Dear CH304,
This weekend's assignment. Last week's assignment can be found here.
In the current course of Chemistry, you have learnt that MgCl2 exists as a giant ionic lattice structure. While, H2O is a simple discrete molecule. This implies that ionic bond exist in MgCl2, while covalent bond exists in H2O. Suggest an explanation to why MgCl2 is not covalent. In addition, suggest why H2O is not ionic. You are highly encouraged to use the following articles as a stepping stone.
This weekend's assignment. Last week's assignment can be found here.
In the current course of Chemistry, you have learnt that MgCl2 exists as a giant ionic lattice structure. While, H2O is a simple discrete molecule. This implies that ionic bond exist in MgCl2, while covalent bond exists in H2O. Suggest an explanation to why MgCl2 is not covalent. In addition, suggest why H2O is not ionic. You are highly encouraged to use the following articles as a stepping stone.
1) Atomic structure - Introduction
2) Chemical bonding - Interatomic Bonds
Suggested Answer
Mg has two valence electrons while Cl has 7 valence electrons. In order for Cl to obtain the octet configuration it needs 1 more electron. There are two possible ways this may be done: (1) gaining an electron from a donor. (2) Through sharing of electrons, Cl gains the 8th electron. The third way will be for Cl to lose all its 7 valence electron and this is not possible because it would require too much energy.
If Cl obtain its 8th electron through sharing of electrons with Mg, thus resulting in a Mg-Cl covalent bond to be formed between Mg and Cl. This results in Mg to have only 4 electrons; 2 of its own and 2 derived through sharing. This is a far short of octet.
Hence, Mg will prefer to lose 2 electrons. The energy required to remove 2 electrons is compensated by the ionic bond formed between Mg2+ and Cl-. Therefore, MgCl2 exist as an ionic compound.
While in the situation of H2O, the sharing of electrons will allow both H and O to obtain the full shell configuration. Hence, water has the covalent bond, O-H, and it exists as a simple discrete molecule.
If H2O exists as an ionic compound, it is not possible for O to ionise all its valence electrons as that is energetically not feasible. In addition, for H to ionise its electron to give H+ will not be favourable too. This is because H has only 1 quantum shell. The close proximity between the valence electron of H and the nucleus requires a huge amount of energy to ionise the atom H. Therefore, these scenarios explain why ionic bonding cannot occur between H and O in H2O.
Comments
There are some decent attempts in trying to answer, but there are some of the glaring misconceptions or inabilities to account:
(1) Unable to do a proper compare and contrast to why some compounds exhibit ionic bonding while other exhibit covalent.
(2) Those who tried to do the compare and contrast gave very superficial comparisons, like using electronegativity difference, or electronegativity etc, these highlight that one did not really understood how these two interatomic bonds were derived. Electronegativity difference shows us a pattern to why certain bonds between particles are ionic and others are covalent; BUT they are not the reason to why those interatomic bonds are formed.
(3) Did not highlight why it is improbable for covalent bond for MgCl2 and ionic bond for H2O.
(4) Using polarising power to determine bonding is a poor use of principle. This principle is used to explain anomalies. It is not use to explain for the bonding in the first principle.
(5) This activity does highlight that critical analysis of data and principles is something that you would like to work on. The combination of Johanan (for MgCl2 and Sherlyn's (for H2O) answers give a indication of what I consider as critical analysis.
In conclusion, I think these set of assignments will start to allow us to think carefully about a statement and think more carefully about the fundamentals and hence facilitate the learning of this topic.
20 comments:
In MgCl2, Cl is more electronegative than Mg. Therefore, two Cl atoms attract an electron each from Mg in order for the three atoms to attain an octet structure. However, Cl’s and Mg’s difference in electronegativity is so great that the electrons get effectively transferred to both Cl atoms. Therefore, the Cl and Mg atoms form negatively and positively charged ions respectively. This results in ionic bonds forming between Mg and Cl.
In H2O, O is more electronegative than H. Therefore, the O atom attracts an electron each from two H atoms in order to attain an octet structure. However, their difference in electronegativity is small (less than 1.8). Therefore, each of the H atoms is also able to attract an electron from O and the electrons are shared, not transferred. Thus, a covalent bond is formed, unlike in MgCl2.
-Timothy Kwok
Covalent bonds are formed between non-metals when there are electrostatic forces of attraction between shared electrons and nuclei of the atom. Due to the sharing of electrons, orbitals overlap. Hence, covalent bonds are formed also when atomic orbitals overlap.
