Valence electrons determine how atoms interact. An atom is stable with a closed shell of valence electrons, while an open shell makes it reactive. Noble gases are inert due to their closed shells. Ionic bonds form when one atom takes an electron from another, while covalent bonds share electrons.
Valence electrons are those electrons in the outer shell of a given atom, the number of which determines how the atoms interact with each other. An atom is said to have a closed shell when it has enough valence electrons to make it stable; when there aren’t enough of them, it is said to have an open shell. An atom with an open shell is constantly trying to achieve stability, forming one of the basis of many chemical reactions.
An atom is reactive or inert depending on how many valence electrons it has. The most reactive atoms are those that have one or two to lose, or those that have one or two to gain in order to maintain stability. For this reason the noble gases, which in nature all have a closed outer shell, are chemically inert. In general, eight electrons are required for an atom to achieve stability. Two notable exceptions are hydrogen and helium, both of which need two to create a closed shell.
The affinity for atoms to achieve stability by gaining or losing valence electrons provides a basis for two types of chemical bonds: the ionic bond and the covalent bond. Ionic bonds form when one atom “steals” an electron from another. Table salt (NaCl) is one example. Sodium (Na) has one electron to give up. Chlorine (Cl), on the other hand, needs one to be complete.
To achieve stability, chlorine will take an electron from sodium. This allows both elements to achieve a closed shell and stability. The result is that the sodium atom becomes a positive ion and the chlorine atom becomes a negative ion. Opposite charges will attract each other. When in a solution, these molecules also conduct electricity, as the ions are free to move around in the solution.
Water is an example of atoms forming a covalent bond. Hydrogen has one atom to gain or lose, and oxygen needs two to achieve stability. In this application, however, the oxygen does not steal electrons from the two hydrogen atoms. Rather, the oxygen and two hydrogen atoms share electrons, forming a water molecule. Atoms can also use covalent bonds to share electrons with atoms of the same element, such as in a hydrogen (H2) molecule.
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