Hybridization: sp3 hybridization in Methane

We have discussed the formation of CH₄ in the last post. We have seen that in CH₄  molecule bonds are formed by one s and three p orbitals of C. That means it must have two kinds of bonds, one which is formed by s orbital and the others which are formed by p orbitals of C. But the chemical and physical evidences indicate that it has 4 identical bonds. To solve this mystery scientist proposed the hypothesis (virtual theory) of Hybridization. There are no evidences that it exists but it gives an explanation and it is extensively used in the study of carbon compounds.

Let’s have a look on CH₄ molecule again in the light of this hypothesis. Carbon has 1 spherical s orbital and 3 dumbbell shaped p orbitals but it wants 4 identical orbitals so it uses hybridization.

This hybridization is similar to the hybridization you use in your garden, when you hybridise white rose with red rose you get pink rose. Pink rose is a combination of them but is different from them. C applies similar technique to get identical orbitals. But there is a condition that C can hybridize only those orbitals which have similar energy and 2s has lesser energy than 2p. So C supplied some energy to 2s orbital and lifts it to the same level of 2p orbitals. Now it can mix its one spherical 2s and three dumbbell shaped 2p orbitals and get four identical hybridised orbitals named as sp³. These hybridised orbitals are equal in energy and identical in shape. Their shape is neither spherical nor dumb-belled but it’s somewhat between them.

Each sp³ hybridi​s​ed orbital accommodates unpaired electron of C. These sp³ hybridised orbital arrange themselves in a tetrahedral shape to keeps equal and maximum distance from each other.

H has one unpaired electron in its s orbital. H comes closer to C so that its s orbital and sp³ hybridised orbital of C could get overlapped with each other. After overlapping of orbitals they share their electrons and make bonds. Thus CH₄ molecule is formed by bonding between four sp³ orbitals of C and s of four H atoms.

In hybridization we have learnt that:

• Only the orbitals which have similar energy can be mixed by hybridization.
• Number of hybridised orbitals is equal to the number of atomic orbitals that participate in hybridization. For example, one s and three p orbitals hybridised to form four sp³ hybridised orbitals.
• Hybridised orbitals of central atom decide the shape of the molecule.
• Bonding occurs by overlapping of hybridised orbitals of central atom and the orbital of other atoms.

Let’s take another example of BF₃ molecule. Central atom B needs 3 orbitals but it has one s and three p orbitals. In the next post we will see that how does Boron use hybridization to get 3 equivalent hybridized orbitals?

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Valence Bond Theory (VBT)

Linus Pauling proposed the Valence Bond Theory (VBT) to explain how valence electrons of different atoms combine to form a molecule. He said that unpaired electrons (valence electrons) of one atom combines with unpaired electrons of other atoms and thus forms a molecule. I am not going to puzzle you with rules; we will learn the rules by examples.

Let’s take an example of H₂O molecule and see how it is formed by O and H. To understand it we have to look at the electronic configurations of O and H.

⁸O – 1s², 2s², 2p⁴

¹H – 1s¹

Now place these electrons to their corresponding orbitals. You can see here that O is the central atom and it has 6 valence electrons, out of which 2 are unpaired. Each of the unpaired electron gets paired with 1 electron of H. Thus O and H share electrons and form H₂O molecule. In this pairing they also follow the ‘Pauli Exclusion Principle’ which means the spin of both paired electrons must be opposite.

Now take another example of CH₄. Write the electronic configurations of C and H.

⁶C – 1s², 2s², 2p²

¹H – 1s¹

After placing these electrons to their corresponding orbitals you will find that C has 4 valence electrons, out of which only 2 are unpaired.  But to complete its octet it needs 4 unpaired electrons to get paired with 4 atoms of H. To accomplish this C supplies some energy to its paired electrons and promotes one of them to the empty p orbital so that C has 4 unpaired electrons. Now its electronic configuration becomes:

⁶C – 1s², 2s¹, 2p³

This new configuration is called the excited state configuration and the previous configuration is known as the ground state configuration.

