Ionic Bonding (Biology) — Definition & Role

Ionic Bond: Taken, Not Shared

Elements are electrically neutral because they have the same number of electrons and protons.

Well, if an atom gives an electron away, suddenly it has more protons than electrons. This gives the atom a positive charge. Similarly, if an atom accepts an electron from another atom, then it has more electrons than protons. This gives it a negative charge.

If you remember, charged atoms are called ions. Conveniently enough, bonds between ions are called ionic bonds.

These bonds are formed when an electron of one atom is either donated or taken by another atom. When an atom donates an electron, it gains a positive charge. The atom accepting the electron gains a negative charge. Much like magnets, the opposite charges attract and an ionic bond is formed. An electrostatic attraction holds the atoms together.

These bonds are weaker than covalent bonds, and compounds made from them easily dissociate (fall apart) in water. They are formed between metals and electronegative nonmetals. On the periodic table, the metals often are from Group 1 or 2, while the nonmetals are often from Group 16 or 17. For a deeper look at electronegativity, check out the beginning of this explanation on polar and nonpolar bonds.

$$\color{#3DB917}{\text{F > Cl > Br > I > At}}$$ $$(\text{Most Electronegative}) \longrightarrow (\text{Least Electronegative})$$

Nonmetals, shown above, are the electronegative electron acceptors. They need one or more electrons to have a full outer electron shell. Believe me, they really want a full outer shell with eight electrons. Because of this, they will take the electrons from another atom to complete their valence shell.

The metals are electron donors. They have an extra electron in their valence shell that they don't really want anymore, so they just give it away. The full shell beneath becomes the valence shell and the metal is happy.

Moving on, look at the picture below of table salt, $NaCl$. The sodium atom has an electron that it doesn't really want, while the chlorine atom needs one more electron to be happy.

The sodium atom gives its extra electron to the chlorine, which makes them both ions. The protons outnumber the electrons in sodium, so it becomes a positively charged ion. The electrons outnumber the protons in chlorine, so it becomes a negatively charged ion.

Remember how negative and positive charges attract? The newly formed ions are strongly attracted to each other. They form an ionic compound. These structures are sometimes called a crystalline lattice. This is why a grain of table salt is a small crystal.

$$\color{#8925AE}{\text{Vocabulary Review:}}$$

$\color{#8925AE}{\text{Polarity}}$ : Partial positive and negative charges caused by some atoms wanting electrons more than other atoms in a molecule

$\color{#8925AE}{\text{Covalent Bond}}$ : A bond formed when two atoms share an electron pair

$\color{#8925AE}{\text{Ionic Bond}}$ : A bond formed when one atom gives up one or more electrons from a different atom

$\color{#8925AE}{\text{Hydrogen Bond}}$ : Positively charged hydrogens of one molecule are attracted to the negative charge on another molecule

$\color{#8925AE}{\text{Electronegativity}}$ : A way to measure the tendency of an atom to attract pairs of electrons

$\color{#8925AE}{\text{Van der Waals Forces}}$ : A relatively weak intermolecular force that holds molecules together

Chemical Compounds and Their Bonds

Electronegativity and Ionic Bonding

Electronegativity is a metric of how strongly an atom pulls on electrons. As you move diagonally up the periodic table (ignoring noble gases), this value increases.

Francium has the lowest electronegativity with a value of $0.7$ and fluorine has the highest with a value of $4$.

If two atoms have a large enough difference in electronegativity ($\Delta E_{-} \geq 2$), then the atom with a higher electronegativity will steal an electron (or more than one) from the other. The two atoms become oppositely charged ions and then form a bond. An ionic bond is merely the attractive force between these oppositely charged ions.

How Many Electrons Are Transferred?

The type of atom determines the number of electrons that are transferred. Each atom is trying to obtain a full valence shell, which in most cases is eight electrons. The number of valence electrons an atom has is the same as its group number. You can find that out by looking at the periodic table shown above. In order to form this full valence shell, the atom can either lose all of its current valence electrons or gain electrons until it has eight.

Let's look at an example. Oxygen is in group 6, so it has six electrons in its outer shell. This means it either needs to gain two electrons or lose six to have a full valence shell. It is a lot easier to gain two, so oxygen usually forms an $O^{2-}$ ion. In a similar vein, lithium is group 1, so it only has one electron. It needs to either gain seven electrons or lose just one. Since it is easier to lose one than it is to gain seven, it gives up its electron to form a $Li^{+}$ ion.

