CHEMISTRY ELECTRON ARRANGEMENT

Chapter 4

Light and modern atomic theory

Why aren’t e- drawn into nucleus?

Why do atoms of some elements behave the way they do?

Niels Bohr

studied under Rutherford

Worked w/ him on gold foil experiment

Bohr refined Rutherford's idea by

adding that the e- were in orbits

like planets orbiting the sun

each orbit only holding a set number of

e-

Bohr’s Atom

electrons in orbits

nucleus

to aid in finding out answers, scientists analyzed light samples from elements heated in flames

began to see pattern where chemical behavior of elements related to arrangement of e- in atoms

study of light necessary

Nature of Light

EMG- electromagnetic radiation visible light, x rays, infrared, UV waves

described by1. wavelength ( )- distance between 2

consecutive crests2. frequency ( )- # of waves that pass a

certain pt per secondHertz (Hz)- SI unit 1 wave/second

3. amplitude- wave’s height form origin to crest

all EMG radiation travels at the speed of light (c) 3.00 x 108 m/s

speed = wavelength x frequency

higher the frequency, the higher the energy

light can be explained as moving thru space in form of waves, but not in its interactions w/ matter

early 1900s, scientists conducted 2 exp that didn’t line up w/ light being a wave

photoelectric effect- emission of e- from metal when light of certain frequency shines on metal

prob: light as a wave should knock off e- regardless of light’s frequency, but not the case

Max Planck- studied hot obj & their emission of light

did not emit light continuously if light was only in form of waves

Planck suggested obj emits energy in small packets called quanta

proposed quantum theory quantum- minimum amount of energy that

can be lost or gained by an atom 1905, Albert Einstein proposed EMG radiation

has dual wave-particle nature sometimes acts like wave, sometimes acts like

particles Einstein suggested light is made up of streams

of particles carrying a quantum of energy particles called photons

photon- particle of EMG radiation w/ no mass carries quantum of energy

Einstein’s explanation of photoelectric effect

2nd exp involved hydrogen when gases have elec current pass thru them,

some of their atoms will increase in energy they will go from their ground state (lowest

energy state) to the excited state ( state of higher PE)

when excited atom returns back to its ground state, it emits colored light energy (neon lights)

worked w/ hydrogen- emitted pink light passed light thru prism, it separated into 4

specific colors of visible spectrum these 4 bands of light were hydrogen’s

line-emission spectrum spectrum- pattern of radiant energy

(fingerprint) scientists had expected to see a continuous

spectrum if light was in form of waves, but didn’t

looking for explanation of the specific energy states of H e-

enter Bohr’s theory of e- circling the nucleus in only allowed paths or orbits (planetary model)

e- absorb energy, they move into larger orbit (excited state)

when they emit energy, e- return to original orbit (ground state)

Bohr assigned value to each orbit & calculated radius

mathematically speaking, Bohr’s calculated values for the orbits matched & explained the observed spectral lines of H

thought all atoms would follow same pattern, but not the case

remember that light has ability to act like waves & particles

using Einstein’s formula & Planck’s quantum theory, Louis de Broglie proposed that it was possible for e- to have the same properties

lead to Wave-particle model of e- confirmed by exp

in 1924, Edwin Schrodinger devised an equation that treated e- as moving about the nucleus as waves

equation laid foundation for quantum theory

QT- describe mathematically the wave properties of e- & other small particles

e- , like light waves, can be bent or diffracted & they can interfere w/ each other

So where are e- in atoms?

theory was only accepted after Werner

Heisenberg proposed his uncertainty principle- it is impossible to know both the exact position and the velocity of an object at the same time

e- detected by photons, any attempt to locate e- knocks it off its course

quantum numbers- used to describe e- behavior

e- move about the nucleus at extremely high speeds filling the entire area in e- cloud

quantum numbers used to describe e configuration

atomic orbital- a region of space in which the probability of finding an e- is high

4 quantum numbers:

1. principle quantum number, n

describes the energy level an e- occupies

can only be whole numbers as n increases, the distance of main

energy levels from the nucleus increases & energy increases

known elements utilize main energy levels 1-7

2. orbital quantum number, l

indicates the shape of the region in e- cloud the e- occupies

regions referred to as sublevels or a specific kind of atomic orbital

the # of possible orbital shapes is equal to the value of n

a letter has been designated to represent each different kind of sublevel

s = sphere shaped p = dumbell/ p-nut shaped d = double p-nut f = flower shaped

1st energy level has 1 sublevel (s) 2nd energy level has 2 sublevels (s,

p) 3rd e.l. has 3 sublevels (s,p,d) 4th e. l. has 4 sublevels (s, p, d, f)

3. magnetic quantum number indicates the orientation of an orbital about

the nucleus indicated by n2

s sublevel has only 1 possible orientation, therefore only 1 s orbital in each sublevel

p sublevel has 3 different orientations, lobes extend along x, y, z axes

designated as px py pz these have equal energy

d sublevel has 5 different d orbitals

d1, d2, d3, d4, d5

f sublevel has 7 different f orbitals

f1 –f7

each orbital can hold a max of 2 e-

with increasing main energy levels, there are larger # of orbitals

4. spin quantum number

spin of e- and the orientation of the magnetic field produced by the motion of the e-

indicates also the maximum number of e- the energy level can hold

represented by 2(n2)

only 2 possible values -1/2 or +1/2

Electron Configuration

the arrangement of e- in atoms e- tend to arrange themselves in

ways that give them the lowest possible energy

3 rules that govern e- conf:

1. Aufbau principle- an e- occupies the lowest energy orbital that can receive it

aids in applying Aufbau principle

2. Hund’s Rule- orbitals of equal energy are occupied by 1e- before any orbitals are occupied by 2e-

sharing room w/ sibling

(e- to e- repulsion is minimized therefore lowering energy they have)

3. Pauli exclusion principle- no 2 e- in the same atom can have the same 4 quantum #s

the 2 values of the spin quantum # permits 2 e- of opposite spins to occupy the same orbital

3 notations used to indicate e- conf: 1st 2 indicates ground state e- conf

1. Orbital Line Notation

unoccupied orbital is represented by a line

____ arrows are used to denote the e- in

the orbital label lines w/principle quantum #s

& sublevels write name or symbol of element

before lines

2. E- configuration notation

eliminates arrows & lines # of e- in sublevel is shown

by adding superscripts to the sublevel designation

can abbreviate by using the noble gas e- configuration that comes before it

3. E- dot notation

**most imp for representing how atoms form compounds***

shows the e- in the outermost main e- level known as valence e-

use dots arranged around the element’s symbol to represent these valence e-

The end.