Let’s practice it with one more example of NH₃. Write the electronic configurations of N and H.

⁷N – 1s², 2s², 2p³

¹H – 1s¹

In NH₃molecule N is the central atom. N has 3 unpaired electrons to combine with unpaired electrons of 3 atoms of H. Now it can get paired with unpaired electrons of H atoms.

At this stage when VBT explained the formation of molecule a new question arose. The bonding electrons or paired electrons are placed in the orbitals of the central atom, which means the orbitals of the central atom decide the shape of the molecule. In H₂O molecule bonding electrons are placed in two p orbitals of O. These two p orbitals are at right angle to each other but the shape of H₂O molecule is bent and angle is 104°27′. Similarly in CH₄ molecule the question arises that how one spherical s orbital and three dumbbell shaped p orbitals which are at right angle to each other can give tetrahedral shape to the CH₄ molecule?

In answer to these questions scientists proposed the phenomenon of Hybridization. it is similar to the process of Hybridization some of you have used or listened in gardening. In our next post we will learn about Hybridization and see how does this phenomenon solve the mystery of molecule?

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How does electronegativity affect the Shape of a Molecule?

I hope you have understood the role of ghost lone pairs; how they distort the ideal shape of a molecule. Who do you think stands firm against the lone pairs and put a stop to their action? Bonding pairs guard the shape of a molecule and resist any changes made by lone pairs. Think of a situation when these guards are shifted away for some reason, what do you think will happen? Obviously lone pairs will perform their evil doing uninterruptedly. Let’s see a few examples where similar situation happens.

In NF₃ molecule, central atom has 3 bonding pairs and 1 lone pair (like NH₃ molecule). We can arrange them in tetrahedron but lone pair will distort it and we will get a pyramidal shape as we have got in case of NH₃ molecule.

Now focus on NF₃ molecule. Here N is bonded to F. F belongs to the 17^th group which has largest electronegativity and F is the most electronegative element of the periodic table. That’s why when N is bonded to F, F wins the war over bonding electrons and gets maximum share of it.

Here F is the culprit who shifts bonding electrons towards itself thus weakens the defense against ghost lone pairs. In this situation lone pair succeeds to distort the shape a lot. That’s why the angle of F-N-F is found 102°30′ which is much smaller than expected.

While in NH₃ molecule, the electronegativity difference between N and H is comparable. That’s why bonding pairs are present on their guarding posts to guard the shape of the molecule. As a result the lone pairs could cause very little distortion. The angle of H-N-H is found 107°48′ which is just a little smaller than the expected (109°28′).

This similar situation happens in case of H₂O and F₂O molecule. I am sure you will be able to figure it out which one has the smaller angel? Yes certainly angle of F-O-F will be smaller than the angle of H-O-H.

After the VSEPR theory scientist were able to predict the shape of a molecule successfully but they didn’t stop there. They wanted to know what happens inside the atom when it is bonded to another atom and how a molecule is formed. To explain it, Linus Pauling proposed the Valence Bond Theory (VBT) which gave the orbital picture of the formation of a molecule.

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How to Predict the Shape of a Molecule by VSEPR?

In 1957 “Valence shell electron pair repulsion (VSEPR) theory” was developed by Gillespie. This theory is quite successful in predicting the shape of a molecule and bond angles more exactly. VSEPR also uses electron pairs of central atom to predict the shape of a molecule like ​Sidgwick-Powell theory but, it gives equal importance to all electron pairs including bonding pairs and lone pairs.

