What’s The Difference Between Nuclear Fusion and Nuclear Fission?

This question confounded me and most of my friends back when we were in school, preparing to appear in the Physics test. Atom bombs… they use nuclear fission, right? Or is it fusion? Or maybe both? And how about Hydrogen bombs?

The answers to these questions seem pretty basic now, but back in school…

not so much gif

For starters, if you did a quick Google search for the meaning of the word ‘fusion’, this would come up: ‘the process or result of joining two or more things together to form a single entity.’ This definition of the word ‘fusion’ applies very conclusively to the term ‘nuclear fusion’, only here it pertains to atoms’ nuclei.

But before we go further in nuclei-related nitty-gritty, let’s do a quick recap of a couple of basic concepts related to atoms and nuclei, which you’ll be reading a lot about in the subsequent sections.

A little something about Atoms, Nuclei and Atomic numbers

You must already know that atom is the building block of all matter; everything that we see around us is made up of atoms, a lot of them. Consider this, a human body weighing 70 kilograms (154 pounds) contains about 7*10^27 atoms, or the number you get after putting 27 zeroes after the digit 7!


But it so happens that a atom has a large amount lot of empty space within it; in other words, an atom is mostly empty. This is the basis behind a very popular hypothesis that claims that the entire human race can fit inside a single sugar cube! (More about it here). Whatever little space inside an atom is actually ‘used’, is occupied by something known as the ‘nucleus’ of the atom. Electrons revolve around the nucleus in different energy levels. Within the nucleus there are two primary constituents, namely protons and neutrons.

Atomic number is the measure of the number of protons in an atom; if an element has only 1 proton (Hydrogen) inside the nucleus, then it has an atomic number of 1, or two protons, then it’s atomic number is 2 (Helium) and so on.

hydrogen atom helium atom

Needless to say, an atom with more electrons has a high atomic number, and therefore is said to be ‘heavy’. In fact, every element in the periodic table having an atomic number greater than 92 is a ‘heavy element’.

heavy elements meme

Nuclear Fusion

Nuclear fusion occurs when two or more nuclei join to form a new element with a heavier nucleus, or in technical terms, an element with a higher atomic number. During fusion, matter is not conserved; this is because some of the matter is converted to energy (in photons). Such a combination of two or more nuclei to form a new, heavy nucleus releases an enormous amount of energy, which is closely associated with a most popular Energy Mass equivalence formula:

energy mass equivalence formula

The most likely fusion reaction here on Earth occurs between the two isotopes of Hydrogen, namely Deuterium and Tritium. (Yes, the same tritium for which Doc Octopus created such a fuss in Spiderman 2 and gave our beloved Spiderman a hard time!) Given below is the Deuterium-Tritium fusion reaction:

deuterium tritium fusion reaction

Our sun, and all other stars in the entire galaxy are powered by nuclear fusion reactions. That’s why they have been glowing since ages without ever blacking out!

Nuclear Fission

Nuclear fission is a process in which the nucleus of an atom splits into smaller subatomic particles like free neutrons, gamma particles and a lot of energy. Given below is the nuclear fission reaction of a Uranium-235 nucleus to produce fast-moving lighter elements.

nuclear fission uranium

As you can clearly see, nuclear fission is the exact opposite of nuclear fusion. The similarity in both processes, though, is that both release a huge amount of energy. Nuclear fission is the basic principle behind the working of nuclear weapons like atomic bombs (or atom bombs or A bombs).

Now that you have a fair idea about both fusion and fission, let’s look at their salient differences.

Nuclear Fusion vs Nuclear Fission

Natural occurrence

Nuclear fusion reactions occur in celestial bodies like sun and other stars on their own (in the sense that no artificial, external trigger is required for them to occur), whereas in order to carry out a fission reaction, you have to artificially create highly controlled conditions.


radioactive particles

Nuclear fission produces a lot of radioactive particles (Image Source: pixabay.com)

Fission releases a huge number of radioactive particles but fusion produces fewer. However, it can also produce a lot of radioactive particles if the conditions are slightly ‘tweaked’.

Energy requirement

Nuclear fusion, i.e. bringing two nuclei close together to make them fuse into each other takes a colossal amount of energy, the scale of which is readily available on stars. Fission, on the other hand, can be carried out with much less energy.

Amount of energy released

While the amount of energy released during fission is a million times more than that of ‘regular’ chemical reactions, yet the energy released during fusion is 3-4 times greater still than fission.


Fusion (in man-made power plants) mainly uses isotopes of hydrogen (i.e. deuterium and tritium), while fission’s primary fuel is uranium.

Required conditions

While fusion requires an extremely high temperature, high density environment, fission requires high speed neutrons and critical mass of the substance.

Application in Nuclear Weapons

Nuclear explosion atomic war

Weapons made out of either fusion or fission are both very destructive

Fission is used in fission bombs, or more commonly known as ‘atomic bombs’, whereas fusion is used in hydrogen bombs.

Both these processes require absolutely flawless settings and a highly meticulous execution in order to carry out in artificial settings. Needless to say, such processes are incredibly expensive to carry out too. But if used with a conscientious outlook, both nuclear fission and fusion can provide boundless energy to sustain life on Earth.


  1. Wikipedia
  2. Chemistry Libretexts
  3. Galileo
  4. Hyperphysics
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Ashish is a Science graduate (Bachelor of Science) from Punjabi University (India). He spends a lot of time watching movies, and an awful lot more time discussing them. He likes Harry Potter and the Avengers, and obsesses over how thoroughly Science dictates every aspect of life… in this universe, at least.

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