1.5  Types of Bonding

Bonding is what keeps atoms combined together. But why would atoms want to bond together when they can just exist by themselves. They form bonds because they want to be stable. When they have valence electrons they want to react with another substance so that they can get full shells. This allows the atom to release energy and become calm. The best way they can do this is by forming bonds with atoms or compounds. This means that atoms can become more stable by interacting with each other.

 

There are several types of bonding: ionic bonding, covalent bonding and metallic bonding.

Ionic Bonding

Ionic bonds, as the name suggests, is a bond created between ions. Now, what exactly are ions? Ions are basically atoms but instead of being neutral they are charged +vely or -vely. As you probably can recall, an atom as an equal number of protons and electrons; therefore, their charges cancel out. However, when atoms loose or gain electrons the atom's charge balance is disturbed; you get an atom that turns into an ion.

Ionic bonding occurs only between a metal and a non-metal. But why specifically a metal? Metals have what we call free electrons. These electrons are special electrons that are not attached to the atom; they are free to move. Ionic bonding only occurs when electrons are transferred from one atom and accepted by another. Since only metals can do this, ionic bonding is exclusive to metal and non-metal combinations.

To understand ionic bonding, let's look at sodium chloride. Sodium chloride molecules are made up of a sodium atom and chloride atom. If you look at the periodic table you will notice that sodium is a metal and chlorine is a non-metal. lets draw the electron configuration for both of them.

1 electron transferred

Chlorine (2,8,7) can get a full shell (2,8,8) if it could borrow an electron. Whereas Na (2,8,1) can get a full shell (2,8) if it could get rid of an electron . Therefore, sodium can transfer an electron to chlorine which results in sodium turning into a positive ion and chlorine turning into a negative ion. The + and - ions can now attract each other electrostatically and form a bond.

A positive ion is called a cation. A negative ion is called an anion. Chlorine (Cl-) is the anion and sodium(Na+) is the cation.

Ionic bonding usually results in a lattice structure. A lattice is an ordered structure formed by groups of atoms that are held together by the attraction between the positive and negative charges. These type of attraction is known as electrostatic attraction. Electrostatic forces are strong; therefore, these lattices can grow to form large 3D structures that are strong but brittle. The image below shows what a lattice with alternating atoms would look like. 

If a lattice structure is strong, how can it be brittle? First of all what does brittle mean. Brittle is a physical property of a substance where if you apply a large force, it will break into pieces. Examples of brittle objects are glass, clay and ceramics, cast iron, etc. Ionic bonds are completely dependent on the fact that unlike charges attract. When you apply a force on to a structure with ionic bonding, the force slightly overpowers the ionic bonding and causes the area onto which force was applied to stretch out of place. During this phase, if atoms of like charges come in contact they will repel. Similar to attraction, repulsion is also a very strong electrostatic force. As a result the areas where repulsion is stronger than attraction will break away from the lattice structure. This is shown in the picture below.    

Other examples of ionic bonding include magnesium oxide, magnesium chloride and calcium chloride.The mechanism of how ionic bonds form between atoms can be shown using dot and cross diagram. One element's electrons are shown as dots and the other's electrons are shown as crosses. Let's draw a dot and cross diagram for calcium chloride.

2 electrons transferred
2

As you can see, calcium has two valence electrons in its 4th shell. For it to be stable it will have to get rid of these two electrons and become a 2+ cation. However, for chlorine to become stable it only needs 1 of those electrons from calcium. Therefore, calcium will need two chlorine atoms to get a full shell. As a result CaCl2 is formed. The atoms in the final product all have full shells.

Ionic bonding is not limited to atom-atom bonding. It can also be found between compound ions. These are molecular ions formed by a compound or group of atoms that are already combined. The following are the non-metal compound ions that you should memorize because figuring out their charge is not straightforward.

  • Ammonium ion (NH4)+

  • Sulphate ion (SO4)2-

  • Hydroxide ion (OH)-

  • Nitrate ion (NO3)-

  • Carbonate ion (CO3)2-

  • Hydrogen Carbonate ion (HCO3)-

Figuring out the potential charge on the metal ion can be easily determined using the periodic table. However, some of the metal atoms can form ions with two or more different charges. For example Fe (iron) can supply the non-metal with 2 electrons or 3 electrons. Therefore, Fe can have either a 2+ charge or a 3+ charge. If either of them forms a bond with a chlorine atom you could get either FeCl2 or FeCl3. They have different properties.

 

Or if we take a compound anion, sulphate, which has a 2- charge and get it to form a bond with Fe2+ the resulting product is FeSO4. But how can Fe 3+ make a bond with sulphate ion when Fe has to give away 3 electrons but sulphate can only accept 2. In this case the only way this bond will work is if we bring in another Fe 3+ atom. Now Fe can give away 6 electrons; therefore if you bring in 3 sulphate ions then the electrons are accounted for. The final product would then be Fe2(SO4)3 . We will talk more on how to write chemical formulas in the next chapter. For now just remember that Fe and Cu can form either 2+ cations or 3+ cations resulting in different product in each case.

