Atoms contain electrons residing in specific energy levels or shells. The outermost shell is the valence shell, and the electrons within it dictate how an atom interacts with others. Chemical reactions involve the movement—sharing or transfer—of these valence electrons, which drives all chemical bonding and reactivity.
The Goal of Stability
Atoms engage in chemical reactions to achieve maximum stability, corresponding to the lowest possible energy state. Stability is usually achieved by attaining a full valence shell, known as the Octet Rule. This means an atom is most stable with eight electrons in its outermost shell, mimicking the non-reactive Noble Gases.
The decision to lose or gain electrons depends on energetic favorability. For an atom with only one or two valence electrons, it requires less energy to lose those few electrons than to gain six or seven. By shedding these outermost electrons, the atom reveals the next inner shell, which is already full, achieving a stable Noble Gas configuration. This preference for electron loss defines the chemical nature of groups on the left side of the periodic table.
Groups Prone to Electron Loss
The groups on the far left of the periodic table most readily lose electrons because they possess the fewest valence electrons. These elements have low ionization energy, which is the energy required to remove an electron from a neutral atom. The farther left an element is, the easier it is to remove an electron.
Alkali Metals (Group 1)
The Alkali Metals are the most eager to lose electrons. Every element in this group, such as Lithium, has just one loosely held valence electron. Losing this single electron is energetically favorable, quickly resulting in the stable electron configuration of the preceding Noble Gas.
Alkaline Earth Metals (Group 2)
Moving right, the Alkaline Earth Metals are also strong candidates for electron loss, as each has two valence electrons. Elements like Magnesium and Calcium preferentially shed both electrons to achieve stability. This process is far more favorable than trying to gain six electrons.
Boron Group (Group 13)
Elements in Group 13 typically lose three electrons to reach their stable state. Aluminum, for example, has three valence electrons and forms a positive three charge when it loses them. The elements on the left side of the table are metals defined by their strong tendency to lose their few valence electrons. Transition metals also lose electrons, but their behavior is more complex as they often lose a variable number.
Consequences of Electron Loss
When an atom loses one or more electrons, the resulting particle carries a net positive electrical charge because the number of protons remains unchanged. This positively charged atom is called a cation.
The magnitude of the positive charge on the cation equals the number of electrons lost. For instance, a Group 1 element that loses one electron forms a cation with a +1 charge. Elements with low ionization energy, such as the Alkali and Alkaline Earth Metals, are reactive because they require little energy to form these cations.
These cations are attractive to negatively charged ions, called anions, which are atoms that have gained electrons. The electrostatic attraction between a positively charged cation and a negatively charged anion forms a strong chemical link known as an ionic bond. This is the fundamental mechanism behind the formation of salts and other ionic compounds.