Atoms - The Building Blocks of Life:


At its most fundamental level, life is made up of matter. Everything, including living and non-living things, is made up of matter.  Matter occupies space and has mass. All matter is composed of elements. A single fundamental unit of any element is called an atom. Although the most fundamental unit of anything including humans is an atom, a single ‘living’ unit that performs specialized tasks and consumes energy is known as a cell.  A cell is also made up of atoms.


According to an estimate made by engineers at Washington University, there are around 1014 atoms in a typical human cell. Another way of looking at it is that this is 100 trillion atoms. Interestingly, the number of cells in the human body is estimated to be about the same as the number of atoms in a human cell.



To understand the human body, it is therefore essential to study life at the chemical level as well as the cellular level.



Atoms with a constant number of protons have unique properties. A total of 118 elements have been defined, however, only 92 occur naturally, and fewer than 30 are found in living cells. The remaining 26 elements are unstable and therefore do not exist for very long or are  either theoretical or have yet to be discovered.  



Each element is designated by its chemical symbol (such as H, N, O, C, and Na), and possesses its own unique properties. These unique properties allow elements to combine and bond with each other in specific ways.




An atom is the smallest component of an element that retains all of the chemical properties of that element. For example, one hydrogen atom has all of the properties of the element hydrogen, such as it exists as a gas at room temperature, and it bonds with oxygen to create a water molecule. Hydrogen atoms cannot be broken down into anything smaller while still retaining the properties of hydrogen.


If a hydrogen atom were broken down into subatomic particles, it would no longer have the properties of hydrogen.


At the most basic level, all organisms are made of a combination of elements. They contain atoms that combine to form molecules. In multicellular organisms, such as animals, molecules can interact to form cells that combine to form tissues, which make up organs. These combinations continue until entire multicellular organisms are formed.


All atoms contain at least protons, electrons, and neutrons. The only exception is hydrogen (H), which is made of one proton and one electron. A proton is a positively charged particle that resides in the centre of the atom, the nucleus (the core of the atom), and has a mass of 1 and a charge of +1. An electron is a negatively charged particle that travels in the space around the nucleus. In other words, it resides outside of the nucleus. It has a negligible mass and has a charge of –1.


Fig -1   Atoms of Hydrogen and Helium


Neutrons, like protons, reside in the nucleus of an atom. They have a mass of 1 and no charge. The positive (protons) and negative (electrons) charge balance each other in a neutral atom, which has a net zero charge.


Because protons and neutrons each have a mass of 1, the total mass of an atom is equal to the sum of its protons and neutrons.  The number of electrons does not factor into the overall mass, because their mass is so small.

As stated earlier, each element has its unique properties. Each contains a different number of protons and neutrons, giving it its atomic number and mass number. The atomic number of an element is equal to the number of protons that the element contains. The mass number, or atomic mass, is the number of protons plus the number of neutrons of that element. Therefore, it is possible to determine the number of neutrons by subtracting the atomic number from the mass number.

These numbers provide information about the elements and how they will react when combined  with other elements. Different elements have different melting and boiling points and are in different states (liquid, solid, or gas) at room temperature. They also combine in different ways. Some form specific types of bonds, whereas others do not. How they combine is based on the number of electrons present.



This text was adapted from Concepts of Biology © 2013 by OpenStax, which is under a CC BY 4.0 Licence

The text is slightly modified to suit the audience.

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