What type of electrons are found in the valence shell

The combining power or affinity of an atom of an element was determined by the number of hydrogen atoms that it combined with. In methane, carbon has a valence of 4; in ammonia, nitrogen has a valence of 3; in water, oxygen has a valence of two; and in hydrogen chloride, chlorine has a valence of 1.

Chlorine, as it has a valence of one, can be substituted for hydrogen, so phosphorus has a valence of 5 in phosphorus pentachloride, PCl 5.

Valence diagrams of a compound represent the connectivity of the elements, lines between two elements, sometimes called bonds, represented a saturated valency for each element. Valence only describes connectivity, it does not describe the geometry of molecular compounds, or what are now known to be ionic compounds or giant covalent structures.

The line between atoms does not represent a pair of electrons as it does in Lewis diagrams. In this table, you can see that helium has a full valence shell, with two electrons in its first and only, 1n, shell. Similarly, neon has a complete outer 2n shell containing eight electrons.

These electron configurations make helium and neon very stable. Although argon does not technically have a full outer shell, since the 3n shell can hold up to eighteen electrons, it is stable like neon and helium because it has eight electrons in the 3n shell and thus satisfies the octet rule. In contrast, chlorine has only seven electrons in its outermost shell, while sodium has just one. These patterns do not fill the outermost shell or satisfy the octet rule, making chlorine and sodium reactive, eager to gain or lose electrons to reach a more stable configuration.

Bohr diagrams indicate how many electrons fill each principal shell. Group 18 elements helium, neon, and argon have a full outer, or valence, shell. A full valence shell is the most stable electron configuration. Elements in other groups have partially filled valence shells and gain or lose electrons to achieve a stable electron configuration.

Elements are placed in order on the periodic table based on their atomic number, how many protons they have. In a neutral atom, the number of electrons will equal the number of protons, so we can easily determine electron number from atomic number.

Because most filled electron shells have eight electrons in them, chemists called this tendency the octet rule. However, there is another way an atom can achieve a full valence shell: atoms can share electrons. This type of bonding would be a covalent bond. However, there are a few stable molecules which contain an odd number of electrons.

These molecules, called "free radicals", contain at least one unpaired electron, a clear violation of the octet rule. Free radicals play many important roles a wide range of applied chemistry fields, including biology, medicine, and environmental chemistry. For example, combustion of any material will produce OH not hydroxide free radicals. Once inhaled, OH can enter the body and affect cells.

Free radical exposure has been linked to cancer, aging, and a number of autoimmune diseases. The second kind of bond is an ionic bond. Ionic bonds tend to be stronger than covalent bonds. Sodium fluoride NaF is a common example of a compound formed by an ionic bond. When you brush your teeth in the morning, give thanks to sodium fluoride—and its shared valence electrons —in your toothpaste for fighting cavities. Here is a video explaining and visualizing how valence electrons work.

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