Ionic compounds are conductors of electricity only in the molten state or when dissolved in water. This is because for a substance to conduct electricity it needs free electrons or ions to move about and carry charge. Ionic compounds in the solid form have no free electrons or ions that are mobile since the structure is packed. Therefore, they are good insulators in this state. However, when dissolved or molten, their ions are mobile and can carry charge. For example pure water does not conduct electricity but if we had sodium chloride (salt) to water it begins conducting.

Let us look at the properties of ionic compounds before moving onto covalent bonding.

  1. Electrostatic forces are very strong; therefore, they are strong but brittle under a force.

  2. High melting and boiling points [Eg: Salt(NaCl) has a melting point of 800C].

  3. Good electrical conductivity in water or in its melted state.

  4. Good insulators in solid form.

  5. Crystalline structures.

  6. Soluble in water.

Covalent Bonding

Covalent bonding is similar to ionic bonding in that electrostatic forces are at work. The difference in this case is the electrons are shared rather than transferred. Therefore, there are no ions formed in this case. Then how can electrostatic forces be formed? Well they are formed between the nucleus of the atom, which is +ve due to the protons, and the electrons that are shared. This sharing allows both atoms to become stable and form a bond.

This bonding occurs between two non-metals. Non-metals don't have any free electrons; therefore, they can only share their electrons to achieve a stable outer shell. Lets look at one of the simplest covalent bonds: the bond between two hydrogen atoms.

Hydrogen has only one shell. This means the electrons are very close to the protons on the nuclues. Therefore, the bond formed between the two atoms is very strong. The first shell can hold a maximum of two electrons to become a full stable shell like helium, a noble gas. As you can see, the hydrogen molecule has achieved this.

When drawing dot and cross diagram for covalent bonds you don't have to draw all the shells. You can just draw the last shell as this is where the valence electrons are situated.

An atom from a non-metal can form single covalent bonds, double covalent bonds and even triple covalent bonds with another single atom from the same element or different element . A single bond is formed when only one pair of electrons is shared between the atoms. Double bond is formed when two pairs are shared and triple bond is formed when three pairs are shared. A double bond is shorter than a single bond because the attraction is much stronger. Similar a triple bond is shorter than a double bond. A double bond is shown as 2 parallel lines. A triple bond is shown as 3 parallel lines. For example oxygen molecule has a double bond and nitrogen molecule has a triple bond. Their structures are illustrated below.

O=O
= N

if you count the number of electrons each atoms has in its last shell including the shared pairs, they sum to 8 which means it has a full and stable shell.

Now that we understand how covalent bonds are formed, lets try to understand what their 3D structures might look like. How do they arrange themselves in space? What I mean by space is the area around us. Take the water molecule as an example.

The electron configuration is straightforward. Oxygen's group number is 6 which means it has 6 electrons in its last shell and needs 2 more to become stable. Whereas, hydrogen has only 1 electron and needs 1 more to reach astable structure. Therefore oxygen reacts with two hydrogen molecules to form a water molecule. But will the molecule be linear as shown above in space? No, it has an angular shape. The four electrons of oxygen that are not part of the bonds actually repel and push away from each other. However, since the attractive forces of the bonds are much stronger, rather than breaking away the molecule sort of bends as shown below. This is called an angular structure. Other covalent molecules also undergo similar changes in structure due to repulsion between the electrons. There so many types of these structures. There is linear, trigonal planar, angular,tetrahedral, octahedral and trigonal pyramidal. You will learn more about these later on.

Is the octet rule always followed in covalent bonding? Unfortunately for us, chemistry is not so straightforward. There are cases where there more or fewer electrons than 8 in the outer shell. One example is BF3 known as Boron Trifluoride. Look at the structure below. All the fluorine atoms have 8 electrons but boron only has 6. It violates the octet rule.  Boron has too few electrons to get an octet structure therefore it makes do with only having 6 electrons in its outer shell. It can react to form an anion and get a full structure. Aluminum is also similar to Boron in that it has too few electrons in its outer shell to achieve an octet. Molecules with an odd number of electrons do not follow the octet rule.

OK, so molecules such as water and oxygen and methane are formed when the atoms bond using covalent bonds. But then how do the molecules bond together? They actually stay together because of the intermolecular forces. These forces are weaker than the covalent bonds, which are intramolecular forces. Do not mix the two terms up. This is why when you heat ice it  only melts to form liquid water, the water molecule itself doesn't split to give an oxygen atom and 2 hydrogen atoms.

Strength of intermolecular forces increases with the mass of the molecule. Therefore, larger the molecular mass higher the melting and boiling points. 

There are different types of intermolecular forces but we will learn about these later. For now just know that a substance that is made up of molecules will most likely have intermolecular forces. It is also important to note that ionic compounds also have intermolecular forces; however, they are different from the intermolecular forces in covalent compounds.

Unlike ionic compounds, covalent compounds do not conduct electricity. Ionic compounds have ions which can move about if the ionic compound is in molten state or dissolved in water. These ions can freely carry charge. But the electrons in covalent compounds are held strongly by the nucleus since it is this attraction that keeps the whole molecule stable. Therefore, it has no way to transport charge.

