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.
In addition, the position of an element in the periodic table—its column, or group, and row, or period—provides useful information about how those electrons are arranged.
If we consider just the first three rows of the table, which include the major elements important to life, each row corresponds to the filling of a different electron shell: helium and hydrogen place their electrons in the 1n shell, while second-row elements like Li start filling the 2n shell, and third-row elements like Na continue with the 3n shell. In general, the number of valence electrons is the same within a column and increases from left to right within a row.
Group 1 elements have just one valence electron and group 18 elements have eight, except for helium, which has only two electrons total. Thus, group number is a good predictor of how reactive each element will be:. Improve this page Learn More.
For every proton in an atomic center, somewhere, in an orbital, there will be an electron. Atoms differ from one another in the number of protons, neutrons and electrons they contain. However their arrangement always follows the same set of principles. A Matter of Stability. Hydrogen and helium illustrate the fact that not all atoms have the same properties. Both are gasses, but hydrogen gas is violently explosive, whereas helium gas is completely inert and cannot be made to undergo any kind of chemical reactions under normal circumstances.
Atoms of hydrogen have a single proton in their center and a single electron in the lowest energy level. Helium atoms, on the other hand, have two protons and two electrons in the lowest energy level. The lowest energy level is filled with its maximum number of electrons. This is a very stable arrangement, and helium in consequence is an inert gas with few chemical properties.
Hydrogen only has one electron in its lowest energy level. This is a very unstable arrangement, and hydrogen gas undergoes a variety of reactions so as to reach a stable electron configuration where its energy level is either empty of electrons, or filled with electrons. Atoms are at their most stable when their outermost energy level is either empty of electrons or filled with electrons. Sodium atoms have 11 electrons. Two of these are in the lowest energy level, eight are in the second energy level and then one electron is in the third energy level.
If sodium metal and chlorine gas mix under the right conditions, they will form salt. The sodium loses an electron, and the chlorine gains that electron. In the process, a great amount of light and heat is released. The resulting salt is mostly unreactive — it is stable. It will not undergo any explosive reactions, unlike the sodium and chlorine that it is made of. Referring to the octet rule, atoms attempt to get a noble gas electron configuration, which is eight valence electrons.
Sodium has one valence electron, so giving it up would result in the same electron configuration as neon. Chlorine has seven valence electrons, so if it takes one it will have eight an octet. Chlorine has the electron configuration of argon when it gains an electron.
The octet rule could have been satisfied if chlorine gave up all seven of its valence electrons and sodium took them. In that case, both would have the electron configurations of noble gasses, with a full valence shell. However, their charges would be much higher.
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