1.4 BONDING, ELECTRONEGATIVITY & BOND POLARITY

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Hi again! Today, we are going to focus on bonding and electronegativity and bond polarity, as you probably know from the title. You should know the different types of bonding from previous years at school. This is going to be a reminder for you, but we are going to explore it a little bit further by saying and understanding how some of the bonds are polar while the others aren't and what electronegativity is.

Atoms bond together to become more stable. To be stable, atoms need to have a full valence (outer) shell. This means 8 electrons in the outer shell. They can either do this by donating or accepting electrons (ionic bonding) or by sharing electrons (covalent bonding). In this post, we are going to learn the different types of bonds.

Ionic bonding

Ionic bonding - a bond formed by the electrical attraction between positive and negative ions. This happens between a metal and a non-metal.

This essentially means that one atom donates one or more electrons to the other atom. Atoms from group 1-3 want to donate the electron to have a full outer shell and atoms from groups 5-7 want to accept electrons from different atoms to have a full shell. Group 4 atoms can either accept or donate electrons.

When you have an atom that has donated an electron then it becomes positive (because it has one less electron and electrons are negative, so there are now more protons than electrons) and is called a CATION. And when we have an atom that accepts an electron it becomes negative and is called an ANION.

I prepared an example for you with sodium and chlorine. You firstly check what the atomic number is on the periodic table for these two atoms. This is the number of protons and electrons in an atom. Apart from looking at the atomic number, you can also see in what group they are.

If an atom is in group 1, just like sodium, it will have 1 electron in its outer shell.

Whereas, if an atom is in group 7, similarly to chlorine, it will have 7 electrons in its outer shell. A full shell has 8 electrons hence, sodium gives an electron to chlorine.

This means sodium becomes positive and is called a cation. While chlorine becomes negative and is called an anion. They both now have 8 electrons in their outer shells (valence shells) and are 'happy'.

Attractions and repulsions

One last thing now about this picture you see above; There are attractions and repulsions in each ionic bond.

Repulsions: between electrons in both of the atoms;

: between the nucleus of both of the atoms (the nucleus is positive);

Attraction: between the nucleus of one atom and the electrons of the other atoms;

: between the electrons of one atom and the nucleus of the other atom;

The repulsive forces balance the attractive forces therefore, the ionic bonds are created and the atoms do not collapse into each other.

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Covalent bonding and coordinate bonding

Covalent bonding - has a pair of electrons, with opposite spins, shared between the atoms with each atom giving one electron. This happens between a non-metal and a non-metal.

Covalent bonding is between non-metals (one or more electrons are shared between two atoms) which suggest that covalent bonds do not conduct electricity (like non-metals) and have a low melting temperature. You might be thinking, but how does graphite and diamond have a high melting point if there are covalent bonds there. Covalent bonds are usually small molecules and therefore the melting point is low. But both diamond and graphite are large lattices and therefore have high melting points.

And for example: we have two hydrogen atoms that bond covalently. The first shell can only hold 2 electrons. Both of the hydrogens have 1 electron and therefore they share these electrons between each other. This means they now have two electrons in their outer shell, so they are full.

Coordinate bonding - a covalent bond in which both electrons come from one of the atoms.

And to visualise this, I prepared an example for you.

We have boron fluoride and ammonia that have a coordinate bond.

Boron is already bonded to three fluorine atoms and has no more free electrons.

Nitrogen is bonded to three other hydrogens and still has one pair of electrons left for bonding.

Ammonia shares these two electrons with boron fluoride and therefore they create a coordinate bond.

You can define covalent bonds by either POLAR or NON-POLAR. The cause for this is that the electron pair is usually not shared equally between two atoms. One atom has more control over the electron than the other.

1. Let's begin with the non-polar covalent bonds:

- the electrons are shared equally between two atoms.

- e.g. between two the same atoms (covalent bond between hydrogen and hydrogen, like our example higher up in the post).

2. Now, the polar covalent bonds:

- the electrons are shared unequally. One of the atoms in the bond has greater control over the electrons and therefore is slightly negative (δ-). The other atom on the other hand, which has less control over the electrons is slightly positive (δ+).

- an example of this is between hydrogen and chlorine.

The question is, how do I recognise if a covalent bond is polar or not?

And this is exactly what we're going to have a glance at now.

The effect of one atom having more control over the other is caused by the electronegativity difference between them.

Electronegativity - the ability of an atom in a molecule to attract electrons.

*Electronegativity increases across a period - more protons are in the nucleus, therefore greater attraction of electrons.

*Electronegativity decreases down a group - the outer electrons are further away from the nucleus (from the positive nucleus), therefore attraction decreases.

Linus Pauling- electronegativity scale

Linus Pauling created a radioactivity scale. It shows if bonding is covalent or ionic and the degree of polarity of a covalent bond according to electronegativity values they have.

The table with Pauling's electronegativity scale is above in the picture. If the resulting number between electronegativity values is below 2.1, then the bond is covalent. If it is over 2.1, then the bond is ionic. Inside the table, you will also find an example under each electronegativity value.

In the exam, you will be given the electronegativity values (devised by Linus Pauling) and you use them and the table above to find if the bond is covalent or ionic. You do this by taking away the lower number from the higher number.

e.g. Electronegativity values:

H= 2.1, F= 4.0, I= 2.5

HF= 4.0-2.1= 1.9 .:we have a strong polar covalent bond.

HI= 2.5-2.1= 0.4 .: we have a covalent non-polar bond.

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Metallic bonding

Metallic bonding- the attraction between nucleus and electron cloud. It happens between two metals.

In this bond, metal contributes one of its outer electrons to a delocalised sea of electrons. These delocalised electrons form a bond with the positive nucleus of an atom. And this is what a metallic bond is.

Furthermore, this type of bond has most properties that metals have. Here is a mind map that shows some of them and let me explain them to you.

Properties:

1.Good electrical conductors - free moving electrons (delocalised electrons) in the electron cloud conduct electricity.

2.Ductile - in metallic bonding layers slide over each other because there are electrons that move between the layers.

3.High melting and boiling points- due to the strong attractions between atoms. Between the positive nucleus and the negative cloud of electrons.

Of course, there are many more other properties of metals and they are also the properties of metallic bonds. Some of them are: malleable, they have high density, conduct heat, etc.

So, this is all for this post. There is going to be another post on this topic. Be sure to come back and visit the other one too.

PS. Please remember, I am only a student, and as anyone, I can make mistakes. If you think you can see one, don't hesitate and comment (either here on on my youtube channel) Thank you!