Classification of Elements and Compounds

In nature, pure substances exist as either elements or compounds. Depending on the basic unit, elements exist in either atomic or molecular form, while compounds can be subcategorized as either molecular or ionic.

Atomic elements have single atoms as their basic units. Helium, neon, and krypton are examples of monatomic elements that can exist as stable individual atoms.

Molecular elements, on the other hand, contain two or more atoms bonded together, forming a molecule as the basic unit. Diatomic hydrogen with two hydrogen atoms and polyatomic sulfur with eight sulfur atoms are examples of molecular elements.

In comparison to molecular elements, molecular compounds are formed when atoms of two or more nonmetallic elements combine by sharing valence electrons through covalent bonds. 

The combination lowers the overall potential energy of the system, thereby driving the formation of a stable molecule. Hence, the basic units of a molecular compound are discrete molecules composed of constituent atoms.

Ethanol is a molecular compound, which contains basic units of distinct ethanol molecules comprising two carbon, six hydrogen, and one oxygen atoms.

Ionic compounds are formed when atoms of metallic elements combine with atoms of one or more nonmetallic elements. The metal atoms readily lose their valence electrons to form positively charged cations while the non-metal atoms gain electrons forming negatively charged anions.

Strong electrostatic attractions or ionic bonds are formed between the oppositely charged ions, thereby lowering the total energy of the system and driving the assembly of stable, ordered, three-dimensional ionic lattice structures. 

The smallest array of ions that form electrically neutral basic units of an ionic compound is called the formula unit. Table salt is an ionic compound with a formula unit NaCl containing an alternating three-dimensional array of sodium ions and chloride ions.

However, many ionic compounds are composed of ions that are not singular, like sodium or chloride, but made of multiple covalently bonded atoms with a net negative or positive charge — called polyatomic ions. 

For instance, sodium acetate is composed of positively charged sodium ions and polyatomic acetate ions carrying a net negative charge.