Electron configuration

• electron shell

• subshell

  • Electrons with the same $n$ and $\ell$ are said to be in the same subshell
  • Orbitals with the same $n$ and $\ell$ are said to be in the same subshell (or sublevel)
  • For every subshell (given by $n$ and $\ell$):
    • The number of orbitals is $2\ell +1$.
    • The maximum number of electrons is $2(2\ell +1)$.

• energy level

• atomic orbital

  • every atomic orbital can hold a maximum of two electrons
  • electrons with the same $n$, $\ell$, and $m_l$ are in the same orbital. (they have the same energy level, shape, and orientation)

• quantum numbers

  • principal quantum number ($n$)
    • $n=1,2,3,4,\dots$
    • determines the electron shell
    • electrons with the same $n$ are in the same shell
    • orbitals with the same $n$ are in the same shell
    • The energy level of an electron is determined by the principal quantum number
    • The number of orbitals in a shell is $n^2$.
    • The maximum number of electrons in a shell is $2n^2$.
  • azimuthal quantum number ($\ell$) (or orbital angular momentum quantum number)
    • determines the shape of the orbital
    • $\ell =0,1,2,3,\dots ,n-1$
  • magnetic quantum number ($m_l$)
    • Determines the orientation of the orbital
    • $m_l=-\ell ,\dots ,0,\dots ,\ell$
  • spin quantum number ($m_s$)
    • determines the orientation of the electron’s spin

• valence electrons (אלקטרוני ערכיות)

  • electrons in the valence shell (outermost shell) of the atom.
  • number of valence electrons
  • valence (or valency) of an atom

• The core electrons are the electrons in the inner shells of the atom.

• energy level

• Aufbau principle (or Aufbau rule)

  • Electrons fill the lowest energy

• Pauli exclusion principle

  • No two electrons in an atom can have the same set of four quantum numbers.

• Hund’s rules

  • Electrons fill orbitals singly before pairing up.

• octet rule

  • ”atoms usually react in such a way as to obtain a noble gas configuration” (Denniston, 2022)
  • “elements in groups 1A to 7A (1, 2, 13 to 17) react with other elements by forming ionic or covalent bonds to produce a stable electron arrangement, usually eight electrons in the outer shell.” (Timberlake, 2017)
  • “In covalent bond formation, atoms go as far as possible toward completing their octets by sharing electron pairs.” (Jones, 2016)

• lone pair (or unshared pair, non-bonding pair, or electron pair)

  • “Two paired electrons localized in the valence shell on a single atom. Lone pairs should be designated with two dots. The term ‘nonbonding electron pair’ is more appropriate, and is found in many modern text books.” (IUPAC Gold Book)

• electron density (or electronic density)

• isoelectronicity

$\ell$	subshell letter	electrons $2(2\ell +1)$	shells containing it	orbitals
$0$	$s$	$2$	Every shell	$s$
$1$	$p$	$6$	2nd shell onwards	$p_x$, $p_y$, $p_z$
$2$	$d$	$10$	3rd shell onwards	$d_{xy}$, $d_{yz}$, $d_{xz}$, $d_{x^2-y^2}$, $d_{z^2}$
$3$	$f$	$14$	4th shell onwards	…
$4$	$g$	$18$	5th shell onwards (theoretically)	…

shell name	$n$	$\ell =0$	$1$	$2$	$3$	shell max electrons $2n^2$
$K$	$1$	$1s$				$2$
$L$	$2$	$2s$	$2p$			$8=2+6$
$M$	$3$	$3s$	$3p$	$3d$		$18=2+6+10$
$N$	$4$	$4s$	$4p$	$4d$	$4f$	$32=2+6+10+14$
$O$	$5$	$5s$	$5p$	$5d$	$5f$	$50=2+6+10+14+18$

References

• Denniston (2022). General, Organic, and Biochemistry. McGraw-Hill Education.
• Timberlake, Karen C. (2017). Chemistry: An Introduction to General, Organic, and Biological Chemistry. Pearson.
• Jones, Loretta (2016). Chemical Principles. W. H. Freeman.