I need help with my homework. It is Science and I am doing Covalent Bonding. Any information would help me, preferably, how Covalen Bonding is formed. Credits are offered for good help.
i got a D in science but overall at the end i got a C- dont remember that stuff,, i would rather do history then science
Ionic and covalent bonding are really just models at the extremes of a continuum of types of bonding. Where the dividing line between them lies is arguable, but a generally accepted definition is that if the difference between the electronegativity values of two elements is over 1.7, then the bonding between them is regarded as ionic, otherwise it is covalent. The smaller the difference, the less polar the covalent bond is, until at differences of less than about 0.5 the bonding is considered to be non-polar (pure covalent, if you like). ADDED Ionic bonding involves complete transfer of the valence electrons from the metal to the non-metal, giving both atoms a stable octet (full outer shell). In covalent bonding, both atoms are in the position of needing an electron or 4 to achieve an octet, so they "share" by a combining of electron clouds known as a covalent bond.
Covalent Bonding Ionic substances: * usually brittle * high melting point * organized into an ordered lattice of atoms, which can be cleaved along a smooth line the electrostatic forces organize the ions of ionic substances into a rigid, organized three-dimensional arrangement The vast majority of chemical substances are not ionic in nature * gases and liquids, in addition to solids * low melting temperatures G.N. Lewis reasoned that an atom might attain a noble gas electron configuration by sharing electrons A chemical bond formed by sharing a pair of electrons is called a covalent bond The diatomic hydrogen molecule (H2) is the simplest model of a covalent bond, and is represented in Lewis structures as: The shared pair of electrons provides each hydrogen atom with two electrons in its valence shell (the 1s) orbital. In a sense, it has the electron configuration of the noble gas helium When two chlorine atoms covalently bond to form Cl2, the following sharing of electrons occurs: Each chlorine atom shared the bonding pair of electrons and achieves the electron configuration of the noble gas argon. In Lewis structures the bonding pair of electrons is usually displayed as a line, and the unshared electrons as dots: The shared electrons are not located in a fixed position between the nuclei. In the case of the H2 compound, the electron density is concentrated between the two nuclei: The two atoms are bound into the H2 molecule mainly due to the attraction of the positively charged nuclei for the negatively charged electron cloud located between them For the nonmetals (and the 's' block metals) the number of valence electrons is equal to the group number: Element Group Valence electrons Bonds needed to form valence octet F 7A 7 1 O 6A 6 2 N 5A 5 3 C 4A 4 4 Examples of hydride compounds of the above elements (covalent bonds with hydrogen: Thus, the Lewis bonds successfully describe the covalent interactions between various nonmetal elements Multiple bonds The sharing of a pair of electrons represents a single covalent bond, usually just referred to as a single bond In many molecules atoms attain complete octets by sharing more than one pair of electrons between them. Two electron pairs shared a double bond Three electron pairs shared a triple bond Because each nitrogen contains 5 valence electrons, they need to share 3 pairs to each achieve a valence octet. * N2 is fairly inert, due to the strong triple bond between the two nitrogens * The N - N bond distance in N2 is 1.10 Å (fairly short) From a study of various Nitrogen containing compounds bond distance as a function of bond type can be summarized as follows: * N-N 1.47Å * N=N 1.24Å * N=N 1.10Å As a general rule, the distance between bonded atoms decreases as the number of shared electron pairs increases
The stability of a molecule is a function of the strength of the covalent bonds holding the atoms together. How do we measure the strength of a covalent bond? Bond-dissociation energy (i.e. "bond energy") Bond energy is the enthalpy change (DH, heat input) required to break a bond (in 1 mole of a gaseous substance) What about when we have a compound which is not a diatomic molecule? Consider the dissociation of methane: There are four equivalent C-H bonds, thus we can that the dissociation energy for a single C-H bond would be: D(C-H) = (1660/4) kJ/mol = 415 kJ/mol Note: * The bond energy for a given bond is influenced by the rest of the molecule. * However, this is a relatively small effect (suggesting that bonding electrons are localized between the bonding atoms). * Thus, the bond energy for most bonds varies little from the average bonding energy for that type of bond Bond energy is always a positive value - it takes energy to break a covalent bond (conversely energy is released during bond formation) Average bond energies: Bond (kJ/mol) C-F 485 C-Cl 328 C-Br 276 C-I 240 C-C 348 C-N 293 C-O 358 C-F 485 C-C 348 C=C 614 C=C 839 The more stable a molecule (i.e. the stronger the bonds) the less likely the molecule is to undergo a chemical