Primary Bonds
We will discuss about what is known as the primary bonds. So bonds between atoms or molecules can be categorized into two distinctive category. The first is known as the primary bond, the second is the secondary bond. Primary bonds - what are the basic features of primary bond. Primary bond are stronger bonds than secondary bond that is very characteristic property about primary bond and point of difference with secondary bond. Primary bond also known as chemical bonds involve the valence electrons. The valence electrons of the atoms comes into picture when we discuss about primary bond. The nature of the bond depends on the type of atom we are talking about what that means is that primary bond is not just a single type of bond. There are multiple types of primary bond and what kind of primary bond will be formed between two atoms will depend on the nature of both the atom.
Types of Primary Bond
Now what are the types of primary bond that are present. Primary bonds can be divided into three categories. First and foremost it's called ionic bond, the second is called covalent bond and finally what we have is known as the metallic bond. We'll discuss about all three of them in details.
Ionic Bond
Ionic bond are the type of bond which normally exists in a compound having one metallic atom and another nonmetallic atom. Put differently the two atoms participating in an ionic bond generally have large distance of electronegativity. The difference of electronegativity gives rise to ionic bond and the reason as we will discuss now. We will discuss it through an example of sodium and chlorine. Now you might be knowing that sodium has 11 electrons and chlorine has 17 electrons. So the configuration is like the first orbit really be using the Bohr's model for the simplification. But we can also write 1s2 2s2 2p6 and 3s1. So this gives 10 electrons and this is the 11th electron, this 10 electrons lie in the two inner orbit only this single electron in the outer shell. Similarly chlorine has a configuration of 1 is 2s2 2p6 and 3s2 3p5. So there are a net of 7 electrons in the outermost shell from your basic chemistry understanding. Might be knowing that in order to have a stable configuration, in order to have a noble gas configuration or the outermost orbit should have eight electrons in it for stability. Now how can this two atoms attain stability.
Let's suppose that the single electron in sodium atom here is transferred to the outer orbit of the chlorine atom. What will that give rise to that, will give rise to eight electrons here because this orbit no longer exists. This orbit will have eight electrons and the stable configuration will be attained that is exactly what happens. So the electron is donated by the metal to the nonmetal and that gives rise to something known as sodium chloride. Since sodium has lost electron it becomes singly positive charged and chlorine, since it cleaned the electron it became single negative charge and the compound which we got is sodium chloride. This kind of bond that is developed by transfer of electron from one atom to another is known as a ionic bond. This helps attain the novel gas configuration as I said.
Properties of Ionic Bond
What are the properties of an ionic bond? There is columbic force of attraction between the atoms why so because there is a positive charge here and a negative charge here. There is electrical attraction between the two atoms now and that binds the two atoms together. It is non directional in nature. What we mean by non directional nature of an ionic bond is that it does not matter if we have a sodium positively charged atom. The chlorine atom can be anywhere around here because charge spreads is attraction in a spherically symmetric manner. There is no fixed direction in which it attracts more, it attracts in all the directions. Similarly so it is non directional in nature. As we will discuss in our discussion about covalent bond, covalent bond is a directional nature bond that is not the case of ionic bond. The bond energy is relatively large in ionic bonds and this large bonding energy results in a higher melting point for ionic systems. I think bonds normally gives rise to hard brittle and good insulators, hard materials and brittle because of the strong bond, good insulators because there are no free electrons isn't it and also the charged particles. The atoms are charged but they are bound together. Thereby they are electrically neutral, thereby they are good insulators ceramic materials. The sodium chloride dissolves in something dissolves in water a polar material this being polar it will dissolve in water. Thereby we will have charge distribution there will be sodium plus ion and chlorine minus ion and that will result in conductivity. So the ionic material then not dissolved in water are good insulators. But once it dissolves it starts to conduct a letter and ceramics mainly have ionic bond. This gives you a holistic picture about what ionic bonds are.
Covalent Bond
Let us discuss the second kind of primary bond that is the covalent bond. Covalent bond are formed by sharing of electrons. Ionic bonds were formed by either accepting electron or donating electron, that is a major point of difference between the two. Then both atoms contribute electron to the bond since there is sharing both participating atoms will contribute electron to it.
