Chlorine has the same electron configuration as fluorine, just one shell larger. This allows chlorine a broader palette of chemical reactions, given the presence of a d-orbital. Like sulfur, this gives rise to more possible molecular geometries. Chlorine can sustain multiply bonded atoms, but it takes bonding atoms with stronger electronegativity than chlorine in order to draw it into multiple bonds. This limits to list of contenders essentially to oxygen and fluorine, as we see in the chlorate (ClO3–) ion. (The wireframe indicates the boundary of the n=3 shell.)
The outer spheres above simply indicate the directions of maximum electron density. The orbitals themselves will be more like spherical tetrahedra that can only occupy volume within their shell. The entire shell will be filled with electron density. It will be highest at the center of the face of each orbital and will decrease toward the nodal regions that divide the shell into four equal volumes, where electron density will be lowest. Like argon, chlorine features two nested spherical tetrahedra, the inner 2nd shell comprising 4 di-electrons, the outer 3rd shell comprising 3 di-electrons and 1 single electron. In this case the three orbitals containing the di-electrons (dark pink) will occupy slightly more volume than the one containing an unpaired electron.
Chlorine is keen to obtain an extra electron to fill its third shell and it can bond with many atoms on the periodic table. Chlorine can make one or more covalent bonds or gain an electron in ionic interaction in order to reach the stability of the 3s23p6 noble gas configuration of argon, which is a multi-di-electron state with three concentric full shells. That is why chlorine forms a 1– ionic state. The negative ion is larger than its neutral version because electrons now outnumber protons. This results in a lower effective nuclear charge — a lower average attraction by the nucleus on each electron.
When sodium and chlorine interact, chlorine takes the electron from sodium that sodium is keen to lose, forming both atoms into their ions and allowing both to achieve full shell configurations. The ions can then stick to each other because of their opposite charges, forming sodium chloride (NaCl) crystals. This process is called ionic bonding.
Sodium chloride (NaCl) dissolves in water because the polar H2O molecules and the ions in the crystal attract each other. The water molecules can therefore tug ions off the crystal and still satisfy the ion’s desire to attract their opposite polarity.
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