What are Atoms and Molecules?
An atom is the smallest unit of an element that possesses all the properties of that particular element. Molecules are the smallest unit of a substance that possess all the properties of that particular substance.
Atoms
In our day-to-day life, we deal with many substances of different physical and chemical properties. Each of them is unique in its own way. For example, take an apple and an orange; both are classified as fruits. However, they differ in their color, shape, smell, taste, and so on. This difference originates from the very basic building units of these fruits. They have different building blocks; thus, they are different in nature. These smallest building blocks of matter are called atoms.
Consider the example of oxygen gas and hydrogen gas. Both have different chemical and physical properties. Given a bottle full of oxygen gas and a bottle full of hydrogen gas, the bottle with oxygen gas will contain only oxygen atoms and the bottle with hydrogen gas will contain only hydrogen atoms. Obviously, that is the reason behind their different character; they are made up of different atoms.
The word ‘atom’ was coined by the great Greek philosopher Democritus. He derived it from the Greek word ‘atomos,’ which means indivisible. Before that, an Indian philosopher, Maharishi Kanadan, mentioned the indivisible building blocks of matter. He named it ‘paramanu’. Both the concepts were similar and concerned the indivisible building blocks of matter. Later, in 1808, an English chemist, John Dalton, proposed an atomic theory that put forward the major concepts about atoms. Some of his concepts were proved wrong later. For example, he stated that an atom is the smallest particle of matter and it is indivisible. However, there exist smaller particles than atoms, which means atoms can be further broken down into component particles.
Structure of an Atom
Atoms are composed of smaller particles. There are three constituent particles for an atom, also known as subatomic particles:
- Protons
- Neutrons
- Electrons
Protons are positively charged particles and electrons are negatively charged particles. Neutrons, on the other hand, do not have any charge; they are neutral. Protons and neutrons together form a nucleus. Electrons are present outside the nucleus. The size of the nucleus is really small compared to the size of the atom. Consider a football placed at the center of a large football field. In this scenario, if the football represents the nucleus, the field would be the entire atom.
The structure of an atom can be compared to the solar system. In a solar system, the sun stays at the center, with all other planets revolving around the sun. In an atom, the nucleus is placed at the center, similar to the sun, with the electrons revolving around the nucleus just like planets. The positively charged protons and the neutrons with no charge make up the nucleus, so the charge of the nucleus is positive. The number of positively charged protons and the number of negatively charged electrons are the same in an atom. This makes the total charge of an atom neutral.
The electrons are kept outside of the nucleus. They are positioned in certain paths called orbits or shells. The electrons revolve in these shells around the nucleus. Each shell will contain a certain number of electrons. The loss or gain of electrons will lead to the formation of ions with a positive charge and negative charge. Positive ions are called cations and negative ions are called anions.
The number of protons represents the atomic number of an element. The sum of protons and neutrons represents the atomic mass of the element. The elements are arranged on the basis of atomic number and mass in the periodic table.
Elements with the same atomic number and different mass numbers are called isotopes. The difference in mass number is caused by a difference in the number of neutrons. The number of protons will be the same so that their atomic number remains the same. For example, carbon is known to have two major isotopes: carbon-12 and carbon-14. Carbon-12 contains six protons and six neutrons, thus, the atomic number is six and atomic mass is twelve. Carbon-14 contains six protons and eight neutrons, thus, the atomic number is six and the atomic mass is fourteen.
Molecules
Molecules are formed by the combination of two or more atoms. The combining atoms can be the same or different. For example, consider beads in a chain as atoms and the chain as a molecule. We can make a chain using the same beads and different kinds of beads. Just like that, a molecule can be made with the same atoms or different atoms.
When a molecule is made with the same atoms, it is called a homoatomic molecule, and when the molecule is made up of different atoms, it is called a heteroatomic molecule. For example, consider two vessels. One is filled with hydrogen gas (H2) and the other is filled with water (H2O). The vessel filled with hydrogen gas will contain hydrogen atoms only. However, the vessel filled with water will contain both hydrogen atoms and oxygen atoms. The hydrogen gas is made up of hydrogen molecules and one hydrogen molecule is made up of two hydrogen atoms. Water is made up of water molecules and each water molecule is made up of one oxygen and two hydrogen atoms.
Thus, hydrogen is a homoatomic molecule and water is a heteroatomic molecule.
Let’s check another example. Oxygen gas (O2)is a homoatomic molecule while carbon dioxide (CO2) is a heteroatomic molecule. This is because O2 is made up of only oxygen atoms and CO2 is made up of two oxygen and one carbon atom.
Some elements remain as a single atom in their free state. They are referred to as monoatomic elements. Some will have more than one atom in their free state; they are referred to as diatomic, triatomic and so on. Carbon, sodium, and many other metals are monoatomic elements. Chlorine, hydrogen, oxygen and many other elements are diatomic elements.
Common Mistakes
Do not confuse atoms and homoatomic molecules. Some elements are monoatomic while others are diatomic, triatomic, and so on. For example, gaseous hydrogen (H2) and oxygen (O2) are diatomic molecules while elements like carbon and neon are monoatomic.
An atom is the smallest unit of a substance while a molecule is the smallest unit with all the properties of that particular substance.
Practice Problem
Arrange the following as atoms, homoatomic molecules, and heteroatomic molecules.
C, N2, CO2, Cl2, NH3, Cu, H2O, Na, H2
Atoms: C, Cu, Na
Homoatomic molecules: N2, Cl2, H2
Heteroatomic molecules: CO2, NH3, H2O
Context and Applications
This topic is significant in the professional exams for both undergraduate and graduate courses, especially for
- B.S. Chemistry
- M.S. Chemistry
- B.S. Biochemistry
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