The atom is the fundamental unit of matter, composed of a dense, central nucleus surrounded by negatively charged electrons. These electrons determine how atoms interact to form all substances. Specifically, valence electrons are the key players in almost every chemical process. Understanding their location and behavior is the first step toward predicting chemical reactions.
Defining the Outer Shell Electron
Atoms are structured with electrons residing in distinct energy levels, often visualized as shells surrounding the nucleus. The innermost shells hold core electrons, which are tightly bound to the nucleus and generally do not participate in chemical reactions. Valence electrons occupy the outermost energy level, referred to as the valence shell. Because they are farthest from the nucleus, the attractive pull is weaker, giving them the highest energy and making them the most likely to be involved in interactions.
Valence Electrons and the Periodic Table
To determine the number of valence electrons for main group elements (Groups 1, 2, and 13 through 18), a chemist can use the element’s group number on the Periodic Table. Elements in Group 1, such as sodium, have one valence electron, and those in Group 2 have two. For Groups 13 through 18, the number is determined by the last digit of the group number. For instance, elements in Group 15 have five valence electrons, and those in Group 17 have seven. All elements within the same vertical column share the same number of outermost electrons, which explains their similar chemical properties.
The Rule of Eight and Chemical Stability
The behavior of valence electrons is governed by the drive toward chemical stability. Atoms are most stable when their outermost shell is completely full, resembling the nonreactive noble gases. For most elements, this stable state requires eight electrons in the valence shell, known as the octet rule. Atoms lacking a full outer shell are chemically active and will readily engage with others to attain stability. They achieve a full octet by either gaining, losing, or sharing their valence electrons.
Predicting Chemical Behavior and Bond Type
Knowing the number of valence electrons allows prediction of an atom’s chemical behavior and the type of bond it will form. Metals (one, two, or three valence electrons) tend to be electron donors, losing them to become positively charged ions. Non-metals (five, six, or seven valence electrons) are electron acceptors, gaining electrons to form negatively charged ions. The transfer of electrons between these types of atoms results in an ionic bond, held together by electrostatic attraction. If two non-metal atoms share their valence electrons, a covalent bond is created, which forms the basis for most molecules found in living organisms.