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Is calcium carbide an ionic or covalent compound?
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Malcolm Muhd
Is calcium carbide an ionic or covalent compound?
Myth debunking time!
First, there is no such thing as a purely ionic bond and no such thing as a purely covalent bond. Each bond (yes, even the bond in $\ce{H2}$) has a partially covalent and a partially ionic character. However, it is true that we can classify bonds and interactions as predominantly covalent or predominantly ionic in many cases especially those that occur at a beginner’s level.
While it is a nice explanation for the creation of e.g. sodium chloride to think of a full electron transfer — not an exchange because only one electron per ion pair is transferred and only in one direction — this cannot explain the structures of e.g. sodium sulphate well. Instead, you should think of ionic compounds as those composed of differently charged ions between which the interactions are predominantly electrostatic (non-directional). This is not true for all ionic compounds (ammonium fluoride being an exception) but gets you very far.
Now that I have established that (those that can be classified as predominantly) ionic compounds are composed of charged particles that interact electrostatically, you may be able to understand that not each ionic compound consists of exactly one atom type as the anion and exactly one atom type as the cation. In fact, I want to postulate that the vast majority of ionic compounds do not follow this rule. These compounds contain polyatomic ions meaning that many atoms connected together by covalent bonds form one charged molecule (or molecular ion). One typical example is the sulphate anion $\ce{SO4^2-}$ that I mentioned earlier but also the ammonium cation $\ce{NH4+}$. Thus, a compound can be held together by both covalent and ionic bonds, if for example it is composed of molecular ions.
This is the case in calcium carbide. We do indeed have a calcium cation $\ce{Ca^2+}$. The counterion is a diatomic anion, the ethynediide anion $\ce{C2^2-}$. It consists of two carbon atoms connected by a triple bond with each carbon atom carrying an additional lone pair and thus formal negative charge for an overall charge of $2-$. The bond between the two carbon atoms is covalent. However, the bond between calcium and the $\ce{C2^2-}$ fragment is ionic.
Therefore, calcium carbide can be classified as an ionic compound.
If you dig deeper, you may come across the crystal structure of calcium carbide; see for example the following taken from Wikipedia:
(This structure actually depicts barium peroxide but calcium carbide crystallises in the same structure.)
The interatomic distances given by Atoji and Medrud[1] are:
$$\begin{array}{lccc}\hline
\text{atoms} & \ce{C#C} & \ce{C-Ca}\text{ (lin)} & \ce{C-Ca}\text{ (orth)}\\ \hline
d/\mathrm{pm} & 120 & 259 & 282 \\ \hline\end{array}$$
The linear $\ce{(C#)C-Ca}$ distances are actually short enough to also consider a covalent interaction. Other acetylides can also be characterised with an intermediate or predominantly covalent bond. The picture becomes much more fuzzy once you take a closer look. The interaction of calcium with the $\ce{C2}$ fragment’s π system should also be considered as a potentially bonding interaction; think in the direction of transition metal complexes.
However, the similarity of both $\ce{C-Ca}$ distances (which becomes even more similar if you take the distance from calcium to the centre of the $\ce{C2}$ fragment) can also be seen as support for the argument of non-directional ionic bonding.
Considering all this experimental data, it is clear that the compound should have significant covalent bonding. Significant ionic bonding is not out of the question. The bonds are likely somewhere in the middle between covalent and ionic. Whatever the ‘final result’ e.g. of calculations is, it will not tip the scales strongly.
Reference:
[1]: M. Atoji, C. Medrud, J. Chem. Phys.1959, 31, 332. DOI: 10.1063/1.1730352.
First, there is no such thing as a purely ionic bond and no such thing as a purely covalent bond. Each bond (yes, even the bond in $\ce{H2}$) has a partially covalent and a partially ionic character. However, it is true that we can classify bonds and interactions as predominantly covalent or predominantly ionic in many cases especially those that occur at a beginner’s level.
While it is a nice explanation for the creation of e.g. sodium chloride to think of a full electron transfer — not an exchange because only one electron per ion pair is transferred and only in one direction — this cannot explain the structures of e.g. sodium sulphate well. Instead, you should think of ionic compounds as those composed of differently charged ions between which the interactions are predominantly electrostatic (non-directional). This is not true for all ionic compounds (ammonium fluoride being an exception) but gets you very far.
Now that I have established that (those that can be classified as predominantly) ionic compounds are composed of charged particles that interact electrostatically, you may be able to understand that not each ionic compound consists of exactly one atom type as the anion and exactly one atom type as the cation. In fact, I want to postulate that the vast majority of ionic compounds do not follow this rule. These compounds contain polyatomic ions meaning that many atoms connected together by covalent bonds form one charged molecule (or molecular ion). One typical example is the sulphate anion $\ce{SO4^2-}$ that I mentioned earlier but also the ammonium cation $\ce{NH4+}$. Thus, a compound can be held together by both covalent and ionic bonds, if for example it is composed of molecular ions.
