Electron Energy Levels
• Not all electrons in an atom have the same energy
• They exist in discreet energy levels
• These levels are arranged in shells (n = 1, n = 2…)
• Electrons closer to the nucleus have lower energy
(stabilized by positively charged nucleus)
• Maximum number of electrons in a shell = 2n2,
where n = the shell #
• Electrons fill shells from lowest E up
Electron Shells
Electromagnetic Radiation
• Electromagnetic radiation consists of photons
(particles) that travel as waves
• Examples: light, x-rays and radio waves
• Distance between peaks is called the wavelength
• Longer wavelength = lower energy
• Electrons can jump to a higher energy level by
absorbing energy (light, heat…)
• When they drop back down to their original E,
sometimes visible light is emitted (neon lights)
Electromagnetic Spectrum
Subshells
• Electrons within each shell are not all the same E
• They are arranged into subshells (s, p, d, f)
• The E order of the subshells goes s < p < d < f
• Number of subshells = shell number
• n = 1 has only s, n = 2 has s and p, etc.
• As n increases, main E levels are closer, so there is
some overlap (4s subshell is lower E than 3d)
Orbitals
• Each subshell is made up of orbitals
• An orbital is a region of space in which electrons
of that E level are most likely to be found
• The s subshell has only one orbital (spherical)
• The p subshell has three orbitals (dumbell
shaped)
• Each orbital can contain 0, 1 or 2 electrons
• So s has a max of 2 electrons and p has a max of
6 electrons (2 for each orbital)
Electron Configurations
• The electron configuration shows how the
electrons are arranged in the subshells of an atom
• Written as the shell number and subshell symbol,
with number of electrons in the subshell as
superscript
• Examples: Li = 1s22s1 S = 1s22s22p63s23p4
• Use periodic table to get electron configuration
• (check that superscripts add up total number of
electrons = atomic number)