Atom is the smallest unit of matter, or we can say the fundamental unit of matter. We can predict the chemical properties of an element using an atom. The term “atom” came from a Greek word that means invisible. An atom consists of protons, neutrons, and electrons. Protons and neutrons are composed of quarks. Protons and neutrons together form the nucleus of an atom. The electrons in an atom revolve around the nucleus in fixed orbitals. In this article, we look at the history, definition, and structure of atoms.
History of atom
The history of atoms was started in 450 BC by a Greek philosopher, named Democritus. He wondered if an apple is cut into smaller pieces, we will reach a point where we won’t be able to cut it further. This uncuttable piece is called an atom.
Democritus’ atomic theory
For centuries it was widely believed that matter was made from Earth, air, fire, and water. But, Democritus came up with a theory that explains that atoms exist individually and they combine together to form matter.
Dalton’ atomic theory
In 1803, a British scientist by the name of John Dalton, proposed his atomic theory.
- Atoms of each element have unique weights and properties.
- Atoms cannot be created or destroyed. (law of conservation of mass)
- The matter is formed by the combination of atoms.
Plum-pudding model
In 1897, a British physicist by the name of Thomson discovered electrons inside atoms. In 1899, he exposed the “plum pudding model”. In this model, he includes a larger number of electrons are suspended in something to produce a positive charge making the atom neutral.
Rutherford’s model
The next model of the atom was given by a scientist named Rutherford who was a student of Thompson. In 1911, he published another version of the atom which includes a positively charged nucleus that is orbited by electrons. He came up with this model when Rutherford and his assistance fired alpha particles on thin sheets of gold. They noticed that a small percentage of alpha particles were scattered in large angles to the original direction. He was able to say the approximate size of the nucleus of the gold atom which was very smaller than the entire size of the atom. His model of the atom is still used as the basic model of the atom.
Other models
Several other scientists such as Niel Bohr, Erwin Schrodinger and others came up with their own theories about the structure of an atom.
Definition of atoms
Atoms are the fundamental unit of matter that can define the structure of the elements. Atoms are too small for naked eyes to see. Atom is further composed of smaller particles called quarks. Atoms of different elements have different atomic compositions. The number of protons and electrons will be different. The properties of an atom can be roughly predicted using its configuration.
Structure of atoms
An atom is composed of a nucleus(positively charged) placed at the center with electrons(negatively charged) revolving around it. The nucleus is composed of protons (positively charged) and neutrons (neutral). Neutral atoms have equal number of protons and electrons. However, atoms may gain or lose electrons and get charged. Such a charged atom is called an ion.
Protons
- They are positively charged particles that form the part of the nucleus.
- Protons have a weight of weight is 1amu.
- Protons are composed of two up quarks and one down quark
- The charge of a proton is approximately 1.602 × 10-19 C
- The mass of a proton is approximately 1.672 × 10-24 Kg
Electrons
- Electrons are negatively charged particles
- Electrons being negatively charged are naturally attracted to the positively charged protons by an electromagnetic force
- Electrons can escape from their orbit with the help of an external force
- The charge of an electron is approximately -1.602 × 10-19 C
Neutrons
- Neutrons do not possess any charge
- Isotopes are produced when neutron numbers are varied
- The number of electrons can also differ in the same element
There are about 120 known elements in the periodic table. The atoms of the various elements have varying numbers of electrons, protons, and neutrons. Each element has a different atomic configuration and has an atomic number(which is the number of protons in each atom of the element).
Atomic Structure of Isotopes
Each atom of an element has a unique number of protons. This is called an atomic number. For example, all the atoms of oxygen will have 8 protons. The number of protons in an atom is unique but not the number of neutrons. An isotope atom will have a different number of neutrons in its atom. For example, oxygen has 3 stable isotopes:16O, 17O, 18O. 16,17 and 18 represent the atomic mass = no. of protons + no. of neutrons.
- 16O will have 8 protons and 8 neutrons
- 17O will have 8 protons and 9 neutrons
- 18O will have 8 protons and 10 neutrons
Let us take another example, an hydrogen atom:
A hydrogen atom has an atomic number of 1. It has only one proton and has no neutron. Hydrogen atom has 3 stable isotopes: 1H(protium), 2H(deuterium), 3H(tritium). The isotope atoms vary with respect to stability but generally have similar chemical properties due to the unique number of protons.
- 1H (protium) has 1 proton and 0 neutron
- 2H(deuterium) has 1 proton and 1 neutron
- 3H(tritium) has 1 proton and 2 neutrons
How small is an atom?
The size range of Atoms varies from 62 pm for helium atom to 520 pm for cesium atom. They are invisible to the naked eye. Just for comparison, 1 picometer is 100 millionth the breath of a human hair.
Can we see atoms?
It is very difficult to see atoms with our eyes or even with conventional optical microscopes. The wavelength of visible light is greater than the size of an atom. Hence, we cannot use optical microscopes. Recently, atoms have been imaged using advanced techniques like scanning tunneling microscopes.
