Bohr Diagrams, Ions, and Chemical Compounds

Cations (Positive Ions)

Metals typically have only a few electrons in their valence shells, making it easier for them to lose those electrons rather than gaining many more. When an atom loses electrons, it ends up with more protons than electrons, resulting in a positive charge. These are called cations.

• Lithium ($Li$): Loses 1 electron to become $Li^+$.
• Magnesium ($Mg$): Loses 2 electrons to become $Mg^{2+}$.

Anions (Negative Ions)

Non-metals often have nearly full valence shells, so they tend to gain electrons to complete them. When an atom gains electrons, it has more electrons than protons, resulting in a negative charge. These are called anions.

• Chlorine ($Cl$): Gains 1 electron to become $Cl^-$.
• Oxygen ($O$): Gains 2 electrons to become $O^{2-}$.

Chemical Compounds: Ionic vs. Non-Ionic

Once ions are formed, they can combine to create chemical compounds. The way these atoms interact depends on whether they are transferring or sharing electrons.

Ionic Compounds

Ionic compounds are formed through the electrostatic attraction between a positive ion (usually a metal) and a negative ion (a non-metal). This process involves a complete electron transfer.

A classic example is Sodium Chloride ($NaCl$). In this reaction, a Sodium ($Na$) atom transfers its valence electron to a Chlorine ($Cl$) atom. The Sodium becomes a positive ion ($Na^+$) and the Chlorine becomes a negative ion ($Cl^-$), and the opposite charges pull them together into a stable bond.

Non-Ionic (Covalent) Compounds

When two non-metals combine, they do not transfer electrons. Instead, they form non-ionic (covalent) compounds by electron sharing. By sharing electrons, both atoms can effectively "fill" their valence requirements without one atom completely taking from the other.

Writing Formulas and Naming Compounds

Naming Ionic Compounds

To write a correct chemical formula for an ionic compound, you must ensure the total charge of the compound is zero.

The Process:

1. Write the symbols for the ions.
2. Determine the charges.
3. Use the charges to find the ratio of atoms needed to balance to zero.

Examples:

• Magnesium Oxide: $Mg$ is $+2$ and $O$ is $-2$. They balance in a 1:1 ratio, so the formula is $MgO$.
• Aluminum Chloride: $Al$ is $+3$ and $Cl$ is $-1$. To balance the $+3$ charge, you need three $Cl^-$ ions, resulting in $AlCl_3$.

If the compound contains a polyatomic ion (a group of atoms acting as a single unit with a charge), simply name the metal followed by the name of the polyatomic ion.

• $Mg(OH)_2$: Magnesium hydroxide.
• $CaCO_3$: Calcium carbonate.

Naming Molecular (Covalent) Compounds

Because molecular compounds consist only of non-metals and involve sharing, they do not have charges. To name them, we use Greek prefixes to indicate exactly how many atoms of each element are present in the molecule.

Examples:

• $CO_2$: Carbon dioxide.
• $N_2O_4$: Dinitrogen tetroxide.
• $P_4O_{10}$: Tetraphosphorus decoxide.