Formation of Ions

The charge of an ion is directly related to its Group Number in the periodic table, which indicates the number of valence electrons available for loss or the number of vacancies to be filled.

Metals (Groups 1, 2, and 3): These elements have 1, 2, or 3 electrons in their outer shell, respectively. It is energetically favorable for them to lose these electrons, forming ions with charges of $+1$, $+2$, or $+3$.

Non-metals (Groups 5, 6, and 7): These elements have 5, 6, or 7 electrons. They tend to gain electrons to reach a total of 8, forming ions with charges of $-3$, $-2$, or $-1$.

Transition Metals: Unlike main-group elements, transition metals can often form multiple different ions (e.g., Iron can form $Fe^{2+}$ or $Fe^{3+}$), usually indicated by Roman numerals in their names.

Polyatomic ions are clusters of atoms covalently bonded together that carry an overall net charge. They act as a single unit during chemical reactions and formula writing.

Common examples include the Ammonium ion ($NH_4^+$), which is the only common positive polyatomic ion, and various negative ions like Hydroxide ($OH^-$), Nitrate ($NO_3^-$), Sulfate ($SO_4^{2-}$), and Carbonate ($CO_3^{2-}$).

When writing formulas involving polyatomic ions, if more than one of the ions is required to balance the charge, the entire ion must be placed inside brackets with a subscript outside (e.g., $Mg(OH)_2$).

The fundamental rule for ionic compounds is that they are electrically neutral; the total positive charge must exactly equal the total negative charge.

The Swap-and-Drop Method is a reliable technique: take the numerical value of the charge of the cation and make it the subscript of the anion, and take the numerical value of the charge of the anion and make it the subscript of the cation.

For example, to combine Aluminum ($Al^{3+}$) and Oxygen ($O^{2-}$), the '3' from Aluminum becomes the subscript for Oxygen, and the '2' from Oxygen becomes the subscript for Aluminum, resulting in $Al_2O_3$.

Always simplify the subscripts to the lowest whole-number ratio. For instance, $Mg^{2+}$ and $O^{2-}$ would technically 'swap' to $Mg_2O_2$, but this must be simplified to $MgO$.

The 'Electron-Charge' Inverse: Always remember that losing electrons (negative particles) makes an atom more positive. A common mistake is thinking 'loss' means a 'negative' charge.

Roman Numerals: If a question mentions 'Copper(II)', the Roman numeral is telling you the charge ($2+$), not the number of atoms in the formula.

Conductivity Logic: If asked about electricity, remember that ionic solids do not conduct because ions are locked in a lattice. They only conduct when molten or aqueous because the ions are free to move.

Verification: After writing a formula, multiply the charge of each ion by its subscript. The sum must be zero. For $Al_2O_3$: $(2 \times +3) + (3 \times -2) = 6 - 6 = 0$.