Lone pairs are unpaired electrons in the valence shell of an atom that affect the shape of molecules. They are studied in chemistry because they explain why molecules with the same number of bonds have different shapes. The valence shell electron pair repulsion theory (VSEPR) explains how electrons repel each other, affecting the shape of molecules. Tin chloride has a bent shape due to a lone pair of electrons, resulting in a smaller angle between the atoms.
A lone pair refers to a pair of electrons in the valence shell of an atom that are not bonded to another atom or molecule. Because only those electrons in the valence, or outer, shell of an atom participate in bonding, lone pairs are studied in chemistry to explain the different shapes of molecules with the same number of bonds. Because electrons repel each other, molecules that have central atoms with a lone pair will have a different shape than those that don’t.
Electrons orbit the nucleus of an atom in various shells. Each shell can contain a certain number of atoms, and the electrons always orbit in pairs, spinning in opposite directions. The electrons in the atom’s outer shell, called the valence shell, can be shared with other atoms to form bonds and make molecules. In some molecules, all of the valence shell electrons in the central atom are bonded to another atom, but in others, only some are bonded. A pair of electrons in an atom that are not bonded to another atom is called a lone pair.
In chemistry, loan pairs are studied because they affect how certain molecules are shaped, which in turn can affect the behavior of molecules. Explained by the valence shell electron pair repulsion theory (VSEPR), electrons naturally repel each other, which explains the shapes of various molecules. For example, beryllium chloride (BrCl2) consists of a beryllium atom bonded to two chlorine atoms. Each chlorine atom is attached to the beryllium atom by a pair of beryllium electrons through a covalent bond. Since no unbound electrons remain in the valence shell, the furthest the electrons holding the chlorine atoms move away from each other is 180°, creating a linear molecule.
Tin chloride (SnCl2), however, has a borrowed pair of electrons. Just like beryllium chloride, tin chloride has two chlorine atoms bonded to one tin atom by electron pairs. Unlike beryllium chloride, tin chloride also has an additional pair of unbonded electrons, a lone pair, in the valence shell. This results in the tin chloride molecule having a bent shape as all three pairs of electrons try to move a maximum distance away from each other.
The resulting angle between the chloride atoms should therefore be 120°. However, scientific research has found that it is actually 95°. This discrepancy is a result of the loan pair orbital. The orbital of a loan pair is larger than the orbital of a bonded pair, which leads to a smaller angle between the atoms.
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