In the case of MgCl2, magnesium is not a non metal. Electrons are not shared in MgCl2 but are in fact donated from the cation to anion. Orbitals do not overlap. Hence, MgCl2 is not covalent.
Ionic bonds are formed because the electropostive atom [cation] has donated an electron to the electronegative atom [anion]. The electrostatic attraction between oppositely charged ions creates the ionic bond.
In the case of H20, electrons are not donated from cation to anion. Instead, the electrons are shared and orbitals overlap. Hence, H2O is not ionic but is instead covalent.
The reason why magnesium chloride exists as a giant ionic lattice structure while dihydrogen oxide exists as a simple discrete molecule:
The difference in electronegativity between magnesium (1.31) and chlorine (3.16) is 1.85. However, the difference in electronegativity between hydrogen (2.20) and oxygen (3.44) is smaller at 1.24.
Therefore, in the compound magnesium chloride, the chlorine atoms can each pull an electron into their valence shells as they are more electronegative than magnesium by 1.85. As a result each chlorine atom gains an electron, forming chloride ions in the process. Magnesium atoms are less electronegative than chlorine atoms and each lose two electrons, forming magnesium (2+) ions in the process. Therefore magnesium chloride exists as a giant ionic lattice.
However, in the compound dihydrogen oxide the difference in electronegativity between hydrogen and oxygen is too small and therefore the more electronegative oxygen cannot pull electrons into its valence shell, although electrons are attracted to oxygen more than hydrogen. Hence they share electrons instead, forming a covalent bond.
The bond formed between magnesium ions and chloride ions in MgCl2 is ionic. Ionic bonds form between oppositely charged ions. Mg is a metal, hence it tends to form positive ions while Cl is a non-metal, it tends to form negative ions.
In H2O molecules, one oxygen atom forms covalent bond with two hydrogen atoms as they share valence electrons to obtain the noble gas configuration. Since both hydrogen and oxygen have high electronegativity, they both tend to attract electrons rather than remove them (to form ions, hence give rise to ionic bond).
the difference in electronegativity bewteen two elements determines the type of bond.
in MgCl2, Mg and Cl have a big difference in electronegativity. the difference is quite big. and since Cl has a greater electronegativity than Mg, the Cl takes an electron from Mg. Mg has such a small electronegativity than it can give away yet another electron to yet another Cl. this produces an Mg2+ cation (with empty valence shell) and two CL- anions (with full valence shell) and the electrostatic attraction between the atoms would cause the atoms to be attracted to each other. but the big difference in electronegativity is not big enough to disrupt the rest of the electron cloud of Mg, so there is very little overlapping of orbitals, if at all, meaning that this bond is not covalent, but mostly ionic.
in H2O the difference in electronegativity between H and O is small. therefore neither atom can totally pull an electron from another. but O is more electronegative than H, so it distorts the electron cloud of H and pulls it toward itself. this means that the orbitals are now overlapping. the electrons are thus shared. there is also electrostatic attration bewteen the atoms now. since there is no transfer of electrons to each other, this bond has little ionic characteristics and is mostly covalent.
Mg (metal) has to lose 2 electrons to gain the nearest octet configuration, while Cl (non-metal) needs to gain only one. Thus, MgCl2 is an ionic bond as Mg gives off 2 electrons to 2 Cl atoms, thus becoming electrostatically attracted to each other. The large electronegativity difference between Mg and Cl leads to them forming ionic bonds.
In H2O, both atoms are non-metals, and have a small electronegativity difference. H needs one electron to gain the duet configuration, while oxygen needs two electrons. Thus, they are covalently bonded by O sharing a pair of electrons with each H atom.
MgCl2 is not covalent because 1 Mg atom donates 1 electron to each Cl atom. There is a transfer of electrons. There is no sharing of electrons between the atoms. Hence, there is no overlapping of atomic orbitals and no covalent bond is formed.
H2O is not ionic because there is no transfer of electrons from one atom to another atom. Both H atoms and O atoms are not charged. Hence, no oppositely charged ions are formed and no ionic bond is created.
