Matter is broadly categorized into mixtures, which can be physically separated, and pure substances, which have a fixed chemical identity. Determining whether helium is a pure substance requires examining its atomic structure and chemical behavior. This analysis uses the principles of chemical composition to place helium within the established hierarchy of matter classification.
The Definition of a Pure Substance
A pure substance is defined in chemistry as matter that possesses a constant and uniform composition throughout and exhibits distinct, unchanging physical and chemical properties. This constancy means that any sample of the substance, regardless of its source, will behave identically under the same conditions. Pure substances are distinct from mixtures, which consist of two or more components physically combined in variable proportions.
Pure substances are further divided into two subcategories: elements and compounds. Elements are the most fundamental type of pure substance, consisting of only one type of atom and unable to be broken down into simpler substances by ordinary chemical means. Compounds, conversely, are pure substances made up of two or more different elements chemically bonded together in a fixed ratio, such as water or salt.
What Makes Helium Unique
Helium (He) is an element with an atomic number of 2, meaning every atom contains exactly two protons in its nucleus. It is the second-lightest element in the universe, only heavier than hydrogen. Under normal conditions, helium is a colorless, odorless, and tasteless gas.
Helium is characterized by having the lowest melting and boiling points of any known substance, becoming liquid only at extremely low cryogenic temperatures near absolute zero. This elemental gas is positioned in Group 18 of the Periodic Table, classifying it as one of the noble gases.
Classification: Why Helium is a Pure Element
Helium is unequivocally classified as a pure substance because it fulfills the criteria for being an element. A sample of helium gas consists only of helium atoms, each containing two protons, and there are no other types of atoms present in its ideal form. This singular atomic composition gives it the fixed chemical identity required of a pure substance.
Because helium atoms cannot be separated into simpler chemical components, it satisfies the definition of an element. If a sample of helium were mixed with oxygen, for example, it would become a mixture, but the isolated helium itself remains a pure element.
The Monatomic Nature of Noble Gases
Helium’s stability and consistent purity are directly linked to its unique atomic structure as a noble gas. Like all elements in Group 18, helium has a complete outer valence electron shell, containing two electrons, which is a stable configuration called a duplet. This full shell configuration eliminates the need for the atom to gain, lose, or share electrons with other atoms.
Consequently, helium is chemically inert, meaning it is highly non-reactive under normal conditions. It does not readily form chemical bonds with itself or other elements. Helium gas naturally exists as single, unbonded atoms, which is known as being monatomic, represented by the symbol He.