The outermost shell of an atom, known as the valence shell, holds the electrons that dictate an atom’s chemical personality. These valence electrons are the farthest from the nucleus and are the least tightly held, making them the primary participants in interactions with other atoms. Altering the number or energy state of these electrons is the fundamental mechanism driving all chemical reactions and many physical phenomena. A neutral atom seeks stability, and manipulating these outer shell electrons determines whether an atom forms a compound, changes its electrical properties, or emits light.
Altering the Net Charge of the Atom
One consequence of altering the outer shell electrons is the creation of an electrically charged particle, a process called ionization. This occurs when an atom loses or gains one or more valence electrons, resulting in an imbalance between the number of electrons and protons. When a neutral atom loses an electron, the positive charge from the protons outweighs the negative charge, forming a positively charged ion, known as a cation.
Conversely, when a neutral atom gains valence electrons, the atom acquires an excess of negative charge, forming an anion. This electron transfer is often driven by the tendency of atoms to achieve the stable configuration of noble gases, which typically involves having eight electrons in the outermost shell (the octet rule).
The resulting ion has different chemical and physical properties compared to the original neutral atom. For example, neutral sodium is a highly reactive metal, but its corresponding cation is an unreactive component of table salt. The formation of these charged particles is the first step toward creating compounds, as these oppositely charged ions are strongly attracted to one another.
Forming Stable Relationships with Other Atoms
The transfer or sharing of valence electrons is the mechanism by which atoms join together to form molecules and compounds. The complete transfer of electrons leads directly to the formation of an ionic bond. This occurs most readily between a metal atom, which tends to lose electrons, and a non-metal atom, which tends to gain them.
The resulting cation and anion are held together by a strong electrostatic force of attraction, forming an ionic lattice. Common table salt, sodium chloride, is an example where the positively charged sodium ions and negatively charged chloride ions are locked into a crystal structure. This bonding is characterized by a significant difference in the elements’ tendency to attract electrons.
Alternatively, atoms can achieve stability by sharing their valence electrons, resulting in the formation of a covalent bond. This sharing occurs between two non-metal atoms that are both seeking to fill their valence shells. The atomic orbitals of the two atoms overlap, allowing the shared electrons to be simultaneously attracted to both nuclei.
The sharing of electrons creates a molecule, which is a distinct chemical species with unique properties. For instance, a water molecule forms when an oxygen atom shares electrons with two hydrogen atoms, satisfying the valence requirements for all three. Covalent bonds can involve the sharing of one, two, or three pairs of electrons (single, double, or triple bonds), which influences the shape and reactivity of the resulting molecule.
Changing the Atom’s Energy State
A final way to alter a valence electron is by changing its energy level without causing it to leave the atom entirely, a process known as excitation. When an atom absorbs energy, such as from heat or light, a valence electron temporarily jumps from its normal, low-energy orbit (the ground state) to a higher-energy orbit. This jump requires the absorbed energy to precisely match the difference in energy between the two orbital levels.
The atom is unstable in this higher-energy state, and the excited electron quickly falls back down to a lower energy level. As the electron returns to its ground state, the atom releases the excess energy. This energy is often emitted as a photon, which is a particle of light or other electromagnetic radiation.
The color of the light emitted depends directly on the energy difference between the higher and lower orbits, as this difference determines the photon’s wavelength. This alteration of the electron’s energy state is the basis for technologies like neon signs and the colors seen in fireworks. This temporary change does not alter the atom’s charge or identity, but rather its ability to emit light.