元素：化学記号と同位体

Some elements have symbols derived from their names in other languages because the symbols were established historically before standardization. For example, iron's symbol Fe comes from its Latin name ferrum, and mercury's symbol Hg derives from its Latin name hydrargyrum. This practice reflects the international nature of chemistry and helps maintain consistency across different scientific communities.

An element is a pure substance containing only one type of atom, defined by its atomic number or number of protons. An isotope is a variation of that element where the number of neutrons differs, resulting in different mass numbers and atomic masses. For example, carbon-12 and carbon-13 are both carbon but have different numbers of neutrons, making them different isotopes of the same element.

The mass number, which represents the total of protons and neutrons, is written as a superscript to the left of the element symbol. For example, in the notation 24Mg, the 24 is the mass number positioned as a superscript. This notation clearly identifies the specific isotope. The atomic number is sometimes omitted since it's already defined by the element symbol itself.

Radioactive isotopes are variations of an element whose nuclei decay by emitting energy, transforming into other substances. Stable isotopes, like standard hydrogen and deuterium, remain unchanged under natural conditions. Tritium, a radioactive isotope of hydrogen with two neutrons, spontaneously decays. The instability arises from an imbalance in the proton-to-neutron ratio, causing the nucleus to release energy to reach a more stable state.

Capitalization distinguishes element symbols from compound notations. Only the first letter of a symbol is capitalized; if a second letter exists, it is lowercase. This prevents confusion—for example, Co represents cobalt, while CO represents carbon monoxide, a compound containing two different elements. Proper capitalization ensures clear communication in chemical writing and prevents misidentification of substances.

Hydrogen has three naturally occurring isotopes with different neutron counts. Standard hydrogen contains no neutrons, deuterium contains one neutron, and tritium contains two neutrons. All three have the same atomic number of one proton, but their varying neutron numbers create different mass numbers and atomic masses. While standard hydrogen and deuterium are stable, tritium is radioactive and decays over time.

At minimum, each square on the periodic table contains the atomic number and the chemical symbol. For isotopes, the mass number is also included to identify the specific isotope. The atomic number, representing the number of protons, defines the element's identity. Additional information like atomic mass may be included, but the symbol and atomic number are essential for identifying and distinguishing elements.