Here's why:

* Atomic Number: The atomic number of an element is defined by the number of protons in its nucleus.

* Nuclear Fusion: This is a process where two or more atomic nuclei combine to form one or more different atomic nuclei and subatomic particles (neutrons or protons).

During nuclear fusion, the combination of nuclei often results in the creation of heavier elements, which have a higher atomic number than the original nuclei.

Example:

* The fusion of two hydrogen nuclei (each with 1 proton) produces a helium nucleus (with 2 protons).

* This process increases the atomic number from 1 (hydrogen) to 2 (helium).

Other processes that can change the number of protons, but not directly increase the atomic number:

* Radioactive decay: Some isotopes can decay by emitting protons, decreasing the atomic number.

* Particle bombardment: Bombarding a nucleus with particles like neutrons can lead to changes in the number of protons, but the result isn't always a simple increase.

Key takeaway: Nuclear fusion is the primary process responsible for increasing the atomic number of elements. This process is responsible for the creation of all elements heavier than hydrogen in the universe.