Let’s take an example of NH₃ molecule. Its name is Nitrogen trihydride, it is known as ammonia. Draw the Lewis dot structure for NH₃. You will find 1 lone pair of N and 3 bonding pairs. Now you have to find a best possible arrangement for 1+3= 4 electron pairs. Yes, of course tetrahedral arrangement is best suited for them. But Lone pair raise objection on this arrangement. It says it must get privilege as it is exclusively owned by central atom while bonding pairs are shared between central atom and other atoms, and the judgment goes in favour of lone pair. When all 4 pairs are arranged in tetrahedral shape, lone pair gets freedom to dominate bonding pairs and it start pushing nearby bonding pairs and distort the tetrahedron. Lone pairs are like ghost because their presence influences the shape but they aren’t visible in the shape of the molecule. In tetrahedron 3 corners are occupied by bonding pairs and 4th place is occupied by lone pair. Because lone pair is not visible that’s why the NH₃ molecule looks like a pyramid.

Now let’s have a fresh look on H₂O molecule. In Lewis dot structure you must have spotted 2 lone pairs and 2 bonding pairs. Again you have 4 electron pairs to arrange in a shape. Obviously we try to arrange them in a tetrahedral shape. But this time situation is more difficult because central atom has 2 lone pairs. These ghosts are really aloof (unfriendly), they push bonding pairs as well as each other. So that the tetrahedron in which 2 corners are occupied by lone pairs and 2 by bonding pairs is distorted a lot. That’s why the resulting shape of H₂O molecule is bent shaped.

The VSEPR theory uses number of electron pairs of central atom to predict the shape of a molecule and also consider the presence of lone pairs. And it suggests that the repulsion between lone pairs-lone pair is greater than the lone pair-bonding pair which in turn greater than the repulsion between two bonding pairs. Thus the presence of lone pair distorts the ideal shape and results in decreased bond angle.

I hope you have understood the role of ghost lone pairs; then try to predict the shape of NF₃ molecule. It has 3 bonding pairs and 1 lone pair (you must have found similarity with the NH₃ molecule). We can arrange them in tetrahedron but lone pair will distort it and we will get a pyramidal shape like we have got in case of NH₃ molecule. But the shape of these two molecules are not identical, the angle of F-N-F is smaller than the H-N-H. Can you spot the difference between these two molecules? In the next post we will see how VSEPR solves this puzzle.

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What Will be The Shape of a Molecule?

Two different elements combine together and form an entirely different molecule. This fact has surprised us and it has puzzled scientists too. Whenever they came closer to solve one mystery, the next question arose and in an effort to solve the mystery of molecules, scientists have given different theories. Each theory tried to solve this mystery but left few unanswered questions and then scientists proposed a new theory. We will try to reveal its mystery in this post and the coming posts. With each successive theory we will be able to understand these molecules more closely.

Covalent molecules are more mysterious than ionic molecules because in covalent molecules bonding electrons are shared between bonded atoms and they also share a bit of atomic orbital. We will discuss covalent and ionic molecule in different posts. Here we will discuss covalent molecules.

In covalent molecules there is one central atom which is surrounded by atoms of other elements. Central atom acts like the king who governs the shape of the molecule. Valence electrons help central atom to make bond with other elements and bonding electrons keep them bonded together. These valence electrons of central atom and bonding pairs play an important role in deciding the shape of a molecule.

Let’s take an example of a simple (three atomic) molecule BeCl₂. I guess you will be able to name it correctly, Beryllium dichloride. In BeCl₂molecule Be is the central atom, it has 2 bonding pair of electrons. Each bonding pair repel other and struggle to go far away from each other. If you imagine Be atom with the arms of ‘bonding pairs’ you must be able to guess the position in which they can be placed at maximum distance. Yes, you guessed correct; when Be atom fully stretches its arms, both bonding pairs are far way from each other.  That’s why the shape of BeCl₂ molecule is linear and the angle of Cl-Be-Cl is 180°.

