Download ELECTRONIC STRUCTURE OF ATOMS

ELECTRONIC STRUCTURE OF ATOMS
 Electronic structure refers to the arrangement of electrons.
Understanding of the structure has come from the study of light.
 Light is a form of electromagnetic radiation. EMR carries energy
through space and can be categorized by frequency and
wavelength.
 All types of EMR move through a vacuum at the speed of light,
3x108 m/s
 For EMR, the product of the frequency() and the wavelength()
equals the speed of light: c =  x 
 Wavelength is the distance from node to node. Frequency is the
number of wavelengths that pass a given point in a period of time.
 When solids are heated , they emit radiation i.e. the red glow of a
stove burner or the light from a light bulb.
 Max Planck (1900) hypothesized that energy can be released or
absorbed by atoms in certain amounts. He called these amounts
quanta. A quantum is the smallest amount of energy that can be
absorbed or released as EMR
 The relationship is: E = h where h is 6.626 x 10-34 J*s (Plancks
constant)
 Using Plancks hypothesis Einstein explained the photoelectric
effect. Experiments had shown that light shining on a metal
surface caused it to emit electrons. For each metal there is a
minimum frequency below which no electrons are emitted.
 Einstein assumed light travels in packets(photons) that behaved
like particles. Einstein deduced that each photon had energy
proportional to the frequency of the light. Thus radiant energy
was quantized. This explains the difference in effect between xrays(high frequency, high energy) vs radio waves ( low frequency,
low energy)
 Radiation composed of one wave length is called monochromatic.
 When white light is separated, it forms a continuous spectrum
 Not all radiation is continuous. A spectrum where we see only
specific wavelengths is called a line spectrum.
 Bohr used this observation to hypothesize that electrons were
confined to specific energy states , orbits.
 The ground state is the lowest energy state . An electron in a
higher state is said to be in an excited state.
 Colors from excited gases(street lights) arise because electrons
move between energy states. Since energy states are quantized,
the light emitted must be quantized and appear as a line
spectrum.
 Bohrs model was important since it quantized energy states for
electrons. However, it only worked for atoms and ions with one
electron.
 Since we know light has a particle nature, does matter have a
wave nature?
 Louis de Broglie used Planck’s equation to derive:  =h / mv.
 The momentum, mv, is a particle property while  is a wave
property. In one equation de Broglie summarized waves and
particles as they apply to low mass(electron), high speed objects.
 The Heisenberg uncertainty principle states that it is impossible to
determine the position and momentum of an electron at a
particular instant. That means that electron orbitals do not
represent specific orbits(i.e. planets) but instead a probability of
finding an electron in a region of space.
 Schrodinger described this as a probability density.
 The position of an electron is described by its quantum number.
Each electron has 4 quantum numbers which apply to its shell,
subshell, orbital and spin.
 Shell(n) describes the average distance from the nucleus. Higher
values are farther away and will have more energy and be less
stable.
 Subshell(l) describes the shape. The first shell(n=1) has one
subshell,s or l=0.
 Orbitals(ml) describes the orientation of the orbital in space. It
describes the path in a 3 dimensional grid. The s subshell has one
orbital ml=0; the p subshell has three orbitals ml= -1, 0 +1
 Spin(ms) can be either +1/2 or –1/2
 The Aufbau principle states that electrons are placed in orbitals,
subshells, and shells in order of increasing energy.
 The Pauli exclusion principle state that within an atom no 2
electrons can have the same set of quantum numbers.
 Effective nuclear charge is the net positive charge experienced by
an electron in a multiple electron atom. The farther away from
the nucleus the greater the screening effect.
 Valence electrons are in the outer most shell. They are gained or
lost in reactions.