Science Entrance Practice Exam 2026 - Free Science Practice Questions and Study Guide

An atom is the smallest unit of an element that still retains all the chemical properties of that element. Each element on the periodic table is composed of atoms, which consist of a nucleus surrounded by electrons. The unique arrangement and number of protons, neutrons, and electrons in an atom determine the identity and characteristics of the element. For example, a single atom of carbon exhibits the properties associated with carbon, such as its ability to form various compounds.

Ions, molecules, and compounds refer to different structures involving atoms. An ion is an atom or molecule that has gained or lost one or more electrons, resulting in a net charge, but it can change the properties of the original neutral atom. A molecule consists of two or more atoms chemically bonded together and may represent a substance with distinct properties that differ from the individual atoms. A compound is formed when two or more different kinds of atoms bond together, creating a new substance with its own unique properties. Therefore, only an individual atom can fully encapsulate all properties inherent to that particular element.