What is an Atomic Element?

An atomic element, or more precisely a chemical element, is a pure substance whose atoms all contain the same number of protons in their nuclei. This number of protons is called the element’s atomic number, and it uniquely identifies each element. For example, all atoms with 8 protons are oxygen, while those with 79 are gold[1][2][5][6][7][8].

Elements are the fundamental building blocks of matter. They cannot be decomposed into simpler substances by ordinary chemical means[2][5][8]. Each element is represented on the periodic table, and there are currently 118 known elements[5][3]. Atoms of the same element can have different numbers of neutrons—these are called isotopes—but as long as the proton count remains the same, the atom is still considered the same element[1][6].

Elements One Nucleon Apart: Surprising Differences

The nucleus of an atom consists of protons and neutrons, collectively known as nucleons. Two elements that are only one nucleon apart in mass number may have dramatically different properties, especially if that nucleon is a proton (which changes the element) rather than a neutron (which creates an isotope of the same element)[1][6].

Here are some striking examples of elements that are only one nucleon (specifically, one proton) apart but have surprisingly different properties:

Sodium (Na, Atomic Number 11) vs. Magnesium (Mg, Atomic Number 12)

– Sodium is a highly reactive, soft, silvery metal that reacts explosively with water.
– Magnesium is a harder, less reactive metal that burns with a bright white flame but does not react violently with water.
– Adding just one proton transforms sodium into magnesium, changing its chemical behavior and role in biology and industry.

Chlorine (Cl, Atomic Number 17) vs. Argon (Ar, Atomic Number 18)

– Chlorine is a toxic, greenish-yellow gas and a highly reactive nonmetal, essential for disinfection and biological processes.
– Argon is a colorless, odorless, and completely inert noble gas used in lighting and welding.
– The addition of a single proton turns a reactive, poisonous gas into an inert, safe one.

Lead (Pb, Atomic Number 82) vs. Bismuth (Bi, Atomic Number 83)

– Lead is a dense, toxic metal with many industrial uses and is known for its cumulative health hazards.
– Bismuth is a brittle, crystalline metal that is non-toxic and even used in medicines and cosmetics.
– Despite being neighbors on the periodic table, their toxicity and applications are dramatically different.

Hydrogen (H, Atomic Number 1) vs. Helium (He, Atomic Number 2)

– Hydrogen is a highly flammable, reactive gas and the simplest element.
– Helium is an inert, non-flammable gas.
– The addition of one proton (and usually two neutrons) creates a completely unreactive element from the most reactive one.

These examples illustrate how the addition of a single nucleon—especially a proton—can result in elements with vastly different chemical and physical properties, underlining the precision and diversity of the periodic table[1][6][7].

Turning Lead into Gold

Turning lead into gold, long the dream of medieval alchemists, has been achieved in modern times through the power of nuclear physics rather than mystical chemistry. At CERN’s Large Hadron Collider, scientists propelled lead nuclei—each with 82 protons—at nearly the speed of light, causing near-miss interactions that generated intense electromagnetic fields. These fields sometimes knocked out exactly three protons from a lead nucleus, transforming it into gold, which has 79 protons. The process is fleeting and produces only minuscule, short-lived amounts of gold—just 29 trillionths of a gram in recent experiments—but it represents a literal, if impractical, fulfillment of the ancient quest for chrysopoeia, offering new insights into nuclear transmutation and the fundamental behaviors of matter under extreme conditions[9][10][13][16].

Turning Mercury into Gold

Turning mercury into gold, a classic goal of alchemy, has been achieved in the modern era through nuclear transmutation. Mercury and gold are neighbors on the periodic table, and by bombarding certain isotopes of mercury (such as Hg-196) with neutrons in a nuclear reactor or particle accelerator, the mercury nucleus can absorb a neutron and become Hg-197, which then undergoes beta decay to form stable gold-197—the only naturally occurring isotope of gold. This process was first demonstrated in 1941, but the gold produced was initially radioactive due to the isotopes formed. While the science is sound, the method is extremely inefficient and costly: producing even a tiny amount of gold from mercury requires vast energy and specialized equipment, making it far more expensive than mining natural gold. Nevertheless, this feat stands as a testament to the power of nuclear physics and the realization—on an atomic level—of the ancient alchemists’ dream.

In summary:
An atomic element is defined by its number of protons, and even a one-nucleon difference can mean the difference between a toxic gas and an inert one, a reactive metal and a safe one, or a poisonous heavy metal and a medicinal ingredient. This is the remarkable subtlety and power of the atomic nucleus in shaping the diversity of matter in our universe.