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\section*{EL3} \section{EL3}  \subsection{Shells of Electrons}  It is more appropriate to talk about shells than energy levels, due to complexity of atoms beyond hydrogen.\\  Each shell has a maximum number of electrons that it can hold. For a higher value of $n$, the shell is further from the nucleus and has greater energy.  \begin{table}[]  \centering  \begin{tabular}{lll}  First Shell & $n=1$ & 2 electrons \\  Second/Third Shells & $n=2$/$n=3$ & 8 electrons \\  Fourth/Fifth Shells & $n=4$/$n=5$ & 18 electrons \\  Sixth/Seventh Shells & $n=6$/$n=7$ & 32 electrons \\  \end{tabular}  \end{table}  The lowest energy shells are filled first. Much of chemistry is decided by the outer shell electrons.  \subsection{Sub-Shells of electrons}  Sub-Shells are labelled \textit{s, p, d, f}. These correspond to the shells:  \begin{itemize}  \item $n=1$ has an s sub-shell  \item $n=2$ has s and p  \item $n=3$ has s, p, and d  \item $n=4$ has s, p, d, and f  \end{itemize}  \begin{table}[]  \centering  \begin{tabular}{ll}  Sub-Shell & Maximum number of electrons \\  s & 2 \\  p & 6 \\  d & 10 \\  f & 14 \\  \end{tabular}  \end{table}  In atoms other than hydrogen, sub-shells within a shell have different energies. The shells of 3d and 4s have an overlap in energies.    \subsection{Atomic Orbitals}  \begin{itemize}  \item S sub-shells have one s-orbital  \item P sub-shells have three p-orbitals  \item D sub-shells have five d-orbitals  \item F sub-shells have seven f-orbitals  \end{itemize}  In an isolated atom, orbitals within the same sub-shell have the same energy.  \begin{itemize}  \item Each orbital can hold a max of 2 electrons  \item Must have opposite spin  \item Corresponds to clockwise or anti-clockwise  \end{itemize}  The position of an electron is mapped with a probablilty function as it cannot be pinpointed exactly.  \subsection{Filling Atomic Orbitals}  The orbitals are filled to give the lowest energy arrangement possible. To do so, they are filled in order of increasing energy.  \begin{itemize}  \item Orbitals will take one electron until all are full  \item 4-s fills before 3-d as it is lower energy  \item Not required to write electron configs for Copper or Chromium.  \item Eg. Scandium: 1s\textsuperscript{2}2s\textsuperscript{2}2p\textsuperscript{6}3s\textsuperscript{2}3p\textsuperscript{6}3d\textsuperscript{1}4s\textsuperscript{2}  \end{itemize}