The Structure of the Atom
An atom is the smallest unit of matter that retains all of the chemical properties of an element. Atoms combine to form molecules, which then interact to form solids, gases, or liquids. Water is composed of hydrogen and oxygen atoms that have combined to form water molecules. Atoms have different properties according to the arrangement and number of their basic particles. Many biological processes are devoted to breaking down molecules into their component atoms so they can be reassembled into a more useful molecule.
Atoms consist of three basic particles: protons, electrons, and neutrons (Figure 1). The nucleus of the atom contains the protons (positively charged) and the neutrons (no charge). The outermost region of the atom is called the electron shell and contain the electrons (negatively charged). The hydrogen atom (H) contains only one proton, one electron, and no neutrons.
Protons and neutrons have approximately the same mass, about 1.67 × 10-24 grams. Scientists define this amount of mass as one atomic mass unit (amu) or one Dalton. Although similar in mass, protons are positively charged while neutrons have no charge. Therefore, the number of neutrons in an atom contributes significantly to its mass, but not to its charge.
Electrons are much smaller in mass than protons, weighing only 9.11 × 10-28 grams, or about 1/1800 of an atomic mass unit. Therefore, they do not contribute much to an element’s overall atomic mass. When considering atomic mass, it is customary to ignore the mass of any electrons and calculate the atom’s mass based on the number of protons and neutrons alone. Electrons contribute greatly to the atom’s charge, as each electron has a negative charge equal to the positive charge of a proton (Figure 2). Scientists define these charges as "+1" and "-1." In an uncharged, neutral atom, the number of electrons orbiting the nucleus is equal to the number of protons inside the nucleus. In these atoms, the positive and negative charges cancel each other out, leading to an atom with no net charge.
Accounting for the sizes of protons, neutrons, and electrons, most of the volume of an atom—greater than 99 percent—is, in fact, empty space. Despite all this empty space, solid objects do not just pass through one another. The electrons that surround all atoms are negatively charged and cause atoms to repel one another--so