Two atoms share an electron as they attract it with comparable electric (electrostatic) force – Electron attributed to both – Shared force/electrostatic interaction on electrons is termed a covalent bond
Hydrogen is the element with atomic number 1, which means it has one proton in its nucleus.
It has one electron in its outer shell. In order to form a stable outer shell that has two electrons, it needs another electron.
Fluorine is the element with the atomic number 9, which means it has nine protons in its nucleus.
It has two electrons in its first shell and seven in its outer shell. In order to form a stable outer shell that has eight electrons, it needs one electron.
A hydrogen atom will share its electron with a fluorine atom and in this way they both complete their outer shell. The electrons will be attracted by both nuclei via an electric force, an electrostatic force. How does that work out in space? Will the shared electrons be in the middle of the distance between the atomic centers? It depends on the strength of the electric force from each side.
The nucleus of fluorine with its 9 protons (positive charges) exerts a much more significant electric force on the electrons than the one-proton hydrogen nucleus. (We say that F is more electronegative than H).
As a result, the shared electrons will shift slightly towards the fluorine.
That means that in the HF molecule, the H side will have a proton on its own and a little further away there will be 9 protons and 10 electrons towards the F side. Therefore, the H side will be slightly positive and the F side with 10-9=1 electrons in surplus will have a negative charge.
Water or H20, constitutes a similar case.
Oxygen is the element with atomic number 8. It has 6 electrons in its outer cell and needs another two.
Two hydrogen atoms will share their electrons. The nucleus of oxygen (8 protons) exerts a much stronger electric force on the electrons than the one-proton hydrogen nucleus. As a result, the shared electrons will shift slightly towards the oxygen.
Two atoms exchange an electron– Atoms are no longer electrically neutral but they become ions – Electrostatic interaction between ions is termed an ionic bond
Sodium is the element with atomic number 11, which means it has eleven protons in its nucleus.
It needs to discard one electron to form a stable outer shell of 8 electrons.
As mentioned, fluorine needs for the same reason one electron.
Sodium gives one electron and becomes a positively charged ion while fluorine gets one and becomes negatively charged. Opposites attract and we say that an ionic bond is formed between the two. The sodium chloride crystal is said to be ionically bonded.
Electric/Electrostatic force between permanent dipoles (Keesom force)
We mentioned that the molecules of water and HF are dipoles. The (+) pole of one molecule attracts the (–) pole of the other. Generally, we expect that the hydrogen atom in all its interactions with strong electronegative atoms e.g. N, O, F will have its electron shifted away and will thereby generate a dipole.
Electric/Electrostatic force between permanent dipole and corresponding induced dipole (Debye force)
One example of an induction-interaction between permanent dipole and induced dipole is the interaction between HCl and Ar. In this system, Ar experiences a dipole as its electrons are attracted (to the H side of HCl) or repelled (from the Cl side) by HCl.
Electric/Electrostatic force between instantaneously induced dipoles generated by electron cloud fluctuations (London dispersion force)
"The third and dominant contribution is the dispersion or London force (fluctuating dipole-induced dipole), which arises due to the non-zero instantaneous dipole moments of all atoms and molecules. Such polarization can be induced either by a polar molecule or by the repulsion of negatively charged electron clouds in non-polar molecules. Thus, London interactions are caused by random fluctuations of electron density in an electron cloud. An atom with a large number of electrons will have a greater associated London force than an atom with fewer electrons. The dispersion (London) force is the most important component because all materials are polarizable, whereas Keesom and Debye forces require permanent dipoles. The London interaction is universal and is present in atom-atom interactions as well. For various reasons, London interactions (dispersion) have been considered relevant for interactions between macroscopic bodies in condensed systems.Hamaker developed the theory of van der Waals between macroscopic bodies in 1937 and showed that the additivity of these interactions renders them considerably more long-range."