Let's take an example in which now we have both atoms as chlorine. Now since this chlorine atom has 7 electrons at the outermost shell. Similarly this has 7 electrons in the outermost shell, there is no way one is going to transfer an atom to the other. Since then it will become further 6 electrons. Our alternative is defined out is that if they share one electron into a common bank then both of them will have 8 electrons and they will get their stable configuration. That is exactly what the chlorine atom stares. Now these two electrons are the shared electrons and this two shared electrons form a single covalent point. If there were a atom like oxygen which had six electrons in the outermost shell, then each of them would have shared two electrons and we would get four electrons shared effectively. Thereby two bonds will be formed two covalent bonds.
We understand now how the sharing happens and as I said in the previous discussion about ionic bond that ionic bonds are non directional in nature. But covalent bonds are directional in nature. Why so because they have sharing of electrons. The electrons lie in between the two atoms thereby they are bounded in the 3d space. They cannot be at any angle they want right in this case since there are just two atoms they always be at 180 degrees. Since they are symmetric in the case of water where we have h2o one oxygen, two hydrogen. There is a angle something like this. Oxygen here hydrogen here the two hydrogen shares one electron each and the oxygen spheres to. Thereby oxygen has another four so it completes his octet. The only exception to the octet rule is for hydrogen where it can take to and from noble gas configuration the helium configuration. Therefore we can see here that the two hydrogen can be with respect to the oxygen is defined by the bonding nature and this is roughly 109 degree for water. That is what we mean when we see the covalent bonds are directional in nature, number of bonds depend on the number of valence electrons.
As I discussed for chlorine we will have single bond for oxygen we have two bonds and for nitrogen three bonds. Now when is the covalent bond formed? It is normally formed when the two participating atoms have closer values of electronegativity for ionic bond. The two participating atoms had a huge difference electronegativity. Covalent bonds should have closer electronegativity. Thereby they should be willing to share electrons and this is a important point that though we have discussed, as if ionic nature exists independent of covalent nature or covalent nature and ionic nature cannot coexist. But in reality since there is at least some electronegativity difference between most of the atom except for the cases when both are same atom. Then there is no hundred percent covalent or no hundred percent ionic bonds rather they are partially ionic or partially covalent. Maybe more covalent if they are closer in electronegativity more ionic, if they are further in electronic that is for the covalent bond nature the strength of the bond really varies. It can be very strong bond in a covalent bond or it can be a very weak bond like in this example, bismuth has a very low melting point because it's bond strength is quite weak. Whereas diamond has a very high melting point because it has one of the strongest covalent bond known to mankind.
Metallic Bond
Now let us discuss the third kind of primary bond that is metallic bond. Metallic bond are founded metals are metallic alloy systems. We know that in the metals normally only one two or three electrons exist in the outermost shell. So it is not possible for them to share electrons and gain the octet stability nor is it possible for them to transfer electrons and gain octet stability. So instead what they do is that they share electron to the whole bulk and thereby attain octet stability at the atomic level. So if we have a single let's say sodium or some metal what it will do. It will have a ionic core which will remain positive and the electrons become part of a large sea of electrons drifting throughout the metal.
That is how the metallic bond is found and again here the bonding nature is non directional metallic bonds. Similar to ionic bonds are non directional and the bond energy and the melting point varies greatly. As we know metals like mercury are a liquid at room temperature whereas other metals like tungsten has a very high melting point. Metallic bonds are always strong or always weak. This gives you a rough idea about the third kind of primary bond to briefly summarize. We have three kinds of primary bond ionic bond which is formed by transfer of electrons from one atom to another, covalent bond which is formed by sharing of electrons, metallic bonds which are in metal systems by giving up its valence electrons in the sea of electron.
Bond and Property Relation
What are the bond and property relations? The property of a material is highly influenced by the type of bond. As we saw that something which has a strong bond will find it very difficult to be in the molten state. A lot of energy will be required to melt it. So let us discuss the bond property relation using one simple example of electrical conductivity covalent bonds, covalent materials are insulator. Why so because they have no free electrons available. This is a general statement there may be cases where covalent bonds are electrical conductive conducting. But generally: materials are very good insulators, ionic materials are generally insulators again because there are no free electrons. But when you dissolve some ionic material in water, then ions will be formed separate out and that can conduct electricity. Finally metallic materials normally they have a sea of electrons which are free to move throughout the body of the material. Thereby they are very good conductors of electricity. So we see by understanding the nature of the bond. How we can understand what property it should have and also if we see a material and have a property of it. We can derive back what kind of bonding it has? This brings us to a conclusion on our discussion about the different kind of primary bonds.
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