The ions then form compounds (sometimes called "salts") such that the overall charge is $0$. What this means is that if we look at $NaCl$, it is composed of $Na^{+}$ and $Cl^{-}$. These ions have equal and opposite charges, so the overall charge is $0$. Similarly, with $Na_{2}O$, we have two $Na^{+}$ ions and one $O^{2-}$ ion. If we sum these charges, we get a total charge of $0$.

$$\color{#8925AE}{\text{Review}}$$

$\color{#8925AE}{\text{Electronegativity}}$: The extent to which an atom is able to hold on to electrons

$\color{#8925AE}{\text{Ion}}$: An atom with either too many or too few electrons such that it has an electric charge

$\color{#8925AE}{\text{Ionic bond}}$: The attractive force between oppositely charged ions

$\color{#8925AE}{\text{Ionic compound}}$: A compound held together by the electric attraction between complementary ions

Opposites Attract

Ionic bonds form when electrons are completely transferred from one atom to another. Think of this in contrast to covalent bonds, where electrons are shared between atoms.

Take a look at the electron configuration diagram above. The goal of an atom is to fill up its outermost shell of electrons. Atoms that need just one or two electrons to fill up their outer shells have high electronegativity. This means that they are the most likely atoms to snatch an electron from another atom. Atoms with a large electronegativity are atoms that are greedy for electrons.

Highly electronegative atoms are the ones that are most commonly involved in ionic bonding. This should make sense; this type of bond is all about taking electrons from other atoms.

What Is an Ion?

Recall that electrons have a negative charge.

So, an atom that gains an electron becomes negatively charged. An atom that gives up its electron to the greedier atom becomes positively charged.

Atoms that have a charge because of the loss or gain of an electron are called ions.

Positively charged ions (the atoms that lost electrons) are called cations (pronounced "cat"-"ion"). Negatively charged ions (the atoms that gained electrons) are called anions (pronounced "ann"-"ion").

Once the electron has been transferred, we end up with two ions: one with a positive charge and one with a negative charge.

The phrase "opposites attract" is true for atoms. The positively charged ion (cation) and the negatively charged ion (anion) are attracted to each other. It is this attraction, created by the transfer of electrons, that forms the ionic bond.

The transfer of one electron creates a single bond. The transfer of two electrons creates a double bond. The transfer of three electrons creates—you guessed it—a triple bond.

What Is an Ionic Compound?

Ionic compounds are the chemical compounds formed from ionic bonding between cations (positive ions) and anions (negative ions). Unlike in molecules, the attractions that keep ions in compounds together are weak electrostatic forces.

A popular example of an ionic compound is sodium chloride. Ever heard of it? You may not have heard it by that name, but you've surely heard it called by its more common name: salt. NaCl is what you put on your fries, soft pretzels, popcorn, and whatever else you eat when you want a salty snack.

Looking at the name, we can see that sodium chloride is made of one sodium atom and one chlorine atom. The name "chlorine" changes to "chloride" to let us know that it has become an anion. In other words, chlorine is gaining an electron from sodium, making it more negative.

Polar Covalent Bonds vs. Ionic Bonds

Both ionic bonds and polar covalent bonds involve some sort of charge. This can make it tricky to understand the differences between them. So, what are the differences?

First off, polar covalent bonds have partial charges on different parts of the molecule. Ionic bonds are formed when two oppositely charged ions attract each other. Sure, ions can have charges that are greater than $+1$ or less than $-1$, but these are all full charges. They're not the partial charges found on atoms in a polar covalent bond.

Second, ionic bonds are weaker than covalent bonds. In covalent bonds the electrons are shared, and the interaction between the participating species (atoms) is a balance between attractive and repulsive forces. In polar covalent bonds, the attractive and repulsive forces don't balance exactly, so you get polarity. But in an ionic bond, the attraction between participating species (ions) is a weak electrostatic force. Electrostatic interactions, also known as van der Waals interactions, are the attraction between oppositely electrically charged things.

Finally, molecules are made from covalent bonds, while compounds or salts are made from ionic bonds. Salts are a bunch of ions latticed together in a crystal structure. This is a very different three-dimensional shape than what molecules form.

Key Vocabulary:

1. Ionic bond
2. Ionic compound
3. Ion
4. Cation
5. Anion

(Video) Types of Chemical Bonds

by CrashCourse

This video explains the different types of chemical bonds.

In ionic bonding, atoms either take or give up electrons to form a compound. The atoms in the compound become either positively charged or negatively charged ions. A positive ion loses electrons while a negative ion gains electrons. Whether an atom will gain or lose electrons depends on its electronegativity. This is a measure of how strongly an atom wants electrons.