This is the case in calcium carbide. We do indeed have a calcium cation $\ce{Ca^2+}$. The counterion is a diatomic anion, the ethynediide anion $\ce{C2^2-}$. It consists of two carbon atoms connected by a triple bond with each carbon atom carrying an additional lone pair and thus formal negative charge for an overall charge of $2-$. The bond between the two carbon atoms is covalent. However, the bond between calcium and the $\ce{C2^2-}$ fragment is ionic.
Therefore, calcium carbide can be classified as an ionic compound.
If you dig deeper, you may come across the crystal structure of calcium carbide; see for example the following taken from Wikipedia:
(This structure actually depicts barium peroxide but calcium carbide crystallises in the same structure.)
The interatomic distances given by Atoji and Medrud[1] are:$$\begin{array}{lccc}\hline\text{atoms} & \ce{C#C} & \ce{C-Ca}\text{ (lin)} & \ce{C-Ca}\text{ (orth)}\\ \hlined/\mathrm{pm} & 120 & 259 & 282 \\ \hline\end{array}$$
The linear $\ce{(C#)C-Ca}$ distances are actually short enough to also consider a covalent interaction. Other acetylides can also be characterised with an intermediate or predominantly covalent bond. The picture becomes much more fuzzy once you take a closer look. The interaction of calcium with the $\ce{C2}$ fragment’s π system should also be considered as a potentially bonding interaction; think in the direction of transition metal complexes.
However, the similarity of both $\ce{C-Ca}$ distances (which becomes even more similar if you take the distance from calcium to the centre of the $\ce{C2}$ fragment) can also be seen as support for the argument of non-directional ionic bonding.
Considering all this experimental data, it is clear that the compound should have significant covalent bonding. Significant ionic bonding is not out of the question. The bonds are likely somewhere in the middle between covalent and ionic. Whatever the ‘final result’ e.g. of calculations is, it will not tip the scales strongly.
Reference:
[1]: M. Atoji, C. Medrud, J. Chem. Phys.1959, 31, 332. DOI: 10.1063/1.1730352.
Starts like debunking, but "we can classify bonds and interactions as predominantly covalent or predominantly ionic in many cases — practically all cases that occur at a beginner’s level." is well... Most compounds of transition metals and even ones like very CaC2 arent obvious at all, or are predominantly covalent.More
Myth debunking time!
First, there is no such thing as a purely ionic bond and no such thing as a purely covalent bond. Each bond (yes, even the bond in $\ce{H2}$) has a partially covalent and a partially ionic character. However, it is true that we can classify bonds and interactions as predominantly covalent or predominantly ionic in many cases especially those that occur at a beginner’s level.
While it is a nice explanation for the creation of e.g. sodium chloride to think of a full electron transfer — not an exchange because only one electron per ion pair is transferred and only in one direction — this cannot explain the structures of e.g. sodium sulphate well. Instead, you should think of ionic compounds as those composed of differently charged ions between which the interactions are predominantly electrostatic (non-directional). This is not true for all ionic compounds (ammonium fluoride being an exception) but gets you very far.
Now that I have established that (those that can be classified as predominantly) ionic compounds are composed of charged particles that interact electrostatically, you may be able to understand that not each ionic compound consists of exactly one atom type as the anion and exactly one atom type as the cation. In fact, I want to postulate that the vast majority of ionic compounds do not follow this rule. These compounds contain polyatomic ions meaning that many atoms connected together by covalent bonds form one charged molecule (or molecular ion). One typical example is the sulphate anion $\ce{SO4^2-}$ that I mentioned earlier but also the ammonium cation $\ce{NH4+}$. Thus, a compound can be held together by both covalent and ionic bonds, if for example it is composed of molecular ions.
This is the case in calcium carbide. We do indeed have a calcium cation $\ce{Ca^2+}$. The counterion is a diatomic anion, the ethynediide anion $\ce{C2^2-}$. It consists of two carbon atoms connected by a triple bond with each carbon atom carrying an additional lone pair and thus formal negative charge for an overall charge of $2-$. The bond between the two carbon atoms is covalent. However, the bond between calcium and the $\ce{C2^2-}$ fragment is ionic.
Therefore, calcium carbide can be classified as an ionic compound.
If you dig deeper, you may come across the crystal structure of calcium carbide; see for example the following taken from Wikipedia:
(This structure actually depicts barium peroxide but calcium carbide crystallises in the same structure.)
