the size of a big room, then the nucleus would be a barely visible speck in the center of the room. In labeled atom diagrams, things are not drawn to scale. That's why, you do not realize how small the nucleus really is. It is actually confined within a length, that is less than 10-15 meters.
Figuring out the nature of the electrons orbiting the nucleus was easier than finding out the structure of the nucleus. After years of ingenious research and investigation, it was known that the nucleus was made up of particles called protons and neutrons.
Electrons are the lightest of all three subatomic particles. The mass of an electron is 9.1 x 10-31 Kg and it has a negative charge (- 1.6 x 10-19 Coulomb). Electrons are held in orbit around the atomic nucleus by force of attraction, exerted by the positively charged protons in the atomic nucleus.
The number of electrons in the outermost orbit of an atom determine its chemical properties. They are spin ½ particles and hence fermions. The antiparticle of an electron is the positron (same mass, but opposite charge of electron). Electron is considered as a 'point particle' as it has no known internal structure. Electrons interact with other charged particles, through the electromagnetic and weak forces, and are affected by gravity. However, they are unaffected by the strong force that operates within the confines of the nucleus.
Protons have a positive charge (1.6 x 10-19 Coulomb) and have a mass of 1.67 x 10-27 Kg. That makes them about 1836 times more massive than electrons. They are the nuclei of Hydrogen atoms, which have the atomic number to be 1. It is a spin ½ fermion which interacts through the strong, weak, electromagnetic, and gravitational forces, with other particles. The antiparticle of a proton is the antiproton. The structure of an atomic nucleus is made up of protons and neutrons.
The neutron, unlike protons and electrons, has no charge. It has a mass which is slightly greater than a proton at 1.675 x 10-27 Kg. This makes them the most massive of the parts of an atom. They interact with other particles in nature through the strong, weak, and electromagnetic forces, as well as the gravitational force.
Every one of these particles has certain inherent properties, which makes them bind with each other under the influence of fundamental forces, to create atoms. If you think that protons, neutrons, and electrons are the end of the story, you are in for a big surprise. Though electrons are indivisible, protons and neutrons are not the ultimate building blocks of matter. They are further known to be made up of fundamental particles called quarks.