Describe what atoms are and how they are related to elements, using one example to illustrate.

Atoms are the basic units of matter and the defining structure of elements. They are the smallest units of an element that retain all the chemical properties of that element.

Atoms are composed of three types of particles: protons, neutrons, and electrons. Protons and neutrons are heavier and stay in the nucleus at the center of the atom. The electrons are much lighter and exist in a cloud orbiting the nucleus. The number of protons in an atom's nucleus determines the atomic number and thus the identity of the element.

Elements are pure substances that are made from a single type of atom. They cannot be broken down into any simpler substances by chemical means. Each element has a unique number of protons, neutrons, and electrons. The number of protons (and therefore electrons, in a neutral atom) defines the element.

For example, let's consider the element hydrogen. A hydrogen atom has one proton in its nucleus, and one electron orbiting the nucleus. Therefore, the atomic number of hydrogen is 1. No other element has an atomic number of 1, so no other element has just one proton in each atom. This is what makes hydrogen unique and different from other elements.