reaction Bond Energies and the Enthalpy of Reactions If we know which bonds are broken and which bonds are made during a chemical reaction, we can estimate the enthalpy change of the reaction (DHrxn) even if we don't know the enthalpies of formation (DHf°)for the reactants and products: DH = S(bond energies of broken bonds) - S(bond energies of formed bonds) Example: The reaction between 1 mol of chlorine and 1 mol methane Bonds broken: 1 mol of Cl-Cl bonds, 1 mol of C-H bonds Bonds formed: 1 mol of H-Cl bonds, 1 mol of C-Cl bonds DH = [D(Cl-Cl) + D(C-H)] - [D(H-Cl)+D(C-Cl)] [242 kJ + 413 kJ] - [431 kJ + 328 kJ] -104 kJ Thus, the reaction is exothermic (because the bonds in the products are stronger than the bonds in the reactants) Example: The combustion of 1 mol of ethane bonds broken: 6 moles C-H bonds, 1 mol C-C bonds, 7/2 moles of O=O bonds bonds formed: 4 moles C=O bonds, 6 moles O-H bonds DH = [(6*413) + (348) + (7/2*495)] - [(4*799) + (6*463)] = 4558 - 5974 = -1416 kJ (the reaction is exothermic) Bond strength and bond length Bond Bond Energy (kJ/mol) Bond Length (Å) C-C 348 1.54 C=C 614 1.34 C=C 839 1.20 As the number of bonds between two atoms increases, the bond grows shorter and stronger
Additional Information: Covalent bonding is a form of chemical bonding characterized by the sharing of one or more pairs of electrons between atoms, in order to produce a mutual attraction, which holds the resultant molecule together. Atoms tend to share electrons in such a way that their outer electron shells are filled. Such bonds are always stronger than the intermolecular hydrogen bond and similar in strength to or stronger than the ionic bond. Covalent bonding most frequently occurs between atoms with similar (high) electronegativities, where to completely remove an electron from one atom requires too much energy. Covalent bonds are more common between non-metals, whereas ionic bonding is more common between a metal atom and a non-metal atom. Covalent bonding tends to be stronger than other types of bonding, such as ionic bonding. Unlike ionic bonds, where ions are held together by a non-directional coulombic attraction, covalent bonds are highly directional. As a result, covalently bonded molecules tend to form in a relatively small number of characteristic shapes, exhibiting specific bonding angles.
COVALENT BONDING - SINGLE BONDS This page explains what covalent bonding is. It starts with a simple picture of the single covalent bond, and then modifies it slightly for A'level purposes. It also takes a more sophisticated view (beyond A'level) if you are interested. You will find a link to a page on double covalent bonds at the bottom of the page. A simple view of covalent bonding The importance of noble gas structures At a simple level (like GCSE) a lot of importance is attached to the electronic structures of noble gases like neon or argon which have eight electrons in their outer energy levels (or two in the case of helium). These noble gas structures are thought of as being in some way a "desirable" thing for an atom to have. You may well have been left with the strong impression that when other atoms react, they try to achieve noble gas structures. As well as achieving noble gas structures by transferring electrons from one atom to another as in ionic bonding, it is also possible for atoms to reach these stable structures by sharing electrons to give covalent bonds. Some very simple covalent molecules Chlorine For example, two chlorine atoms could both achieve stable structures by sharing their single unpaired electron as in the diagram. The fact that one chlorine has been drawn with electrons marked as crosses and the other as dots is simply to show where all the electrons come from. In reality there is no difference between them. The two chlorine atoms are said to be joined by a covalent bond. The reason that the two chlorine atoms stick together is that the shared pair of electrons is attracted to the nucleus of both chlorine atoms. Hydrogen Hydrogen atoms only need two electrons in their outer level to reach the noble gas structure of helium. Once again, the covalent bond holds the two atoms together because the pair of electrons is attracted to both nuclei. Hydrogen chloride The hydrogen has a helium structure, and the chlorine an argon structure. Covalent bonding at A'level Cases where there isn't any difference from the simple view If you stick closely to modern A'level syllabuses, there is little need to move far from the simple (GCSE) view. The only thing which must be changed is the over-reliance on the concept of noble gas structures. Most of the simple molecules you draw do in fact have all their atoms with noble gas structures. For example: Even with a more complicated molecule like PCl3, there's no problem. In this case, only the outer electrons are shown for simplicity. Each atom in this structure has inner layers of electrons of 2,8. Again, everything present has a noble gas structure. Cases where the simple view throws up problems Boron trifluoride, BF3 A boron atom only has 3 electrons in its outer level, and there is no possibility of it reaching a noble gas structure by simple sharing of electrons. Is this a problem? No. The boron