Reactivity and Ionisation Energy in Elements

The Role of Electron Configuration in Element Reactivity

The reactivity of an element is fundamentally related to its electron configuration, particularly the valence electrons in the outermost shell. Elements tend to react in ways that will achieve a stable electron configuration, often resembling that of the nearest noble gas. Sodium (Na), for example, with one electron in its outermost shell, readily loses that electron to form a Na+ ion, thus achieving the electron configuration of neon (Ne), a noble gas. Conversely, noble gases like neon have complete outer shells and are inert because they do not need to gain or lose electrons to achieve stability.

Ionisation Energy and Atomic Reactivity

Ionisation energy is the amount of energy required to remove an electron from an atom in the gaseous state. It is a key indicator of an element's reactivity. The first ionisation energy is the energy needed to remove the outermost, or least tightly bound, electron. Atoms with low ionisation energies tend to lose electrons and form cations easily, making them highly reactive. Conversely, atoms with high ionisation energies hold their electrons more tightly and are less likely to form cations and react.

Electron Shells and Chemical Stability

Electrons are arranged in shells around an atom's nucleus, with each shell having a maximum capacity. Atoms strive for a full outer shell configuration, which is typically more stable. Sodium, with one electron in its outer shell, can easily lose that electron to achieve a full shell, resulting in high reactivity. Aluminum, with three electrons in its outer shell, requires more energy to lose all three electrons to achieve stability, making it less reactive than sodium.

Significance of the First Ionisation Energy

The first ionisation energy is the energy required to remove the first electron from an isolated gaseous atom. This energy reflects the atom's reactivity, with a lower first ionisation energy suggesting a greater ease in losing an electron and thus higher reactivity. The first ionisation energy is often used to predict how an element will react, particularly in the formation of cations.

Concept of Second Ionisation Energy

The second ionisation energy is the energy needed to remove a second electron from an atom that has already lost one electron and is now a +1 ion. This energy is typically higher than the first ionisation energy because the electron is being removed from a positively charged ion, which holds onto its remaining electrons more tightly. The trend of increasing ionisation energy continues with each successive electron removed, reflecting the increasing nuclear charge over the fewer remaining electrons.

Factors Affecting Ionisation Energy

Ionisation energy is influenced by several factors, including the nuclear charge, the distance of the electron from the nucleus, and electron shielding. A higher nuclear charge increases the attraction between the nucleus and the electrons, resulting in higher ionisation energy. The greater the distance between the nucleus and the electron, the weaker the attraction and the lower the ionisation energy. Electron shielding occurs when inner electrons reduce the effective nuclear charge experienced by outer electrons, thereby decreasing ionisation energy.

Trends in Successive Ionisation Energies

Successive ionisation energies increase for each electron removed from an atom or ion. This is because the ion becomes more positively charged after each electron is removed, which increases the electrostatic attraction between the nucleus and the remaining electrons. The first ionisation energy is always the lowest because it involves removing an electron from a neutral atom. As more electrons are removed, the ion becomes more positively charged, and the increased nuclear attraction makes it progressively more difficult to remove additional electrons.