Are hydrogen bonds a type of covalent bond?

Answer 1

Hi Sally,

I would like to give you my input on your question.

There are two types of bonds involving negatively charged electrons, there is either sharing of electrons which are the covalent bonds, or there is a transfer of electrons, these are known as ionic bonds. Atoms bond with one another because they do not have a full complement of electrons necessary to balance the positively charged protons of their own nucleus. Electrons are needed to complement the protons of an atom (a positively charged atom), or if there are too many electrons (a negatively charged atom), these are what we refer to as valence. To understand this idea we need to study the periodic table (please find one on line), and understand orbitals. Orbitals are the spaces that electrons exist in. The outer most orbital is known as the valence shell. It is this valence shell that contains the missing or additional electrons, compared with the number of protons in the nucleus. So as we start from one proton and go up, we will need as many electrons in the orbitals as protons in the nucleus to be stable. The first orbital will require two electrons for stability, the orbitals thereafter, generally, from the second one up require 8 electrons for stability. In nature you will note that stability comes in multiples of 2.

Let's focus on hydrogen (H). Hydrogen is found in the first column on the period chart, has an atomic number of 1 and an atomic weight of 1.008. The atomic number corresponds to the number of protons in the nucleus, the atomic weight corresponds to the weight of the protons, neutrons and electrons. In the case of hydrogen there is only 1 proton, no neutrons in the nucleus, and one electron. Because it is found in the first column, Hydrogen has a valence of +1. The elements in the second column such as calcium (Ca), the so called divalent cations, have a valence of +2. The elements in columns 3-12 have variable positive valence charges based on the environment. Skipping to carbon (C) in Column 14, has a valence of 4, but can either donate or accept electrons to become more stable. The elements in columns 15-17 decrease in negative charge from -3 to -1 respectively. Column 18 is our stable noble elements, which are freely stable.

Hydrogen has only 1 electron, so is missing an electron to have a stable valence shell containing two electrons. Thus in nature, hydrogen, and many other elements exist as a pair of molecules, by sharing the one valence electron between the two make a stable valence shell, with each nucleus of one proton being balanced by the electrons. Thus by sharing a pair of electrons between the two hydrogen nuclei, we have a stable molecule! Cool! So hydrogen exists as H₂, but it may also exist as a free element in nature. Elements may readily give up or take electrons, to be more stable in their naturally existing forms. Hydrogen often is dissolved in water exists as a +1 proton. Carbon has a valence of 4. Carbon can participate as either a donor or acceptor of electrons. Oxygen has a valence of -2. These elements give up or harness electrons because of the property of electronegativity and the easiest path to stability of the valence shell. Thus, ionized forms are one path to stability, combining with other elements is another.

What is this property called electronegativity?? This concept has to do with how strongly a nucleus attracts electrons. In very basic terms electronegativity is how well a nucleus 'shields' the positive charges to the electrons in the external orbitals. This shielding involves the neutrons, important because they allow these positively charged protons to occupy a nucleus in close proximity. As we know like charges repel one another, thus neutrons are sort of a 'glue' that allows the protons to associate closely with one another in the nucleus. It is partially the interactions of the neutrons and protons which manifest in the concept of electronegativity. So the more positive charge that escapes the influence of the neutron-proton interactions, the higher the electronegativity value becomes.

We find that the electronegativities of carbon and hydrogen are around 2, while the electronegativity of oxygen is around 3. So when atoms combine to make molecules, atomic constituents of these molecular regions will share the electrons of the covalent bonds equally, or unevenly, based on the electronegativity of the atoms of the shared bond. Thus a bonds forming between carbon and hydrogen, such as that in methane (CH₄) having equal electronegativity, will share the electrons equally. This symmetrical electronic sharing is termed non-polar. However in bonds forming between oxygen and hydrogen, two atoms of differing electronegativities, will not share the electrons equally. The electrons will favor the electron cloud of the oxygen more. Thus in a molecule such as water, H₂O, the electrons will favor the oxygen nucleus, a partial negative charge, and the hydrogen nucleus will have its orbital occupied by electrons for less time than the oxygen nucleus, a partial positive charge So for water we have a polar molecule do to these unequal electron charge distributions. Partial charges in biochemistry is called dipole moment. It is these polar molecules that participate in your hydrogen bonding.

Hydrogen bonding in an environment of molecules, such as that in a solution, where partial positive and negative charges will associate complimentary with one another, due to electrostatic attraction. In water for instance the partially positively charged hydrogen electron clouds are attracted to the partially negatively charged oxygen electron cloud of adjacent water molecules. Thus, the surface tension of water is a force contained by hydrogen bonding. Proteins and other polar molecules also demonstrate the association of these complementary charges, sometimes within the polymer or between other polar molecules, this is extremely important for folding of proteins, or for the association of the two antiparallel strands of DNA, where the hydrogen bonds of the nitrogenous bases, hold the strands together in that perfect double helix. In aqueous environments, these preceding examples are also called hydrophilic (water loving) interactions. Conversely, and equally important are hydrophobic interactions, where non-polar substances (such as fats) will 'hide' away from polar environments. So these non-charged species will associate with one another and avoid the charged aqueous environments. Such as an oil droplet on the surface of water, or the hydrophobic (non-polar) tails of phospholipid bilayer forming the inner non-polar sea of the nuclear membrane. Covalent bonds, hydrogen bonds, and hydrophobic-hydrophilic interactions are fascinating and essential to life.

Answer 2

No. Hydrogen bonds are a type of intermolecular forces that hold certain molecules together. The covalent bond is an intramolecular force that holds atoms together, but doesn't hold molecules together.