has formed the maximum number of bonds that it can in the circumstances, and this is a perfectly valid structure. Energy is released whenever a covalent bond is formed. Because energy is being lost from the system, it becomes more stable after every covalent bond is made. It follows, therefore, that an atom will tend to make as many covalent bonds as possible. In the case of boron in BF3, three bonds is the maximum possible because boron only has 3 electrons to share. Note: You might perhaps wonder why boron doesn't form ionic bonds with fluorine instead. Boron doesn't form ions because the total energy needed to remove three electrons to form a B3+ ion is simply too great to be recoverable when attractions are set up between the boron and fluoride ions. Phosphorus(V) chloride, PCl5 In the case of phosphorus 5 covalent bonds are possible - as in PCl5. Phosphorus forms two chlorides - PCl3 and PCl5. When phosphorus burns in chlorine both are formed - the majority product depending on how much chlorine is available. We've already looked at the structure of PCl3. The diagram of PCl5 (like the previous diagram of PCl3) shows only the outer electrons. Notice that the phosphorus now has 5 pairs of electrons in the outer level - certainly not a noble gas structure. You would have been content to draw PCl3 at GCSE, but PCl5 would have looked very worrying. Why does phosphorus sometimes break away from a noble gas structure and form five bonds? In order to answer that question, we need to explore territory beyond the limits of current A'level syllabuses. Don't be put off by this! It isn't particularly difficult, and is extremely useful if you are going to understand the bonding in some important organic compounds. A more sophisticated view of covalent bonding The bonding in methane, CH4 Warning! If you aren't happy with describing electron arrangements in s and p notation, and with the shapes of s and p orbitals, you need to read about orbitals before you go on. Use the BACK button on your browser to return quickly to this point. What is wrong with the dots-and-crosses picture of bonding in methane? We are starting with methane because it is the simplest case which illustrates the sort of processes involved. You will remember that the dots-and-crossed picture of methane looks like this. There is a serious mis-match between this structure and the modern electronic structure of carbon, 1s22s22px12py1. The modern structure shows that there are only 2 unpaired electrons for hydrogens to share with, instead of the 4 which the simple view requires. You can see this more readily using the electrons-in-boxes notation. Only the 2-level electrons are shown. The 1s2 electrons are too deep inside the atom to be involved in bonding. The only electrons directly available for sharing are the 2p electrons. Why then isn't methane CH2? Promotion of an electron When bonds are formed, energy is released and the system becomes more stable. If carbon forms 4 bonds rather than 2, twice as much energy is released and so the resulting molecule becomes even more stable. There is only a small energy gap between the 2s and 2p orbitals, and so it pays the carbon to provide a small amount of energy to promote an electron from the 2s to the empty 2p to give 4 unpaired electrons. The extra energy released when the bonds form more than compensates for the initial input. Note: People sometimes worry that the promoted electron is drawn as an up-arrow, whereas it started as a down-arrow. It simply makes the diagram look tidier - nothing very sophisticated is going on! Now that we've got 4 unpaired electrons ready for bonding, another problem arises. In methane all the carbon-hydrogen bonds are identical, but our electrons are in two different kinds of orbitals. You aren't going to get four identical bonds unless you start from four identical orbitals. Hybridisation The electrons rearrange themselves again in a process called hybridisation. This reorganises the electrons into four identical hybrid orbitals called sp3 hybrids (because they are made from one s orbital and three p orbitals). You should read "sp3" as "s p three" - not as "s p cubed". sp3 hybrid orbitals look a bit like half a p orbital, and they arrange themselves in space so that they are as far apart as possible. You can picture the nucleus as being at the centre of a tetrahedron (a triangularly based pyramid) with the orbitals pointing to the corners. For clarity, the nucleus is drawn far larger than it really is. What happens when the bonds are formed? Remember that hydrogen's electron is in a 1s orbital - a spherically symmetric region of space surrounding the nucleus where there is some fixed chance (say 95%) of finding the electron. When a covalent bond is formed, the atomic orbitals (the orbitals in the individual atoms) merge to produce a new molecular orbital which contains the electron pair which creates the bond. Four molecular orbitals are formed, looking rather like the original sp3 hybrids, but with a hydrogen nucleus embedded in each lobe. Each orbital holds the 2 electrons that we've previously drawn as a dot and a cross. The principles involved - promotion of electrons if necessary, then hybridisation, followed by the