Unlike planets orbiting the Sun, electrons cannot be at any kind of arbitrary distance from the nucleus; they have the right to exist just in certain certain locations called permitted orbits. This property, an initial explained by Danish physicist Niels Bohr in 1913, is another result of quantum mechanics—specifically, the requirement that the angular momentum of an electron in orbit, like every little thing else in the quantum world, come in discrete bundles referred to as quanta.

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In the quantum mechanical version of the Bohr atomic model, every of the permitted electron orbits is assigned a quantum number n the runs from 1 (for the orbit closest to the nucleus) come infinity (for orbits really far from the nucleus). Every one of the orbitals that have the very same value that n consist of a shell. Within each shell there might be subshells equivalent to various rates the rotation and orientation of orbitals and also the spin direction of the electrons. In general, the farther away from the cell core a covering is, the much more subshells it will certainly have. See the table.


This setup of possible orbitals explains a great deal about the chemistry properties of various atoms. The easiest way to view this is come imagine structure up complicated atoms by starting with hydrogen and adding one proton and also one electron (along through the appropriate variety of neutrons) in ~ a time. In hydrogen the lowest-energy orbit—called the soil state—corresponds to the electron located in the covering closest to the nucleus. There are two feasible states because that an electron in this shell, matching to a clockwise spin and also a counterclockwise rotate (or, in the slang of physicists, rotate up and spin down).

The next most-complex atom is helium, which has two protons in the nucleus and also two orbiting electrons. These electrons to fill the two available states in the shortest shell, creating what is referred to as a fill shell. The next atom is lithium, with 3 electrons. Due to the fact that the closest shell is filled, the third electron goes into the next greater shell. This shell has actually spaces because that eight electrons, so that it takes an atom with 10 electron (neon) to to fill the first two levels. The next atom after neon, sodium, has actually 11 electrons, so that one electron goes into the next highest possible shell.

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In the progression hence far, three atoms—hydrogen, lithium, and also sodium—have one electron in the outermost shell. As stated above, it is this outermost electron that determine the chemistry properties of an atom. Therefore, this three elements should have similar properties, as undoubtedly they do. For this reason, they show up in the same tower of the regular table that the elements (see routine law), and the same principle determines the position of every element in that table. The outermost covering of electrons—called the valence shell—determines the chemical behaviour of one atom, and also the number of electrons in this shell counts on how countless are left over after every the inner shells are filled.