Because its 2n shell is filled, it is energetically stable as a single atom and will rarely form chemical bonds with other atoms. For instance, lithium ( Li \text Ne start text, N, e, end text ), on the other hand, has a total of ten electrons: two are in its innermost 1 s 1s 1 s 1, s orbital and eight fill the second shell-two each in the 2 s 2s 2 s 2, s and three p p p p orbitals, 1 s 2 1s^ 2 1 s 2 1, s, squared 2 s 2 2s^ 2 2 s 2 2, s, squared 2 p 6 2p^6 2 p 6 2, p, start superscript, 6, end superscript. The number of bonds that an atom can form can often be predicted from the number of electrons needed to reach an octet (eight valence electrons) this is especially true of the nonmetals of the second period of the periodic table (C, N, O, and F). Elements in the second row of the periodic table place their electrons in the 2n shell as well as the 1n shell. After the 1 s 1s 1 s 1, s orbital is filled, the second electron shell begins to fill, with electrons going first into the 2 s 2s 2 s 2, s orbital and then into the three p p p p orbitals. The second electron shell, 2n, contains another spherical s s s s orbital plus three dumbbell-shaped p p p p orbitals, each of which can hold two electrons. Hydrogen and helium are the only two elements that have electrons exclusively in the 1 s 1s 1 s 1, s orbital in their neutral, non-charged, state. On the periodic table, hydrogen and helium are the only two elements in the first row, or period, which reflects that they only have electrons in their first shell. This is written out as 1 s 2 1s^ 2 1 s 2 1, s, squared, referring to the two electrons of helium in the 1 s 1s 1 s 1, s orbital. Helium has two electrons, so it can completely fill the 1 s 1s 1 s 1, s orbital with its two electrons. This can be written out in a shorthand form called an electron configuration as 1 s 1 1s^ 1 1 s 1 1, s, start superscript, 1, end superscript, where the superscripted 1 refers to the one electron in the 1 s 1s 1 s 1, s orbital. Hydrogen has just one electron, so it has a single spot in the 1 s 1s 1 s 1, s orbital occupied. The 1 s 1s 1 s 1, s orbital is the closest orbital to the nucleus, and it fills with electrons first, before any other orbital. Total number of protons in the nucleus is called the atomic number of the atom and is given the symbol Z. The first electron shell, 1n, corresponds to a single 1 s 1s 1 s 1, s orbital. Fluorine is a chemical element with atomic number 9 which means there are 9 protons in its nucleus.
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