MgCl2 is not covalent as Mg, which is the electropositive atom, donates an electron each to the two Cl, which is the electronegative atom. As such, the Mg atom becomes a positively charged 2+ ion while the two Cl atoms become two negatively charged 1- ions. Thus, the positive Mg ion and the 2 negative Cl ions are attracted by electrostatic forces of attraction, resulting in the formation on ionic bonds, also known as electrovalent bonds. As the ionic bonds are non-directional, they exert their electrostatic forces of attraction all around them, thus resulting in the formation of the giant ionic lattice structure.
H2O is not ionic as the 2 H atoms share their electrons with the single O atom. This is because the single electron in the s-orbital of the H atom is very close to the positive nucleus of the H atom. Thus, that single electron is strongly attracted to the positive H nucleus. Therefore, the electron would require an extremely large amount of ionization energy to be able to be removed from the Hydrogen atom s-orbital. When the atomic orbital of the O atom overlap with the atomic orbital of the two H atoms, the positive nucleus of the H atom exerts a large electrostatic force of attraction on the electrons of the O atom. Likewise, the positive nucleus of the O atom exerts a large electrostatic force of attraction on the electrons of the H atom. Hence, the orbital of the O atom and the orbital of the two H atoms overlap and form covalent bonds.
Magnesium has a low electro negativity of 1.2 while chloride has that of 3.0.
Firstly, the reason an ionic bond is formed for MgCl is due to the fact that Mg is an anion and is in group II. This means that its valence shell of electrons has only 2 electrons, thus it has two more electron than a stable octet structure.
On the other hand, chloride has 7 electrons in is valance shell.
Mg would want to donate its electron to another atom so that it can achieve an octet structure.
Therefore Cl would want to gain 1 electron from another atom in order to achieve a stable structure.
This results in one (out of 2) electrons from Mg is donated to the valence shell of each Cl. This would then create a positive charge on Mg as the nuclear charge is greater than that expressed by the electrons.
It also creates a negative charge on Cl (the charge of the number of electrons accepted would be greater than the positive charge of the nucleus).
This would thus be the only way to achieve a stable octet structure in the valence shell, thus ionic bonds would exist between these 2 elements.
Finally, covalent bonding usually takes place between 2 non metals with similar electronegativity. As Mg and Cl does not satisfy either part of the criteria, it is ionic.
H2O is not ionic as there is an equal sharing of electrons in the valence shells of both the H and O atoms due to the overlapping of the orbital involved in the bonding. It forms a bent-structure atom which is polar in water.
This is possible as hydrogen needs to accept 1 electron in order to achieve a stable duplet structure, while oxygen needs to accept 2 electrons in order to achieve a stable octet structure. Hence by sharing electrons, both species are able to benefit and achieve a stable element.
Ionic bonding would not be possible as when electrons are removed from hydrogen it would not be able to achieve a stable structure. Thus stability can only be achieved through covalent bonds.
*However, in the covalent bonds, there is a net dipole moment due to the higher electronegativity of oxygen as compared to hydrogen. This attracts the electrons along the bonds to move closer to the oxygen, resulting in it being slightly negatively charged. Also, the hydrogen molecules would thus be slightly positively charged. Thus water would exhibit some ionic properties due to this net dipole moment.
Magnesium is a metal and has two valence electrons while chlorine is a non-metal with 7 valence electrons. As such, when magnesium loses one valence electron each to two chlorine atoms (transfer of electrons), the ions of magnesium, Mg2+ and chlorine, Cl- then gain a noble gas configuration. These oppositely charged ions are attracted to each other and an ionic bond is formed between Mg2+ and Cl- ions. Two Cl- ions are required to balance the charge on one Mg2+, hence the resulting ionic compound, MgCl2. MgCl2 is not covalent, as it does not involve the sharing of electrons of the Mg and Cl.
H2O is a covalent compound as there are electrostatic forces of attraction between the shared electrons and the nuclei of hydrogen and oxygen atoms. Hydrogen has 1 valence electron while oxygen has 6 valence electrons. As they both are unable to transfer their electrons to the other atom to achieve a noble gas configuration, they share their electrons.
MgCl2 is not covalent because the difference in electronegativities of Mg and Cl is very large (difference is approx. 1.8). This means that the bond between Mg and Cl extremely polar, thus it becomes an ionic bond which involves a complete transfer of electrons (creating positive Mg2+ ions and negative Cl- ions) instead of a covalent bond.