formation of molecular orbitals - can be applied to any covalently-bound molecule. Note: You will find this bit on methane repeated in the organic section of this site. That article on methane goes on to look at the formation of carbon-carbon single bonds in ethane. The bonding in the phosphorus chlorides, PCl3 and PCl5 What's wrong with the simple view of PCl3? This diagram only shows the outer (bonding) electrons. Nothing is wrong with this! (Although it doesn't account for the shape of the molecule properly.) If you were going to take a more modern look at it, the argument would go like this: Phosphorus has the electronic structure 1s22s22p63s23px13py13pz1. If we look only at the outer electrons as "electrons-in-boxes": There are 3 unpaired electrons that can be used to form bonds with 3 chlorine atoms. The four 3-level orbitals hybridise to produce 4 equivalent sp3 hybrids just like in carbon - except that one of these hybrid orbitals contains a lone pair of electrons. Each of the 3 chlorines then forms a covalent bond by merging the atomic orbital containing its unpaired electron with one of the phosphorus unpaired electrons to make 3 molecular orbitals. You might wonder whether all this is worth the bother! Probably not! It is worth it with PCl5, though. What's wrong with the simple view of PCl5? You will remember that the dots-and-crosses picture of PCl5 looks awkward because the phosphorus doesn't end up with a noble gas structure. This diagram also shows only the outer electrons. In this case, a more modern view makes things look better by abandoning any pretence of worrying about noble gas structures. If the phosphorus is going to form PCl5 it has first to generate 5 unpaired electrons. It does this by promoting one of the electrons in the 3s orbital to the next available higher energy orbital. Which higher energy orbital? It uses one of the 3d orbitals. You might have expected it to use the 4s orbital because this is the orbital that fills before the 3d when atoms are being built from scratch. Not so! Apart from when you are building the atoms in the first place, the 3d always counts as the lower energy orbital. This leaves the phosphorus with this arrangement of its electrons: The 3-level electrons now rearrange (hybridise) themselves to give 5 hybrid orbitals, all of equal energy. They would be called sp3d hybrids because that's what they are made from. The electrons in each of these orbitals would then share space with electrons from five chlorines to make five new molecular orbitals - and hence five covalent bonds. Why does phosphorus form these extra two bonds? It puts in an amount of energy to promote an electron, which is more than paid back when the new bonds form. Put simply, it is energetically profitable for the phosphorus to form the extra bonds. The advantage of thinking of it in this way is that it completely ignores the question of whether you've got a noble gas structure, and so you don't worry about it. A non-existent compound - NCl5 Nitrogen is in the same Group of the Periodic Table as phosphorus, and you might expect it to form a similar range of compounds. In fact, it doesn't. For example, the compound NCl3 exists, but there is no such thing as NCl5. Nitrogen is 1s22s22px12py12pz1. The reason that NCl5 doesn't exist is that in order to form five bonds, the nitrogen would have to promote one of its 2s electrons. The problem is that there aren't any 2d orbitals to promote an electron into - and the energy gap to the next level (the 3s) is far too great. In this case, then, the energy released when the extra bonds are made isn't enough to compensate for the energy needed to promote an electron - and so that promotion doesn't happen. Atoms will
Covalent bonds are forces that hold atoms together. The forces are formed when the atoms of a molecule share electrons. You will learn more about the chemistry of covalent bond formation when you take Chemistry 1. For biological purposes there are several things to remember about covalent bonds * Covalent bonds represent chemical potential energy that can be used in biological reactions. An example of this are the phosphoanhyride bonds of ATP. * The angles formed between covalently bonded atoms are specific and defined. This means that biological molecules formed with covalent bonds have definite and predicable shapes. * In biological systems, covalent bonds are called strong bonds. This means that they are not normally broken under biological conditions unless by enzymic catalysis. This is in opposition to weak bonds like hydrogen and ionic bonds which are easily broken under normal biological conditions of temperature and pressure. * There is a type of covalent bond called a polar covalent bond. In Biological systems, polar covalent bonds are important because these kinds of bonds allow the formation of another kind of weak bond called a hydrogen bond. Water is an example of a molecule that has polar covalent bonds and engages in hydrogen bonding. * In addition to polar covalent bonds, there are nonpolar covalent bonds. In biological systems, if a molecules has a predominance of nonpolar covalent bonds, that substance is hydrophobic.
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