Before I even begin talking about what Covalent bonds are I want to clear up some possible confusion. If you’re learning general chemistry now you’ll thank me later.
The last two graphics above are typically what are used to illustrate what a covalent bond is between atoms. However this picture is not quite accurate. In reality atoms don’t align themselves in a Bohr model form of atoms and electron orbiting around like planets and moons.
Reality it turns out is just a bit weirder than that. Because quantum mechanics is really weird. The reality is that what exists are orbitals, which are a probability distribution of where an electron may be around an atom. These take on particular shapes depending on the element (heavier elements have more and different orbitals). This is because according to Heisenberg’s Uncertainty Principle, it is not possible to know both where an electron is and its exact momentum. Because of this we can only rely on probabilities on where electrons may be.
The first image is a closer representation of the way electrons are distributed than the circular models used in Basic/General Chemistry.
But this isn’t about orbitals. This is about Covalent bonds, so lets begin.
Covalent bonds are essentially what takes place when the valence electrons (the outermost electrons in an atom) are shared between atoms forming a bond. These occur between non-metals. They can form single, double or triple bonds depending on the amount of pairs of electrons that are shared. In more detail, these bonds consist of sigma and pi bonds, but I won’t go into those today.
These covalent bonds lead to stable molecules if they share electrons in such a way to create a noble gas configuration (where the electrons amounts are the same as those for the noble gas elements) for each atom. Essentially, when the ns2 and np6 orbitals are filled, this leads to stable orientation for the molecules. This is what is known as the Octet Rule, as it involves filling 8 electrons up in the mentioned ns2 and np6 orbitals.
There are different types of Covalent Bonds too.
Nonpolar covalent bonds occur between identical, non-metal atoms. These are the simplest types of covalent bonds. Because the atoms are the same, the electrons are shared equally between the atoms.
Examples of Nonpolar covalent bonds are: Oxygen (O2), Iodine (I2) and Fluorine (F2).
Polar Covalent Bonds are covalent bonds in which the shared electrons are not shared equally. Different atoms pull differently on the shared electrons, which will lead to an increased electron density to one side of the molecule rather than the other. The atom that pulls more strongly on the electrons becomes slightly more negative, and the other slightly more positive. This positive and negative end of a molecule is known as a dipole (like a magnet) How much this occurs is related in the numerical value of Electronegativity. Electronegativity in simpler terms is an atoms affinity for attracting electrons to itself.
Examples of Polar Covalent bonds are: Water (H2O), Ammonia (NH3) and Hydrogen Fluoride (HF).
Whether a molecule is polar or nonpolar covalent can be determined by the electronegativity difference of the atoms that compose it. From 0.0-0.2 is nonpolar covalent. From 0.3-1.4 is Polar Covalent. From >1.5 is Ionic (but I won’t talk about that now). Covalent bonds are stronger than Hydrogen bonds, but are in general weaker than Ionic bonds.
If I’ve made any errors please let me know and I’ll correct them ASAP!
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