OK, so now we know that covalent molecules are held together by relatively weak intermolecular forces. As a result, they have low melting and boiling points.

 

Then, what about diamond? It is a covalent compound with one of the strongest naturally occurring structures. What kind of intermolecular forces do they have? They actually do not have intermolecular forces because the diamond structure is not exactly made of molecules combining together. It is a giant covalent structure. You could say that diamond is just one single molecule that has grown to be extremely large. These are called macromolecules. You'll understand if you look at the structure below. As you can see each carbon is bonded to another 4 carbon atoms covalently. This continuously repeats itself. So each piece of diamond you hold is just one humongous molecule.This means that to break it apart you would have to directly attack the covalent bonds which are very strong. As a result, diamond has a melting point of around 4000 C. It is also used to coat drill bits.

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

C

Other examples of giant covalent structures are  sand, basically silicon dioxide, and graphite. Similar to diamond they are also made up of giant lattice structures. The structure of sand is pretty similar to diamond; therefore, it is also very strong. Not as strong as diamond because the covalent bond between carbon and carbon in diamond is much stronger than the bond between silicon and oxygen in silicon dioxide.

 

But graphite is different from the other giant covalent structures.

 

It is also similar to diamond in that it is solely made up of carbon. They are known as allotropes of carbon. An allotrope is another in which an element can exist. Carbon has a lot of allotropes: diamond,graphite and fullerenes are a few examples.

 

But graphite is soft. In fact it is even used as a lubricant. This is because graphite is made up of layered hexagonal type rings. The bonds between the carbon atoms in the ring are covalent but the rings are held together by weak forces. These weak forces can easily be overcome causing the layers to peel or slide off with ease. But the rings themselves are very strong because they are covalently bonded; therefore the melting point is very high for graphite.

Unlike diamond, in graphite, each atom is bonded to only three more carbon atoms. This means that the carbons have spare electrons that can move around and carry charge. Therefore, they are very good conductors of electricity.

Another allotrope of carbon is the fullerene. It sort of looks like a football (soccer ball). Unlike graphite or diamond, fullerene is made up of molecules. Each of the molecules consists of 60 carbon atoms bonded together. Therefore, it is not a giant covalent structure. It is a simple molecule. These C60 molecules are held together by weak intermolecular forces. They have low melting and boiling points relative to diamond and graphite since the covalent bonds do not have to be broken to melt the compound.

Each carbon is bonded to 3 other carbon atoms. This is similar to the structure of graphite which conducts electricity. However, fullerene is unable to conduct electricity because the 4th electron of each carbon atom is limited to within the molecule. The electrons can not move in between molecules; therefore, cannot carry charge in the compound.

Lets summarize the properties of covalent compounds before moving onto metallic bonding.

  1. Covalent bonds are very strong.

  2. Covalent compounds are made up of molecules that have weak intermolecular forces.

  3. These weak forces can easily be overcome; therefore, molecular covalent compounds have low melting and boiling points.

  4. Melting and boiling points increase as molecular mass increases.

  5. Covalent compounds cannot conduct electricity except for graphite.

  6. Giant covalent structures have very high melting and boiling points.

  7. Covalent compounds are insoluble in water. Soluble in organic solvents

Metallic Bonding

Metals are very interesting materials. They are strong but they not brittle. You can hammer them into shape. They are also very good conductors of heat and electricity. The free electrons or delocalized electrons in metallic structures give rise to such advantageous properties.

Metals are made up of atoms that are tightly packed together with electrons that are free to move around the structure. These electrons are mobile in both the solid and molten state of the metal. You could say that metal is made up of +vely charged ions floating in a sea of electrons. The metallic bond is formed between the cations and electrons. As the previous two bonds this too is a strong electrostatic attraction.

Metals also have high melting and boiling points. This is because they form giant lattices similar to the giant ionic compounds and giant covalent compounds. The resulting structure is very strong. The larger the metal atom, the stronger the bond, the more energy required to break the lattice. This can be observed if you look at sodium. Sodium can be cut very easily using a small knife. It is soft even though it is a metal. This is because each sodium atom only has one delocalized electron. Its bond is weak. But when you go down the group to magnesium, it is much stronger and tougher. It has two delocalized electron with 2+ ions. This makes for a stronger bond. The metals get stronger as you go down the group.

Thanks to these delocalized or free electrons metals are very good conductors. Ionic compounds can only conduct electricity when they are molten or dissolved in water. Metals are able to carry charge in any state. They conduct both electricity and heat. Silver is the best conductor because it has a high volume of free electrons but it is very expensive. Therefore, copper is used instead since it is much cheaper. 

As stated earlier although metals are strong like ionic compounds, they do not break easily. When you apply a force to a metal it changes its shape rather than breaking. This is because the bed of cations can sort of float and move around in the sea of electrons. Therefore layers of atoms just slide over each and rather than being repelled by the other ions, like in ionic compounds, the mobile electrons are able to keep the attractive forces between them and the ions intact. As a result, metals are malleable. They are also ductile since you can pull them into wires without breaking the lattice.

End

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