What is an atom in scientific terms?

The correct choice defines an atom as the smallest unit of an element, which is fundamental in chemistry and physics. Atoms consist of a nucleus made of protons and neutrons, surrounded by electrons. Each element on the periodic table is made up of a unique type of atom, which determines its properties and behavior in chemical reactions. This understanding is crucial because atoms combine to form molecules, making them the building blocks of matter.

The other options refer to different scientific concepts. The smallest unit of life is actually a cell, not an atom. The basic unit of heredity refers to a gene, which is a segment of DNA that carries information for traits. A group of molecules bonded together describes a compound or a complex structure, which is formed when different types of atoms are chemically combined, but it does not define an atom itself.