Atoms are the fundamental building blocks of matter, but they are often unstable because their outermost electron shells are incomplete. To achieve a more stable state, atoms gain or lose electrons, transforming them into electrically charged particles called ions. This process significantly alters the electron cloud surrounding the nucleus, meaning the size of the resulting ion is fundamentally different from the size of the original neutral atom. Understanding the relative sizes of these charged particles requires examining the mechanics of electron loss and gain.
Definitions and Formation
Ions are atoms or molecules that carry an electrical charge due to an imbalance between protons and electrons. A positively charged ion is called a cation, and it forms when a neutral atom loses one or more electrons. For example, a sodium atom (Na) easily gives up its single valence electron to form a sodium cation (\(\text{Na}^{+}\)).
Conversely, a negatively charged ion is called an anion, and it forms when a neutral atom gains one or more electrons. A chlorine atom (Cl) readily accepts an electron to become a chloride anion (\(\text{Cl}^{-}\)). Both cations and anions form as atoms seek the stability of a full outer electron shell, often mirroring the electron configuration of a noble gas.
The Role of Effective Nuclear Charge
The size of any atom or ion is determined by the balance between the positive pull from the nucleus and the repulsion between the negatively charged electrons. The net positive charge felt by an electron in the outermost shell is known as the Effective Nuclear Charge (ENC). This ENC is less than the total number of protons because inner-shell electrons shield the outer electrons from the full attractive force of the nucleus.
When an atom forms a cation by losing electrons, the remaining electrons feel a greater positive pull from the nucleus. The nuclear charge remains constant, but the number of repelling electrons decreases. This imbalance means the ENC increases, pulling the entire electron cloud inward and resulting in a smaller particle.
The opposite effect occurs when an atom forms an anion by gaining electrons. The extra electrons increase the electron-electron repulsion within the outermost shell. This greater repulsion pushes the electron cloud farther away from the nucleus. Since the nuclear charge remains unchanged, the added repulsion decreases the ENC felt by the valence electrons, allowing the cloud to expand significantly.
Comparing Ion Size to the Parent Atom
The size comparison between an ion and its neutral parent atom directly relates to the change in electron count. A cation is always smaller than its parent atom because the removal of electrons reduces electron-electron repulsion. Forming a cation often results in the complete loss of the outermost electron shell, significantly decreasing the particle’s radius.
The sodium cation (\(\text{Na}^{+}\)) is much smaller than the neutral sodium atom (Na) because it has one fewer electron shell and a stronger nuclear pull on its remaining electrons.
An anion is always larger than its parent atom due to the increased electron-electron repulsion caused by the added electrons. The incoming electrons occupy the existing outermost energy level, where they repel the other electrons, causing the entire shell to expand. For instance, the chloride anion (\(\text{Cl}^{-}\)) is larger than the neutral chlorine atom (Cl) even though both have the same number of protons.
Anions are generally much larger than cations because anion formation results in expansion, while cation formation results in contraction.
Isoelectronic Series and Comparative Size
A comparison of ionic size involves an isoelectronic series, which is a group of ions and atoms that possess the exact same number of electrons. For example, the ions \(\text{O}^{2-}\), \(\text{F}^{-}\), \(\text{Na}^{+}\), and \(\text{Mg}^{2+}\) all have ten electrons, mirroring the electron configuration of neon. Since the electron count is identical, the repulsion among electrons is constant across the series.
The size of the ions in an isoelectronic series is determined solely by the number of protons in the nucleus (the nuclear charge). As the nuclear charge increases across the series, the stronger positive pull attracts the fixed number of ten electrons more tightly.
Consequently, the largest ion is the one with the fewest protons, \(\text{O}^{2-}\) (8 protons), and the smallest is the one with the most protons, \(\text{Mg}^{2+}\) (12 protons). This pattern confirms that when the number of electrons is equal, the positive charge of the nucleus controls the size of the ion.