Let’s take another example of (four atomic) molecule BCl₃. Its name is Boron trichloride. Its central atom is Boron and it has 3 bonding pairs. Now imagine B atom with 3 arms, all of them repelling each other. Can you guess, what will be the possible arrangement in which they have maximum distance between them? Yes, in a triangular arrangement where each bonding pair stays apart from each other. That’s why the shape of BCl3 molecule is triangular planer and the angle of Cl-B-Cl is 60°.

Let’s take another example of (five atomic) molecule CH₄. Its name is Carbon tetrahydride and it is known as Methane. Carbon is the central atom in this molecule and it has 4 bonding pairs. Now you have to arrange 4 arms in the space at maximum distance. If you guessed square, it isn’t the correct answer. Imagine a tetrahedron arrangement. Tetrahedron is a three dimensional shape that has equilateral triangles as its four faces. When you place carbon at the centre of the tetrahedron and place H at each vertex, you will be able to place them farthest away from each other. The shape of the CH₄ molecule is tetrahedron and angle between H-C-H is 109°28′.

Take another example of six atomic molecule PCl₅. Its name is Phosphorus pentachloride. Phosphorus is the central atom in this molecule and it has 5 bonding pairs. Now think about the possible arrangement for these 5 bonding pairs. If we fix 2 pyramids by their base we will get 5 vertices and we can place Cl at each vertex and central atom P can be placed at the centre of the common surface of the two pyramids. That’s how you can proof the shape of PCl₅molecule is trigonal bipyramidal which has 3 angles Cl-P-Cl is of 120° and 2 angles Cl-P-Cl is of 90°

Take another example of seven atomic molecule SF₆. Its name is Sulphur hexafluoride. Sulphur is the central atom in this molecule and it has 6 bonding pairs. Now think about the possible arrangement for these 6 bonding pairs. If we fix 2 square pyramids by their base we will get 6 vertices and we can place F at each vertex and central atom S can be placed at the centre of the common surface of two square pyramids. That’s how you can proof the shape of SF₆molecule is octahedron (it has 8 faces) which has all angles F-S-F is of 90°.

All the examples we have discussed above are based on Sidgwick-Powell Theory. This theory reviewed the structure of known molecules. In this theory approximate shape of molecules can be predicted from the number of bonding pair of electrons but it is limited to the molecules which have only single bonds.

Let’s try to predict the shape of H₂O molecule. You might say it must be linear like BeCl₂. But actually its shape is bent. Why your prediction went wrong? Examine the molecule of H₂O what makes it different from BeCl₂. Oxygen also has 2 bonding pair but it also has 2 lone pairs. 

Sidgwick-Powell Theory didn’t explain shape of molecules which has lone pairs. In 1957 Gillespie and Nythol improved this theory and gave a new theory “Valence shell electron pair repulsion (VSEPR)” theory” to predict molecular shape and bond angles more exactly. In the next post we will solve the mystery of the shape of H2O molecule with the help of VSEPR theory.

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How to write the formula and name of a molecule?

Name has a great significance in the world of elements. Before we proceed, I would like to tell you a few basic things. When atoms of two different elements bind together, they form one molecule and a number these molecules are collectively called as compound.

Just like some of us have two different types of names, a proper name and a nick name, the molecules have a scientific name and sometimes a common name as well. But unlike our names, a molecule’s name tells us a number of informations about that molecule. It tells us how many elements are present in that molecule and also gives information about the ratio of these elements. To write the name of a molecule in scientific notation is known as the molecular formula.

You have noticed that in a molecule atoms are present in a definite ratio. This ratio is similar to the ratio of their valencies. We will learn first to name simple ionic molecules which are made up of two elements.

If you bond Na with Cl. You know Na belongs to 1^st group and Cl belongs to the 17^th group. They have large difference in EN values that’s why they will form ionic bond. Valency of Na is 1 and valency of Cl is also 1, so they will combine in ratio of 1:1 and form a molecule of NaCl.

NaCl is the formula of the molecule which is formed on bonding of Na and Cl. It also tells us that 1 atom of Na and 1 atom of Cl were combined to form this molecule.

