Ions Knowledge-Based Study

Study notes

1. In an electrically neutral atom, the number of positively charged protons located in the nucleus exactly equals the number of negatively charged electrons surrounding the nucleus. Consequently, the net electrical charge of the atom is zero.

   An ion is an atom or group of atoms that carries a net electrical charge as a result of gaining or losing one or more electrons.

   • Conservation of Nuclear Identity: Chemical processes involve only the transfer (loss or gain) of electrons. The number of protons and neutrons in the nucleus remains strictly unchanged during ion formation.
   • Net Charge Principle: The overall charge is determined by the difference between the fundamental subatomic particles:
     Net Charge = Number of Protons − Number of Electrons

   A cation is formed when a neutral atom loses one or more valence electrons. Because electrons carry a negative charge, their removal leaves an excess of positive protons in the nucleus.

   Characteristics of Cations:

   • The number of protons is greater than the number of electrons ($p^+ > e^-$).
   • Metal atoms typically lose electrons to achieve a stable electronic configuration, forming cations.
   • A cation carries a positive sign written as a superscript to the right of the chemical symbol, accompanied by the magnitude of the charge if greater than one.

   Examples:

   • A neutral sodium atom ($11$ protons, $11$ electrons) loses $1$ electron to form a sodium cation, written as $\\text{Na}^+$ ($11$ protons, $10$ electrons).
   • A neutral magnesium atom ($12$ protons, $12$ electrons) loses $2$ electrons to form a magnesium cation, written as $\\text{Mg}^{2+}$ ($12$ protons, $10$ electrons).

   An anion is formed when a neutral atom gains one or more electrons into its valence shell. The addition of extra negatively charged electrons results in an overall negative charge.

   Characteristics of Anions:

   • The number of electrons is greater than the number of protons ($e^- > p^+$).
   • Nonmetal atoms typically gain electrons to attain a stable valence shell, forming anions.
   • The negative charge is indicated as a superscript following the element symbol. Monoatomic anions commonly adopt the suffix -ide.

   Examples:

   • A neutral chlorine atom ($17$ protons, $17$ electrons) gains $1$ electron to form a chloride anion, denoted as $\\text{Cl}^-$ ($17$ protons, $18$ electrons).
   • A neutral oxygen atom ($8$ protons, $8$ electrons) gains $2$ electrons to form an oxide anion, denoted as $\\text{O}^{2-}$ ($8$ protons, $10$ electrons).

   Standard isotopic notation for an ion is written as $^{A}_{Z}\\text{X}^{q}$, where:

   • $Z$ (Atomic Number): Number of protons ($p^+$), defining the elemental identity.
   • $A$ (Mass Number): Total number of nucleons ($p^+ + n^0$).
   • $q$ (Net Charge): Difference between protons and electrons ($q = p^+ - e^-$).

   Calculation Rules:

   1. Protons: $p^+ = Z$
   2. Neutrons: $n^0 = A - Z$
   3. Electrons: $e^- = p^+ - q$

   Worked Example: Consider the sulfide ion $^{32}_{16}\\text{S}^{2-}$:

   • Atomic Number ($Z$) = $16$, so the ion contains $16\\text{ protons}$.
   • Mass Number ($A$) = $32$, so the number of neutrons is $32 - 16 = 16\\text{ neutrons}$.
   • Charge ($q$) = $-2$, so the number of electrons is $16 - (-2) = 18\\text{ electrons}$.