H2O is not ionic because the difference in electronegativities of H and O is small (difference is approx 1.4). Although the bond is slightly polar, it is still a (polar) covalent bond because the H and O atoms are not polar enough and the transfer of electrons to form positive and negative ions will not occur.
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MgCl2 is ionic because first of all, metal and non-metals have a tendency of forming an ionic bond. As, Mg has a low IE while Cl has a high electronegativity, and can readily accept electrons. Hence an electron transfer occured and an ionic bond is formed rather than a covalent bond.
While for H2O, both are non-metals which does have a tendency to form covalent bonds, and also both are high in electronegativity, hence both attracts electrons, which is why, they finally shared electrons and form a covalent bond, as the attractive forces of their electronegativity can't beat each other to make the other loses electrons. Hence they can;t form ionic bonds and can only form covalent bonds.
Ren_aldy
According to Fajans's rules( I think ), an ionic compound would have more "ionic character" when it has a low charge density ( i.e low charge and large ionic radius).
Magnesium has a relatively low charge density, due to its low charge and rather large ionic radius. As such, it does not have much polarising power, making it hard for it to severely distort the electron cloud around Cl towards it. This is made harder by the fact that Cl is a relatively small ion ( it is about the same size as Mg), resulting in a smaller electron cloud which would experience a much stronger attraction to the Cl nucleus.
As a result of these facts, it would be hard for the atomic orbitals of Mg and Cl to overlap and form a covalent bond, making it ionic bonded. Some covalent character, however, does exist.
On the other hand, in water, the H atom has a relatively high charge: atomic radii ratio. O, which is larger than H, would too have a relatively large electron cloud.
Due to these nature's of the atom, it would be much easier for the atomic orbitals of H and O to overlap and form a covalent bond as compared with MgCl2.
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Something like this anyway. I've probably not made some "links" or details clear..but...ehhh...
Regards,
Chia Wei Jie
To simply put it, the bonding in MgCl2 involves the donating of electrons between oppositely charged atoms, forming a cation (donator) to the anion (receiver)and thus forming the ionic bond.
Hence, it explains why MgCl2 has a higher boiling and melting point as it is bonded by strong ionic bond in a giant crystal lattice structure.
On the other hand,H2O is bonded covalently as it involves the sharing of electrons between 2 similar charged atoms and thus explaining the presence of atomic orbitals overlapping one another. This results in a simple discrete molecule.
To prove it, the boiling point and melting point of H2O is low as the covalent bonds (Van de Walls forces of attraction)present are weak.
Hi Mr.Kwok, here's my answer for this week's assignment.
MgCl2 is not a covalent compound because there is a relatively big difference between the electronegativity of Mg and Cl, causing ionic bonds to be formed. Electrons are easier to be removed from Mg as the ionization energies are relatively low for magnesium, therefore forming a cation, because since magnesium is an element from Group II, it is most stable when able to lose the outside two electrons. Chlorine is highly electronegative, therefore it is very likely to gain electrons to form an anion. Hence, When Magnesium and Chlorine atoms come together, Mg gives up 2 electrons to form Mg2+, while each Cl receives one electron to form Cl-, forming an ionic bond instead of a covalent bond.
H2O is made up of Hydrogen and Oxygen, which are both non metals, hence when they come together they will form a covalent bond. Also, the difference between the Electronegativity of H and O is very small, causing H and O to share electrons, forming a covalent bond. The H atom has just 1 electron in its outermost shell,and to achieve noble gas configuration (stable state), the H atom has to get 1 more electron from O. However, the O atom also needs electrons to fill its outermost shell to gain stability,as it only has 6 electrons in its outermost shell. Therefore, the O atom will form a single covalent bond with each of the H atom to obtain a noble gas configuration instead of transferring electrons because both H and O are in need of electrons.
Magnesium has low polarising power and thus there is no distortion of its electron cloud. Hence electrons are not shared (which forms covalent bond), but transferred to the chlorine atoms (which forms ionic bond). Conversely, hydrogen has a high polarising power, and this causes a distortion of the electron cloud, allowing electrons to be shared with the oxygen atoms.
Firstly, Magnesium Chloride is an ionic compound and not a covalent compound. This is due to the fact that covalent compounds are formed between two non-metallic atoms, with the sharing of electrons between the two atoms. Magnesium is found in group II which shows us that Magnesium is a metal. Furthermore, Chlorine in found in group VII, which means that it is a non metal. Ionic compounds are formed from the transference of electrons between a metal and a non metal, which involves the transference of electrons between the original atoms. Hence, Magnesium Chloride is an ionic and not a covalent compound.