Did you notice that I wrote Na first and then Cl? Whenever you write a formula or name of a compound you will write electro positive (or less electronegative) element first and then the electronegative element.

Now let’s give a name to this molecule of NaCl.

1. Write the of name electro positive element- Sodium.
2. Then write the name of electronegative element- Chlorine,
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Chlorine, it becomes Chloride.
4. Write both of them together- name will be Sodium chloride.

Take another example of Mg and I. They have large EN difference so they will form ionic bond. Valency of Mg is 2 and valency of I is 1. ‘Mg’ needs to donate its 2 electrons and ‘I’ needs to get 1 electron. That means 2 atoms of ‘I’ will be required to fulfil the need of 1 ‘Mg’ atom.

Now write the formula of the molecule which will be formed by bonding of Mg and I.

1. First write the symbol of electropositive element- Mg
2. Write the symbol of electronegative element- I
3. Write the number of atoms in subscript to the symbol of elements (ignore if number of atom is 1) - MgI₂

Now name this molecule of MgI₂.

1. Electro positive element- Magnesium.
2. Electronegative element- Iodine,
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Iodine, it becomes Iodide.
4. Write both of them together- name will be Magnesium Iodide.

Now try one more combination: Ca and Cl.

1. Valency of Ca is 2 and valency of Cl is 1.
2. Formula will be- CaCl₂
3. Name will be Calcium chloride.

Now we will learn to name simple covalent molecules which are made up of 2 elements. Don’t worry, you don’t have to learn different rules of naming. Only a bit more information is to be added when you write the name of a covalent molecule.

In the name of a covalent molecule we have to add the information about number of atoms involved in the formation of molecule. Let’s take an example of B and Cl.

B and Cl both belong to p block. That’s why they will form covalent bond. Valency of B is 3 and valency of Cl is 1. I hope you will be able to understand now that, to fulfil the need of 1 atom of B, 3 atoms of Cl will be required.

In case of covalent molecules we have to add prefix to the name of elements corresponding to the number of their atoms like di, tri, tetra, penta, hexa, hepta, octa, nona, deca respectively for 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10.

Let’s try to name the BCl3.

1. Electro positive element- Boron.
2. Electronegative element- Chlorine.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Chlorine, it becomes chloride.
4. Add prefix ‘tri’ to Chloride because its 3 atoms are involved in the formation of BCl₃ molecule: trichloride.
5. Name of the BCl₃ molecule will be: Boron trichloride.

Let’s practice it with one more example: H and O.

You can work out the formula: H₂O. Now try to name it.

1. Electro positive element- Hydrogen.
2. Electronegative element- Oxygen.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Oxygen, it becomes oxide.
4. Add prefix ‘di’ to the Hydrogen because its 2 atoms are involved in the formation of H₂O molecule: Dihydrogen.
5. Name of the H₂O molecule will be: Dihydrogen oxide.

Let’s practice it with one more example: C and O.

You can work out the formula: CO₂.

Now try to name it.

1. Electro positive element- carbon.
2. Electronegative element- Oxygen.
3. Add suffix ‘ide’ to the electronegative element- after adding suffix ‘ide’ to Oxygen, it becomes oxide.
4. Add prefix ‘di’ to the oxide because its 2 atoms are involved in the formation of CO₂ molecule: dioxide.
5. Name of the CO₂ molecule will be: carbon dioxide.

The aim of bonding is to achieve octet, that means when you count electrons around any bonded atom in  Lewis dot structure it should be 8. In above examples I encircled the electrons around each atom. If you notice Boron in BCl₃ molecule,  it has only 6 electrons . This deficiency gives a special quality to this molecule. I will explain it in the coming post.

