D1.2 Atoms, Molecules, and Ions

A different fundamental aspect of chemistry is that we can better understand elements, compounds, and reactions if we build models based on tiny particles that are constantly in motion: atoms and molecules.

An atom is the tiniest particle of an element that has the chemical properties of that element. Atoms are extremely small, with sizes on the order of 100 pm to 350 pm (1 pm = 10⁻¹² m). The smallest thing discernible with the most powerful optical microscope is more than 1000 times bigger than an atom. A molecule is two or more atoms connected by chemical bonds. Atomic scale or sub-microscopic scale refers to things about the same size as atoms and molecules. Chemists build atomic-scale models to interpret and predict macroscopic phenomena. For example, many substances consist of molecules; the properties of these molecular substances can be predicted if we know which atoms make up their molecules and how those atoms are arranged.

Atoms contain three kinds of particles: protons, neutrons, and electrons. Atoms are identified by the number of protons in the nucleus, the atomic number. Protons are positively charged. Neutrons have no electric charge. Electrons have negative charge with the same magnitude as a proton’s positive charge. Protons and neutrons have much greater mass than electrons and are found in the nucleus, a very small volume in the center of an atom that contains most of the atom’s mass. Electrons, which constitute less than 1/1000 the mass of an atom occupy 99.9999999999999% of the space the atom occupies. When a chemical reaction occurs, atoms remain unchanged, except that a few electrons in the outer part of one atom may transfer to the outer part of another atom. Before and after any chemical reaction the same number of atoms of each type is present; this is known as the law of conservation of matter. When copper metal reacted with oxygen from air to form copper oxide in the video you’ve previously watched, the copper oxide included all the copper atoms that originally were in the copper that reacted. When hydrogen reacted with the copper oxide those copper atoms remained on the surface as copper metal.

Ions can form from atoms or molecules. An ion is an atom or molecule that has gained or lost one or more electrons and therefore has a negative or positive electric charge. For example, when copper reacts with oxygen, the product consists of copper(II) ions and oxide ions. In a copper(II) ion a Cu atom has lost two electrons to form an ion with two units of excess positive charge, Cu²⁺; in an oxide ion an O atom has gained two electrons to form an ion with two units of excess negative charge, O²⁻. When a molecule gains or loses electrons, polyatomic ions form: the molecule NO₂ forms both a positive ion, NO₂⁺ and a negative ion, NO₂⁻, by loss and gain of one electron.