High school basics / The Constituent Particles of Matter 4 / 6

Formation of Ions and Ionic Formulas

Atoms become ions when they exchange electrons. Let us organize positive cations and negative anions.

Basic Knowledge

Atoms exchange electrons in order to obtain the same electron arrangement as a noble gas (the octet rule).
・When an atom loses electrons, it becomes a cation (positive ion): Na→Na⁺, Ca→Ca²⁺, Al→Al³⁺.
・When an atom receives electrons, it becomes an anion (negative ion): Cl+e^-→Cl^-, O+2e^-→O^2-.
The number of electrons lost or received is called the valence. Valence is determined by the number of valence electrons in the original atom or the number of electrons needed in the outermost shell (the octet principle).
There are also polyatomic ions:
・NH₄⁺ (ammonium ion)
・OH^- (hydroxide ion)
・NO₃^- (nitrate ion)
・SO₄^2- (sulfate ion)
・CO₃^2- (carbonate ion)
・PO₄^3- (phosphate ion)
These are essential items to memorize.

Key Terms
Ion（An atom or atomic group with an electric charge, formed by electron exchange）
Cation（An ion that loses electrons and has a positive charge. Metals readily form them）
Anion（An ion that receives electrons and has a negative charge. Nonmetals readily form them）
Valence（The absolute value of an ion’s charge. Mg²⁺ has valence 2）
Polyatomic ion（One ion unit made of several bonded atoms. NH₄⁺, OH^-）
Ionization energy（Energy required to remove one electron from an atom. An indicator of how readily a cation forms）
Electron affinity（Energy released when an atom receives one electron. An indicator of how readily an anion forms）

Further Study (Background and Significance)

Elements with low ionization energy readily form cations (alkali metals, especially Cs). Elements with high electron affinity readily form anions (Group 17 halogens, especially Cl). These trends show beautiful patterns in the periodic table and are applied, for example, to catalyst selection in the Haber–Bosch process.
Body electrolytes (Na⁺, K⁺, Cl^-, Ca²⁺) exist as ions and are responsible for nerve transmission, muscle contraction, and osmotic-pressure regulation. This is why sports drinks advertise “electrolyte replenishment.” Ions are also deeply connected with life science and medicine, and understanding them is a starting point in the organic chemistry and life-science fields pursued by the CEO.

Key Points

Atoms exchange electrons to achieve a noble-gas arrangement
Metals form cations; nonmetals form anions
Valence: Group 1=1, Group 2=2, Group 13=3, Group 16=2−, Group 17=1−
Representative polyatomic ions: NH₄⁺, OH^-, NO₃^-, SO₄^2-, CO₃^2-
Low ionization energy → readily forms cations
High electron affinity → readily forms anions
Body electrolytes are essential for nerves, muscles, and osmotic pressure

Points to Note (Common Confusions)

① Do not confuse ionization energy (atom→cation) with electron affinity (atom→anion). The former absorbs energy; the latter releases energy. ② Whether Be and Mg are included among alkaline-earth metals varies by convention, but in basic chemistry it is acceptable to think of all Group 2 elements as forming 2+ cations. ③ The structures of polyatomic ions such as SO₄^2- may be memorized in basic chemistry. ④ Hydrogen H can become either H⁺ (a cation) or H^- (an anion).

Practice

When Ca forms a stable ion, give its valence and ionic formula.
Give the names of the polyatomic ions NH₄⁺ and OH^-.
What type of ion does an element with low ionization energy readily form?

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Structure of the Periodic Table and the Periodic Law

Electronegativity and Periodicity of Element Properties