Secondly, Water molecules are not ionic but is a covalent compound. Ionic bonds are formed because the electropostive atom has donated an electron to the electronegative atom. For the principal quantum number 1, only two electrons can be filled in the 1s subshell to form a stable configuration, for example, like in Hydrogen. Hence, 2 hydrogen atoms will share electrons with oxygen atom to form a covalent compound, so that hydrogen and oxygen atoms can form stable configuration.
Magnesium has an electronic configuration of 1s2 2s2 2p6 3s2. Thus, the 3s orbital containing the two valence electrons is at a high energy level and the first two ionization energies would be relatively low compared to a non-metal like chlorine. Hence, it is easy to remove those two electrons in order to have a completely filled principle shell, leading to a magnesium ion of charge +2 to be formed. Chlorine has an electronic configuration of 1s2 2s2 2p6 3s2 3p5. It has 7 valence electrons and the energy needed to remove 7 successive electrons in order to have a stable electronic configuration is high. Thus, a much easier solution would be for the chlorine atom to take in one more electron into one of its half filled orbital, achieving a stable octet configuration to form an anion of -1 charge. Since an ionic bond is formed when an electropositive atom (Magnesium) donates an electron to an electronegative atom (chlorine), and there is an electrostatic force of attraction that results in an ionic bond. MgCl2 is not covalent because even if the orbitals overlap, the chlorine atom is much more electronegative compared to the magnesium atom, resulting in the “shared” electron to be pulled so close to itself making a covalent bond hard to form.
H2O is not ionic because an ionic bond can only be formed between a cation and an anion. This requires one atom to transfer an electron completely to the other. In the case of H2O, although Oxygen is more electronegative than Hydrogen, the differences in their electronegativity is small and thus, the electrons from hydrogen are not completely transferred to oxygen but rather are shared in-between the two atoms (closer to the oxygen atom) experiencing an electrostatic force of attraction from the nuclei of both atoms. Thus, cations and anions are not formed meaning H2O is not ionic.
MgCl2 is a metal compound, and we have learnt that most metal+non-metal compounds are ionic and not covalent, and vice versa for non-metal+non-metal compounds like H2O which are more likely to be covalent.
This is because the electronegativity values of metals are generally lower than that of non-metals(electronegativity increases along a period), and therefore are less likely to form covalent bonds and compounds since they have lesser tendency to attract orbitals to overlap.
This is contrasted to the opposing trait of electropositivity, which is the measure of how readily an element gives away an electron. Electropositivity of a metal is therefore higher and so is more likely to give away electron(s) to form ionic bonds or compounds thsn non-metals.
Therefore, MgCL2 is an ionic compound as Mg does not have a high electronegativity value like non-metals to form a covalent bond. On the other hand, H and O have high values of electronegativity which leads to the attraction and overlapping of the bonding orbitals, forming a covalent bond.
MgCl2 is not covalent because the atomic orbtials of Mg and Cl atoms do not overlap with each other and thus they do not share electrons. Therefore, there are no covalent bonds formed because there is no attraction between the nuclei and pair of shared electrons when the pair of shared electrons do not exist.
Covalent compounds share electrons whereas MgCl2 have no need to share electrons especially when Mg is electropositvie and Cl is electronegative.
Between Mg and Cl, Mg is the electropositive atom and Cl is the electronegative atom. Therefore, Mg donates two electrons to 2Cl atoms to achieve its stable octet structure. While each Cl atom receives an electron also to achieve a stable octet structure. Mg then becomes a cation and the 2Cls, anions. The electrostatic attraction between the ions form ionic bonds, thus, MgCl2 is not covalent.
H2O is not ionic because in order for the H atoms to achieve a noble gas configuration, it has to either have 2electrons in its valence shell or have non. However, because the sole H electron is so near the nuclei of the atom, it makes it difficult to remove the electron and the H atom would rather gain an electro than to lose 1. Also, O has the configuration of 1s2 2s2 2p4. In order to be a stable structure, it needs 2 electrons to fill the p- subshells. Therefore, since both types of atoms, H and O requires electrons, they share their electrons so that both H and O get to achieve its noble gas configuration. Neither of them are cations nor anions in the compound, thus there is no electrostatic attraction between them to form any ionic bonds.
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