Chemistry - Are metallic/ionic bonds weaker than covalent bonds?
Quartz and diamond are stronger substances because their molecules form network covalent structures. These structures form a lattice-like structure, much the same as ionic compounds.
This molecular network is also the reason that diamond and quartz form a crystalline structures, just like you'd see in ionic substances such as NaCl. Some other structures you might want to look into are Graphite and Graphene, which are both allotropes of carbon (allotropes are, simply put, different molecular arrangements of an element).
The network structure combines to make the substance stronger than normal covalent bonded substances.
So to answer your question, substances with standard covalent bonds seem to be weaker than those with ionic bonds because the ionic bonds tend to form a lattice structure, that makes them much stronger. You can see this in the fact that the boiling points of ionic salts are much higher than that of a covalent substance like water. However, when covalent bonds form network covalent structures, atoms combine to form a singular macromolecule that is much stronger than singular covalent bonds.
What you learned in your mineralogy class was correct; bond strength decrease in the following order covalent > ionic > metallic. The reasoning for this is as follows. In covalent bonds such as those in methane and oxygen, the valence electrons are shared between the atoms involved in the bond and they (the electrons) spend most of their time in the region between the nuclei involved in the bond; this makes for a strong bond. In ionic materials such as sodium chloride, the electrons are donated from one (the electropositive) atom to the other (the electronegative) atom in order for the atoms to achieve a filled shell structure. The ionic atoms are attracted to one another through electrostatic attraction and the crystal lattices that are formed. The bonds formed through electrostatic attraction are not as strong as those formed from covalent sharing of electrons. Finally, in metals the outermost electrons are donated or "pooled" in the band structure that exists in metals. The electrons are free to travel great distances (hence the conductivity of metals) and serve as a glue to hold all of the positively charged metal nuclei together. So in the case of metals, there are no significant metal-metal bonds and these bonds are therefor the weakest.
It depends, because for covalent there are two types of bonds, network or molecular, or as I have also heard it be called, polar covalent and non polar covalent. But, network covalent consists of a vast network among the atoms and each one are connected, and they mostly made up of one element.
Take a diamond for example, it is only made up of carbon, but since the atoms are connected to each other and have not bonding between molecules, like something such as salt, which is an ionic bond, it is harder to break. However, if it were a molecular covalent bond, then the whole story is different, because they tend to be very weak bonds and easily broken like sugar or otherwise known as glucose, sucrose doesn't matter, it is still a covalent molecular bond because they have molecules while as a diamond is technically one big molecule.
But, since sugar has multiple the bond between each other molecule is weaker than the bonds between the elements themselves, then it is really weak.
This turns out to be a nonsense question. Chemical bonds run the entire gamut from very strong to very weak as evidenced by the amount of energy required to break them. Trying to say that bonds which are either ionic or covalent are stronger is a big mistake, starting with the fact that "ionic" and "covalent" are merely the hypothetical extremes of the bonding continuum and can be considered "ideal" bonds. Real bonds lie along the continuum and have characteristics of both of the ideal bond types. Therefore, your original question has no place in the study of chemistry.
Don't confuse the strength of bonds with the strength of the forces that hold crystalline solids together
There is a reason why the lessons you learned from chemistry are different from the lessons learned in mineralogy: they are not talking about the same things.
The problem is that mineralogy the bonds they talk about are the bonds that hold crystals together but in in chemistry what is often talked about are the bonds that hold the molecules together not the crystals made from the molecules.
This distinction is important. The vast majority of crystals in chemistry consist of discrete molecules held together by weaker intermolecular forces (sometimes called van Der Waals bonds). These are fairly weak compared to covalent bonds and result in crystals that are weak and have low melting points. So a chemist might look at compounds where the bonding in the molecules is covalent and make the generalisation that they typically form crystals much weaker than metals or ionic compounds. But that is because the bonds that make the crystals are not covalent.
A mineralogist will mostly look at compounds which are not made of discrete molecules but are made from ionic lattices or covalent networks (or both). There are no molecules of diamond, the crystal is a network solid held together by an (almost) infinite array of C-C covalent bonds, as silica is held together by an infinite array of O-Si-O bonds. Other minerals are a mix of the two with many silicates containing, for example, sheets of O-Si-O structures with a variety of ions in between. So, to a mineralogist, covalent bonds look strong compared to other types of bond. Ionic bonds are strong but not as strong as purely covalent network solids.
And the problem is further complicate by definitions of strength that are too narrow. Are metals stronger or weaker than diamond-like structures? It depends what you mean by strength. Diamond is harder than any metal but it is also more brittle. If resilience to being struck with a sharp object matters, choose a ductile metal object over a diamond any day. This happens because the crystal structure in some metals can absorb energy by reorganising crystal defects rather than by shattering bonds (pretty much the only option in silica or diamond). So, in one sense, metals are stronger than covalent solids.
The overall lesson is to be careful about definitions. There is no good generalisation of crystal strength based on bond types. Be careful whether you are talking about the bonds within the components of the crystal (molecules) or the bonds that hold those components together (many "covalent" compounds consist of crystals where molecules are held together by much weaker forces). Don't forget that many minerals have both ionic and covalent bonding. And be specific about what you mean by "strength" (eg harness are resilience to impact are not the same thing).