Formation of Ionic Compounds
When you prepare nixtamal, you add lime (cal) to the water with the corn; when you season some beans, you add table salt. Both are solid substances made of a metal and a nonmetal. How do atoms this different join together, and why is salt NaCl but quicklime is CaO? By the end of the page you will know.
Valence Electrons and the Octet
Atoms bond using only the electrons in their outermost shell, the valence electrons. In the representative elements, the number of valence electrons is the last digit of the group in the periodic table: group 1 → 1 electron, group 2 → 2, group 16 → 6, group 17 → 7.
The noble gases (group 18) hardly react: they have 8 valence electrons (helium has 2), and that makes them very stable. The other atoms tend to end up with 8 in their outermost shell. This is called the octet rule.
A Lewis diagram draws only those electrons as dots around the symbol.
Giving and Gaining: Ions
An atom is neutral: it has as many protons (+) as electrons (−). If it loses or gains electrons, it ends up with a charge and is called an ion.
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Metals (few valence electrons) lose electrons easily. When they lose them, they end up with a positive charge: cations.
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Nonmetals (close to the octet) gain electrons. They end up with a negative charge: anions.
Rule of thumb: the metal gives up all its valence electrons; the nonmetal gains the ones it needs to reach 8.
| Atom | Outer electrons | What it does | Ion |
|---|---|---|---|
| Na | 1 | gives up 1 e⁻ | $\text{Na}^+$ |
| Mg | 2 | gives up 2 e⁻ | $\text{Mg}^{2+}$ |
| Ca | 2 | gives up 2 e⁻ | $\text{Ca}^{2+}$ |
| Cl | 7 | gains 1 e⁻ | $\text{Cl}^-$ |
| O | 6 | gains 2 e⁻ | $\text{O}^{2-}$ |
Notice: by giving up its outer shell, the metal is left with the previous shell full (an octet), and the nonmetal completes its own. Both ions end up stable.
Sodium (Na) has 1 electron in its valence shell and chlorine (Cl) has 7. Explain in your own words how table salt (NaCl) forms: which atom gives up electrons, which atom…
Sodium (Na) has 1 electron in its valence shell and chlorine (Cl) has 7. Explain in your own words how table salt (NaCl) forms: which atom gives up electrons, which atom gains them, which ions appear, and why they stay together.
How NaCl Forms
Let's look at table salt step by step.
So the complete answer to the exercise is: sodium gives up an electron, chlorine gains it, the ions $\text{Na}^+$ and $\text{Cl}^-$ appear, and they are held together by the electric attraction between opposite charges.
When the Charges Don't Match
The final compound is always neutral: the total positive charge must equal the total negative charge. If one ion gives up 2 electrons and the other can only receive 1, more than one ion is needed.
A trick for the formula: the charge of one becomes the subscript of the other ("they cross over"). In $\text{Mg}^{2+}$ and $\text{Cl}^-$: the 2 goes to the chlorine and the 1 (which isn't written) to the magnesium.
Magnesium (Mg) has 2 valence electrons and chlorine (Cl) has 7. When magnesium chloride (MgCl₂) forms, how many electrons does each magnesium atom give up, and how many…
Magnesium (Mg) has 2 valence electrons and chlorine (Cl) has 7. When magnesium chloride (MgCl₂) forms, how many electrons does each magnesium atom give up, and how many chlorine atoms are needed to receive them? Explain why the formula is MgCl₂ and not MgCl.
Predicting the Formula of Any Ionic Compound
With the above, you now have a method for any representative metal + nonmetal.
Summary of the method:
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Find the group of each element in the periodic table and get its valence electrons.
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The metal gives up all of its own (positive charge); the nonmetal gains the ones it needs to reach 8 (negative charge).
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Combine the number of ions so that the total charge is zero.
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Write the formula with the metal first.
| Compound | Ions | Ratio |
|---|---|---|
| NaCl | $\text{Na}^+$, $\text{Cl}^-$ | 1 : 1 |
| MgCl₂ | $\text{Mg}^{2+}$, $\text{Cl}^-$ | 1 : 2 |
| CaO | $\text{Ca}^{2+}$, $\text{O}^{2-}$ | 1 : 1 |
| Na₂O | $\text{Na}^+$, $\text{O}^{2-}$ | 2 : 1 |
Calcium (Ca) has 2 valence electrons and oxygen (O) has 6. Predict the charges of the ions formed when both elements react and write the formula of the resulting ionic…
Calcium (Ca) has 2 valence electrons and oxygen (O) has 6. Predict the charges of the ions formed when both elements react and write the formula of the resulting ionic compound (quicklime, widely used in Mexico to nixtamalize corn). Justify your answer.
Go back to the beginning: salt is NaCl because sodium gives up 1 electron and chlorine gains 1 (1 : 1 ratio); quicklime is CaO because calcium gives up 2 and oxygen gains 2. In both cases, the metal and the nonmetal join through the attraction between ions of opposite charge.