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The atomic nucleus is the heart of an atom. It is where protons and neutrons are tightly packed together and held by strong nuclear forces. These protons and neutrons are called nucleons. The number of protons in the nucleus is known as the atomic number, which determines what element an atom represents. For example, an atom with one proton is hydrogen, while an atom with six protons is carbon.

• Protons are positively charged particles.
• Neutrons have no electric charge.

Together, protons and neutrons give the nucleus its weight or mass. This mass is almost all the mass of the atom since electrons, which orbit the nucleus, have much less mass. In transmutations, changes happen at the level of the nucleus. These changes may alter the number of protons, changing the element itself.

Radioactive decay is a natural process by which the unstable nucleus of an atom loses energy by emitting radiation. This occurs because the nucleus wants to reach a more stable state. During radioactive decay, an element changes into another element or a different isotope.
Types of radioactive decay include:

• Alpha decay: The nucleus releases an alpha particle, which contains two protons and two neutrons.
• Beta decay: A neutron turns into a proton, or a proton turns into a neutron, emitting a beta particle (an electron or positron).
• Gamma decay: The nucleus emits gamma radiation, which is high-energy photons, without changing the number of protons or neutrons.

These processes can happen naturally as parts of transmutations where new elements get created from radioactive isotopes, leading to changes in the atomic number and stability.

Nuclear reactions involve the altering of an atom's nucleus and can lead to the transformation of one element into another, a process known as transmutation. This can happen naturally or artificially, with the latter often occurring in labs or reactors. Unlike chemical reactions, which involve electrons, nuclear reactions involve changes in the nucleus.
Key features of nuclear reactions:

• They may involve the collision of nuclei or particles.
• Energy is predominantly released or absorbed.
• They can change the identity of the element.

Artificially induced nuclear reactions can be initiated through particle accelerators or in nuclear reactors, where particles are propelled to collide with target nuclei. This often results in fission or fusion reactions, releasing immense energy and transforming elements through changes in the nucleus. These processes are essential for generating energy and producing medical isotopes.