Ionic Bond Definition

One ionic bond definition is that atoms are held together by charge. Plus sticking to minus. Charged atoms are called ions, and they can clump like a pile of magnets locked together. The charged atoms clump together in a very specific geometric pattern that we often call a crystal structure. Like salt.

The best ionic bond examples include salts. Specifically, common table salt, the stuff we put on food, is held together by ionic bonds. So, when somebody talks about an ionic bond, think about table salt, formula NaCl.

NaCl really means Na+ is stuck to Cl-. There is an ionic bond because of the force of attraction between positively and negatively charged ions in the compound. Ions are atoms that are positively or negatively charged. We say that NaCl is an ionic compound, but note that we don’t write the + and – in the formula (not Na+Cl-). We call it sodium chloride (not sodium chlorine).

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Simple Ionic Bond Definition

The easiest and most straightforward ionic bond definition is that there are both metal and nonmetal elements in the substance. Basically, if a substance contains both metal and nonmetals elements, it’s probably an ionic compound held together by ionic bonds. The periodic table below shows the metal elements in blue and nonmetal elements in gold.

Na, or sodium, is on the left of the periodic table (leftmost column, third row down). Cl, or chlorine, is on the right of the periodic table (second column from the right, third row down). Hence, NaCl has ionic bonds. There are many other ionic bond examples, too. Sometimes table salt is “iodized” for nutritional reasons. In addition to NaCl, it also contains NaI. This really means Na+ is stuck to I-. We call it sodium iodide.

This ionic bond definition is simple. It works well when elements are at opposite ends of the periodic table. The problem, however, is that the boundary between the metals and nonmetals on the periodic table is not really so exact. Elements near the red line in the above figure may not act like proper metals and nonmetals.

Formal Ionic Bond Definition

A more formal ionic bond definition is that there is a big difference in electronegativity between the atoms in a compound. Electronegativity is shown in the below table. It’s a number that ranges from 0 to 4. Atoms with a relatively high electronegativity tend to become negative. Atoms with the relatively low electronegativity tend to become positive. Specifically, “they” say that if the difference in electronegativity is more than two units, an ionic bond will form.

Consider again NaCl. Hypothetically, imagine a neutral Na atom approaching a neutral Cl atom. (This would never actually happen. Just imagine.) Cl has electronegativity of 3.0 compared to Na at 0.9, for a difference of 2.1. An ionic bond would form, as Na becomes Na+ and Cl becomes Cl-. They are now stuck together as ions in a compound NaCl.

But now consider NaI, which definitely does have ionic bonds. Iodine has an electronegativity of 2.5 compared to Na at 0.9, for a difference of 1.6. It doesn’t quite make the cutoff of two for the formal definition of an ionic bond. Houston, we have a problem!

If we could lower the cutoff of electronegativity difference to 1.5, then NaI would meet the criteria of being ionic. But then this would cause the opposite problem. Compounds truly lacking ionic bonds would be classified as having them.

What’s going on here is that this formal definition isn’t really based on any exact science. It’s just an arbitrary cut-off that somebody came up with. A few times over. Some scientists say the cutoff should be 2, some say 1.5, yet others say other things.

You might cleverly note that nonmetal elements have high electronegativity and metal elements have low electronegativity. So, this formal definition based on numbers is really just saying that we have both metals and nonmetals involved in ionic bonds. It’s mostly just an overly complicated restatement of the simple definition above. Just be warned that many science teachers and textbook publishers think the cutoff is real, and you might concede here in the interest of getting a point on an exam. So be it.

Ionic Bond Examples

NaCl

The NaCl bond type is ionic, because it contains a metal and nonmetal. A good real-world example is a crystal of salt on the dinner table. Hopefully you understand that this salt crystal, even though tiny, has way more than just a single Na+ and a single Cl-. There are many, many interlocked Na+ and Cl- arranged in an alternating pattern. We call this grid a lattice, or crystal structure. In the figure below, note that any particular Na+ is not bonded to any particular Cl-. There is a vast network of ionic bonds holding the substance together. It would be extremely uncommon to talk about any one particular ionic bond between one particular Na+ and one particular Cl-. The idea is moreso that all these forces of attraction result in interlocking.

The best evidence of ionic bonds would be the alternating plus-minus crystal structure the atoms. It’s so plain and obvious in the above image. The above image is, however, essentially a cartoon. It’s not generally possible take a picture of atoms and see the alternating plus-minus charge.

Alum

A more complex ionic bond example is alum, chemical formula KAl(SO4)2 when dry. It has a more complex mix of elements than NaCl, yet it still has ionic bonds. Simply, note the metal elements aluminum (Al) and potassium (K) along with the nonmetals sulfur (S) and oxygen (O). Plus, note the obvious, and beautiful, crystal structure for alum in the below photo. Alum definitely has ionic bonds, although the lattice structure would be a bit more complex than for NaCl shown above, as there are more elements present in alum.