The interatomic distances given by Atoji and Medrud[1] are: $$\begin{array}{lccc}\hline \text{atoms} & \ce{C#C} & \ce{C-Ca}\text{ (lin)} & \ce{C-Ca}\text{ (orth)}\\ \hline d/\mathrm{pm} & 120 & 259 & 282 \\ \hline\end{array}$$
The linear $\ce{(C#)C-Ca}$ distances are actually short enough to also consider a covalent interaction. Other acetylides can also be characterised with an intermediate or predominantly covalent bond. The picture becomes much more fuzzy once you take a closer look. The interaction of calcium with the $\ce{C2}$ fragment’s π system should also be considered as a potentially bonding interaction; think in the direction of transition metal complexes.
However, the similarity of both $\ce{C-Ca}$ distances (which becomes even more similar if you take the distance from calcium to the centre of the $\ce{C2}$ fragment) can also be seen as support for the argument of non-directional ionic bonding.
Considering all this experimental data, it is clear that the compound should have significant covalent bonding. Significant ionic bonding is not out of the question. The bonds are likely somewhere in the middle between covalent and ionic. Whatever the ‘final result’ e.g. of calculations is, it will not tip the scales strongly.
Reference:
[1]: M. Atoji, C. Medrud, J. Chem. Phys. 1959, 31, 332. DOI: 10.1063/1.1730352.
Myth debunking time!
First, there is no such thing as a purely ionic bond and no such thing as a purely covalent bond. Each bond (yes, even the bond in $\ce{H2}$) has a partially covalent and a partially ionic character. However, it is true that we can classify bonds and interactions as predominantly covalent or predominantly ionic in many cases especially those that occur at a beginner’s level.
While it is a nice explanation for the creation of e.g. sodium chloride to think of a full electron transfer — not an exchange because only one electron per ion pair is transferred and only in one direction — this cannot explain the structures of e.g. sodium sulphate well. Instead, you should think of ionic compounds as those composed of differently charged ions between which the interactions are predominantly electrostatic (non-directional). This is not true for all ionic compounds (ammonium fluoride being an exception) but gets you very far.
Now that I have established that (those that can be classified as predominantly) ionic compounds are composed of charged particles that interact electrostatically, you may be able to understand that not each ionic compound consists of exactly one atom type as the anion and exactly one atom type as the cation. In fact, I want to postulate that the vast majority of ionic compounds do not follow this rule. These compounds contain polyatomic ions meaning that many atoms connected together by covalent bonds form one charged molecule (or molecular ion). One typical example is the sulphate anion $\ce{SO4^2-}$ that I mentioned earlier but also the ammonium cation $\ce{NH4+}$. Thus, a compound can be held together by both covalent and ionic bonds, if for example it is composed of molecular ions.
This is the case in calcium carbide. We do indeed have a calcium cation $\ce{Ca^2+}$. The counterion is a diatomic anion, the ethynediide anion $\ce{C2^2-}$. It consists of two carbon atoms connected by a triple bond with each carbon atom carrying an additional lone pair and thus formal negative charge for an overall charge of $2-$. The bond between the two carbon atoms is covalent. However, the bond between calcium and the $\ce{C2^2-}$ fragment is ionic.
Therefore, calcium carbide can be classified as an ionic compound.
If you dig deeper, you may come across the crystal structure of calcium carbide; see for example the following taken from Wikipedia:
(This structure actually depicts barium peroxide but calcium carbide crystallises in the same structure.)
The interatomic distances given by Atoji and Medrud[1] are:$$\begin{array}{lccc}\hline\text{atoms} & \ce{C#C} & \ce{C-Ca}\text{ (lin)} & \ce{C-Ca}\text{ (orth)}\\ \hlined/\mathrm{pm} & 120 & 259 & 282 \\ \hline\end{array}$$
The linear $\ce{(C#)C-Ca}$ distances are actually short enough to also consider a covalent interaction. Other acetylides can also be characterised with an intermediate or predominantly covalent bond. The picture becomes much more fuzzy once you take a closer look. The interaction of calcium with the $\ce{C2}$ fragment’s π system should also be considered as a potentially bonding interaction; think in the direction of transition metal complexes.
However, the similarity of both $\ce{C-Ca}$ distances (which becomes even more similar if you take the distance from calcium to the centre of the $\ce{C2}$ fragment) can also be seen as support for the argument of non-directional ionic bonding.
Considering all this experimental data, it is clear that the compound should have significant covalent bonding. Significant ionic bonding is not out of the question. The bonds are likely somewhere in the middle between covalent and ionic. Whatever the ‘final result’ e.g. of calculations is, it will not tip the scales strongly.
Reference:
[1]: M. Atoji, C. Medrud, J. Chem. Phys. 1959, 31, 332. DOI: 10.1063/1.1730352.
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