By Veronica Mitchell Updated Aug 30, 2022
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Atoms react by gaining, losing, or sharing electrons. Their reactivity depends on how easily they can alter their outer electron shell.
Atoms are built from three subatomic particles: protons, neutrons, and electrons. The atomic number—count of protons—identifies the element; for example, any atom with six protons is carbon. Neutral atoms maintain equal numbers of positively charged protons and negatively charged electrons. Electrons orbit the nucleus in energy levels, or shells, that are arranged closest to farthest from the nucleus. Each shell can accommodate only a limited number of electrons, so the outermost electrons—known as valence electrons—are crucial in determining chemical behavior.
Because the number of electrons equals the number of protons, most atoms have a partially filled outer shell. When atoms encounter other species, they tend to achieve a full valence shell, either by losing electrons, gaining electrons, or sharing electrons through covalent bonds. This drive toward a stable configuration allows chemists to predict an atom’s reactivity by examining its electron configuration. Noble gases such as neon and argon are inert because they already possess a complete outer shell, and they rarely participate in reactions unless exposed to extreme conditions.
The periodic table arranges elements so that atoms with similar properties appear in the same column or group. Group 1 elements—sodium, potassium, and others—each contain a single valence electron that is weakly held by the nucleus. Consequently, these atoms readily lose that electron, rendering them highly reactive. In contrast, Group 17 elements have one empty spot in their outer shell; they are eager to accept an electron, which explains their high electronegativity and reactivity.
Ionisation energy (I.E.) is the energy required to remove an electron from an atom. A low first ionisation energy indicates that an atom can easily shed its outer electron. Ionisation energies are measured for successive removal of electrons: the first I.E. removes the outermost electron, the second removes the next, and so on. For example, calcium (Group 2) has a first I.E. of 590 kJ mol⁻¹ and a second I.E. of 1 145 kJ mol⁻¹, but a markedly higher third I.E. of 4 912 kJ mol⁻¹. These values suggest that calcium typically loses its first two electrons during chemical reactions.
Electron affinity (Eₐ) measures how readily an atom accepts an electron. A highly negative electron affinity indicates a strong tendency to gain an electron. Fluorine, the most reactive element, has an electron affinity of –328 kJ mol⁻¹, which makes it exceptionally eager to accept electrons. Like ionisation energies, successive electron affinities reveal how an element will behave when interacting with other species.
