Thus, the two electrons in the carbon 2 p orbitals have identical n, l, and m s quantum numbers and differ in their m l quantum number (in accord with the Pauli exclusion principle). The orbitals are filled as described by Hund’s rule: the lowest-energy configuration for an atom with electrons within a set of degenerate orbitals is that having the maximum number of unpaired electrons. We now have a choice of filling one of the 2 p orbitals and pairing the electrons or of leaving the electrons unpaired in two different, but degenerate, p orbitals. The remaining two electrons occupy the 2 p subshell. Four of them fill the 1 s and 2 s orbitals. As we know, the positively-charged protons in the nucleus of an atom tend to attract negatively-charged electrons. When drawing orbital diagrams, we include empty boxes to depict any empty orbitals in the same subshell that we are filling.Ĭarbon (atomic number 6) has six electrons. Electron configurations are a simple way of writing down the locations of all of the electrons in an atom. There are three degenerate 2 p orbitals ( m l = −1, 0, +1) and the electron can occupy any one of these p orbitals. Because any s subshell can contain only two electrons, the fifth electron must occupy the next energy level, which will be a 2 p orbital. The n = 1 shell is filled with two electrons and three electrons will occupy the n = 2 shell. The fourth electron fills the remaining space in the 2 s orbital.Īn atom of boron (atomic number 5) contains five electrons. Thus, the electron configuration and orbital diagram of lithium are:Īn atom of the alkaline earth metal beryllium, with an atomic number of 4, contains four protons in the nucleus and four electrons surrounding the nucleus.
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