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Ionic Bonding

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Chemistry

Group 8 elements, which you also know as noble gases, are known for being inert. They rarely react with other elements, be it water, oxygen, or metals. This is because they all have a full outer shell of electrons. This is the most stable electron configuration an atom can have. When most atoms react, they share or donate electrons in order to achieve a more stable electron arrangement, but group 8 elements don’t need to do this - they’ve already achieved it.

Ionic Bonding, argon neon electron configuration, StudySmarterThe electron configuration of argon and neon. Note how both have the maximum eight electrons in their outer shell.commons.wikimedia.org

Ionic bonding is one way in which other elements try to obtain a stable electron configuration. They do this by losing or gaining electrons to form ions with full outer shells. Oppositely charged ions attract one another, and an ionic bond is simply the electrostatic attraction between these ions.

How do species form ionic bonds?

There are multiple ways in which atoms can achieve their goal of a noble gas structure. Non-metals often join up in pairs, trios or larger groups, and share their outer shell electrons. (Take a look at Covalent Bond). A group of metal atoms of the same element will lose electrons to form positive ions in a sea of delocalised electrons, (see Metallic Bonding). But when a metal and a non-metal come together, the easiest way for them to both obtain a full outer shell is for one of the species to lose electrons, and the other to gain them.

Remember, species is just a term for any group of identical atoms, ions, molecules, or compounds.

The transfer of electrons forms ions, and oppositely charged ions bond ionically with each other. Let’s explore the process further.

Ions

Ions are atoms that have gained or lost one or more electrons to form a charged particle.

Ionic bonding always occurs between positively charged ions, called cations, and negatively charged ions, called anions. In both cases, the ions have the electron configuration of a noble gas.

  • Cations are formed from atoms that lose electrons. Because electrons are negative, this results in a positive ion.
  • Anions are formed from atoms that gain electrons. This results in a negative ion.
  • In ionic bonding, the cation is always a metal and the anion is always a non-metal. The anion gains the electrons that the cation loses, so both ions have full outer shells of electrons.
  • We call this overall transfer of electrons electrovalence.

Electrostatic attraction

Forming ions is only half the picture - in fact, by definition ionic bonding doesn’t involve the transfer of electrons at all! Rather, it is about the interaction between these ions. When two oppositely charged species are close by, they will attract each other. This is known as electrostatic attraction; you might remember that this is the force that attracts electrons towards the nucleus in an atom. When mixed together, cations and anions will be electrostatically attracted to one another, and ionic bonding is simply another term for this attraction.

An ionic bond is the electrostatic attraction between two oppositely charged ions.

The structure of ions

We now know what an ionic bond is: the electrostatic attraction between oppositely charged ions. But before we look at how they form ionic structures, we need to understand how to work out the charges of the ions involved. Let’s take an example, such as sodium chloride. (See Electron Configuration.)

Sodium chloride is made up of positive sodium cations and negative chloride anions. We can use our knowledge of their electron configurations to work out the charges of the ions they form. Sodium has the electron configuration of . The easiest way for it to achieve a full outer shell is by losing one electron from 3s, so it has the arrangement .As you know, electrons are negatively charged and so this results in a positive ion with a charge of +1. We show this using square brackets, as shown below:

Ionic Bonding, sodium ion, StudySmarterA sodium ion. It now has a full outer shell of electrons due to losing one electron from the 3s sub-shell.StudySmarter Originals

Chlorine, however, has the structure . In order to have a full outer shell it needs to gain one electron. This forms a negative ion with the electron configuration :

Ionic Bonding, negative chloride ion, StudySmarterA negative chloride anion with a full 3p subshell. It has gained the electron sodium lost, shown in blue.StudySmarter Originals

Each sodium atom donates one electron when it forms an ion and each chlorine atom accepts one electron. Therefore, the ions form a compound with a 1:1 ratio of sodium ions to chloride ions, which has the formula . However, some compounds don’t have a simple 1:1 ratio of cations to anions. For example, magnesium chloride () has a ratio of 1:2. Each magnesium atom loses two electrons to form a stable ion whereas each chlorine atom only gains one extra electron. Therefore, we need two chloride ions for each magnesium ion in the compound.

Notice that chlorine is an atom, but chloride is an ion.

