The word “atom” originates from the ancient Greek term atomos, meaning “uncuttable” or “indivisible.” Modern science, however, reveals a more complex structure: an atom consists of a dense nucleus of positively charged protons and neutral neutrons, surrounded by a cloud of negatively charged electrons. This arrangement forms the basic building block of all elements. Atoms are remarkably stable in everyday life, but they can be fundamentally altered under extreme energy conditions.
Atoms in Chemical Reactions
Atoms appear to be indestructible, which is the foundation of the Law of Conservation of Mass. When a piece of wood burns or a metal surface rusts, the atoms involved are not destroyed; they are simply reorganized. Chemical reactions involve the sharing, giving, or taking of outer-shell electrons between atoms, which results in the formation of new chemical bonds.
The number of protons within the nucleus, known as the atomic number, determines the identity of an element, and this number remains unchanged during any chemical process. For example, when hydrogen and oxygen combine to form water, the hydrogen atoms remain hydrogen, and the oxygen atoms remain oxygen; only their molecular partnership changes. The total mass of the reactants before the reaction is precisely equal to the total mass of the products afterward. The apparent indestructibility of matter in our daily experience is really a function of the nucleus being shielded from the relatively low energies involved in chemical rearrangements.
Nuclear Processes That Change Atomic Identity
Any process that changes the number of protons results in the transmutation of one element into another. This change occurs exclusively within the atomic nucleus during high-energy nuclear processes. These processes include radioactive decay, nuclear fission, and nuclear fusion.
Radioactive decay is the spontaneous transmutation of an unstable atomic nucleus. For example, in alpha decay, an unstable nucleus ejects an alpha particle, which is essentially a helium nucleus composed of two protons and two neutrons. The loss of two protons causes the original atom to transform into an entirely new element that is two places lower on the periodic table. Beta decay involves a neutron converting into a proton and an electron (beta particle), which is then ejected. This process increases the atomic number by one, thus changing the element’s identity while keeping the mass number nearly the same.
Nuclear fission involves splitting a heavy atomic nucleus, such as uranium-235, into two or more smaller nuclei, often by bombarding it with a neutron. This division results in new elements, additional neutrons, and a massive release of energy. Conversely, nuclear fusion is the combining of two light nuclei to form a heavier one, an example being the fusion of hydrogen isotopes into helium in the sun. Both fission and fusion reactions release enormous amounts of energy because the final products have a slightly lower total mass than the starting materials.
This measurable loss of mass is described by Einstein’s mass-energy equivalence, \(E=mc^2\). The small amount of mass that disappears is converted directly into the tremendous energy released during the nuclear event. This demonstrates that the mass of the atom is not strictly conserved in a nuclear reaction, proving that atoms are not permanently indestructible.
The Cosmic Cycle of Atomic Creation
The cycling of atoms is primarily driven by the life and death of stars. Stars are effectively cosmic furnaces where lighter elements are continuously fused into heavier ones through stellar nucleosynthesis. In their main phase, stars like our sun fuse hydrogen into helium, while massive stars fuse helium into carbon, oxygen, and progressively heavier elements. This fusion continues until the formation of iron, which is the most stable atomic nucleus and cannot release energy by fusion.
The elements heavier than iron, including gold, platinum, and uranium, are created in the universe’s most energetic events. When a massive star exhausts its fuel, it collapses and explodes as a supernova. The intense neutron flux and extreme pressures during this explosion drive rapid neutron-capture processes that forge the heaviest elements. This cosmic cycling ensures that the atoms forming new stars, planets, and life are a mixture of primordial hydrogen and helium combined with the heavier elements forged in previous generations of stars.
Humans have also learned to perform atomic transmutation artificially in laboratories. Particle accelerators are used to bombard atomic nuclei with high-energy particles, forcing them to change their proton count and convert into different elements. This demonstrates that the ability to change the identity of an atom is not limited to the extreme conditions of space but can be replicated, on a small scale, by utilizing focused, high-energy technology.