The atoms of two different elements combine to form a molecule.  This newly formed molecule is entirely different from parent elements. You can witness it in case of water molecule; hydrogen and oxygen are gases but water is liquid. What makes a molecule so different? Why parent elements lose their identity? In the next post we will try to solve the mystery of a molecule.   

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Covalent Bond Vs Ionic Bond

You have seen how elements are similar to us and as you know them closely you will find some more similarities to our behaviour. You must have experienced the law of attraction in your life. We tend to get attracted towards someone opposite to us. Elements too experience this law of attraction. They are likely to bond with the element quite opposite to their characteristics. You can see, this phenomenon working on the elements placed on two opposite sides of the periodic table (group 1 and group 17).

Group 1 and group 17 have opposite qualities. Group 1 has least Ionization enthalpy (easy to remove electron from valence shell) while group 17 has least Electron gain enthalpy (easy to add electron to valence shell ). Group 1 is least electronegative and group 17 is the most electronegative. That’s why group 1 elements easily donate one of their valence electron to group 17 elements. Thus group 1 elements form M⁺  ion and group 17 elements form X^- ion. And these two oppositely charged ions attract each other and make ionic bond. The force they experience is called as electrostatic attraction. 

Group 2 and group 17 also make ionic bonds for the same reasons.

Now, I want you think of your friends. For a moment, go in the past and think, why did you make him/her your friend? You must have found them quite similar to you. People with common interest like to unite together. Elements also make bonds with elements which have similar qualities.

Group 13, 14, 15, 16 and 17 all are p block elements. They all have a few similarities. They have almost similar values of IE and EGE. They are all on the same ground; no one is capable of donating/ accepting electrons, that’s why they share electrons between them and form covalent bonds. 

In covalent bond, two bonded atoms not only share their electrons but also share the same orbital (molecular orbital), so that the shared pair of electrons can revolve around both of the bonded atoms. This unique sharing makes covalent bond the strongest among all the types of bonds. To understand it you have to look at the orbital picture of covalent bond and we orbital picture of covalent bond and we will discuss it in the coming posts.

These p block elements are dissimilar in one aspect. In electronegativity scale they are on different positions. This dissimilarity adds strength in their bonding. You didn’t understand why? Ok, I’ll explain it to you; the electronegativity difference develops polarity in their covalent bond. This way in a covalent bond two oppositely charged poles are developed, and between them electrostatic attraction starts working, which adds some more strength to the existing covalent bond. So you can conclude that polar covalent bond is stronger than non polar covalent bond.

Covalent bond is the strongest bond. The energy needed to break a single covalent bond is 80kcal/mol while only 8kcal/mol energy is required to break an ionic bond. Ionic bonds are weak bonds even though most of the solid substances are made up of ionic bonds. As you see in above example that BCl₃ and Cl₂ both are gases and NaCl (common salt) is a solid substance.

Now this may contradict your logic, you might have thought that the ionic bonds must be tougher and stronger than covalent bonds. Ionic bond is undoubtedly weaker than covalent bond but electrostatic attraction between ions and the strategy of ionic molecules to arrange them in a network (lattice) give extraordinary strength to the ionic compounds. “Unity in Strength”, this quote is also true in the world of atoms and it is the reason behind the strength of NaCl. NaCl molecules are bound together by electrostatic force to form a cubic structure and a number of such cubes form a network structure (where each Na⁺ is surrounded by 6 Cl^- and each Cl^- is surrounded by 6 Na⁺) which gives strength to the common salt. The electrostatic force is the strongest force working between ions and molecules of ionic substances which gives them extraordinary strength in spite of their weaker bond.

In covalent compounds, forces operating between molecules are not as stronger as electrostatic force. But it doesn’t mean that all covalent compounds are liquids or gases, some are solids too.

At this point, when you have learnt about bonds and their nature, you would like to try different combinations of elements. And you must be curious to know whether it exists or not and what would be the name of that compound? In the next post we will learn how to make a compound? How to work out the formula of the compound and also how to name it?

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