Giant ionic lattices

Ionic compounds don’t form molecules. Instead they form structures known as giant ionic lattices. ‘Giant’ simply means we don’t know exactly how many of each ion it has, just that it has a large number of both - it could stretch on infinitely. However, we do know the ratio of the ions. In sodium chloride, as explored above, the ratio of sodium ions to chloride ions is 1:1. The compound forms a repeating lattice stretching in all directions. Each positively charged ion will bond ionically to all the negative ions surrounding it, not just to one particular ion, and vice versa, as shown below:

Ionic bonding, sodium chloride NaCl lattice structure, StudySmarterA section of the compound NaCl. In reality, the lattice structure is much larger than this and stretches in all directions. The lines joining the ions represent ionic bonds. Note how each ion is theoretically bonded to six oppositely charged ions.StudySmarter Originals

Properties of giant ionic compounds

Because of their oppositely charged ions with strong ionic bonds, giant ionic compounds have certain properties.

Melting and boiling points

Giant ionic compounds have high melting and boiling points because the electrostatic attraction between ions is strong and requires a lot of energy to overcome. Because of this they are generally solid at room temperature.

Solubility

The charged ions in giant ionic compounds can form bonds with polar water molecules. The energy released overcomes the ionic bonds holding the lattice together and dissolves the compound, meaning giant ionic compounds are soluble in water.

Conductivity

When molten or in aqueous solution, ionic compounds can conduct electricity. This is because the ions are free to move and carry a charge.

Strength

Giant ionic compounds are hard and strong due to the high strength of the electrostatic attraction between oppositely charged ions.

Brittleness

Ionic compounds are fairly brittle. If you give them a sharp blow, you may distort the carefully positioned lattice structure. This results in two ions with the same charge adjacent to each other. These ions would repel each other and shatter the compound.

Strength of ionic bonds

As you now know, ionic bonds are very strong and require a lot of energy to overcome. However, there are several factors that affect the strength of the bond. Let’s take a look at two of them together.

Charge on the ion

An ion with a greater charge will experience stronger attraction to oppositely charged ions. This increases the strength of the ionic bond.

For example, aluminium forms 3+ ions whereas magnesium forms 2+ ions. Aluminium therefore forms stronger ionic bonds than magnesium.

Size of the ion

Smaller ions experience stronger ionic bonding. This is because there is less distance between the nucleus and the outer shell electrons and so the attraction between them is stronger.

For example, sodium and potassium both form ions with a charge of 1+. However, sodium is a much smaller ion than potassium and so forms stronger ionic bonds.

Ionic Bonding - Key takeaways

  • Ionic bonding is the electrostatic attraction between oppositely charged ions.
  • Ionic compounds are formed from metal cations and non-metal anions. The metal gives electrons to the non-metal so both have a noble gas configuration.
  • Ionic compounds form giant ionic lattices, not molecules.

Ionic Bonding

An ionic bond is the electrostatic attraction between oppositely charged ions.

Ionic bonds are formed when a metal donates electrons to a non-metal, forming charged ions, which are then attracted to each other.

An ionic bond does not involve electrons. Rather, the bond is the electrostatic attraction between charged ions. These ions are formed through the movement of electrons.

A covalent bond is a shared pair of electrons, whilst an ionic bond is the electrostatic attraction between oppositely charged ions. 

Final Ionic Bonding Quiz

Question

What is ionic bonding?

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Answer

The electrostatic attraction between oppositely charged ions.

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Question

Which two types of elements are needed for ionic bonding?


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Answer

A metal and a non-metal

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Question

What is a cation?


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Answer

A positively charged ion formed when an atom loses one or more electrons.


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Question

Oxygen often forms ions. Are these ions positively or negatively charged?


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Answer

Negatively

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Question

Bromide anions can form ionic compounds. Which of the following ions would they most likely ionically bond to?

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Answer

Sodium ions

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Question

What is an anion?

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Answer

A negatively charged ion formed when an atom gains one or more electrons.


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Question

Decide whether the following compounds contain ionic bonds:

  1. O2.
  2. KF.
  3. CaO.

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Answer

  1. ​No
  2. Yes
  3. Yes

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Question

Use your knowledge of electron configuration to predict the charge of an aluminum ion.


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Answer

+3

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Question

A sodium atom can only form one ionic bond as it only has one electron in its outer shell to donate. True or false? 

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Answer

False

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Question

The reason a bond is formed between chloride ions and sodium ions is because they have opposite charges. True or false? 

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Answer

True

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Question

Explain why ionic compounds are hard.


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Answer

The electrostatic attraction between oppositely charged ions is strong and requires a lot of energy to overcome.


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Question

Explain why ionic compounds are generally not suitable as building materials.


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Answer

They are brittle and shatter easily. They are also generally soluble in water, so would dissolve.

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Question

As the charge of the ion increases, ionic bonding strength ______.


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Answer

Increases

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Question

As the size of the ion increases, ionic bonding strength ______.

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Answer

Decreases

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Question

Predict which of the following compounds will have stronger ionic bonds:

NaCl and KCl.


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Answer

NaCl

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