NOTES: 5.1 Revising the Atomic Model...Bohr’s Atomic Model Defects of Bohr’s Model: 1) Contradicted known physics. 2) Couldn’t explain various intensities of the line spectra.

NOTES: 5.1 –

Revising the Atomic Model

Atomic Models ~ REVIEW!

● ATOM: the smallest particle that has the

properties of an element.

● From the early Greek concept of the atom to the

modern atomic theory, scientists have built on

and modified existing models of the atom.

● Atoms are composed of a positively charged nucleus surrounded by an electron cloud.

-NUCLEUS: (99% of atom’s mass) uncharged neutrons and positively charged protons.

-ELECTRON CLOUD: negatively charged electrons in constant motion creating a “cloud” like a fan.

Electron arrangements…

● the chemical properties of atoms, ions,

and molecules are related to the

arrangement of the electrons within them

● models of the atom will be reviewed with

an emphasis on the electrons

JOHN DALTON:

Dalton’s Atomic Theory:

-All elements are made of tiny atoms.

-Atoms cannot be subdivided.

-Atoms of the same element are exactly alike.

-Atoms of different elements can join to form molecules.

In 1808, this English

schoolteacher proposed his own

atomic theory that became widely

accepted.

● As it turns out, the atom can be divided into

subatomic particles.

● Thomson and Millikan are given credit for the first

discoveries relating to electrons.

● Rutherford discovered the positively charged

nucleus

3 types of particles that make up atoms:

Particles: Mass (amu): Charge:

PROTON 1.00728 + (pos.)

NEUTRON 1.00867 0 (neutral)

ELECTRON 0.00055 - (neg.)

J.J. Thomson (1856-1940)

● proposed the “plum-pudding” model:

-neg. charged electrons are dispersed

throughout a lump of pos. charged

material (like raisins stuck in dough)

How is the atom structured?

● each atom has a tiny, positively charged, massive (very dense) center called an ATOMIC NUCLEUS.

● The area around the nucleus is mostly empty space.

● How do we know?

Rutherford’s “Scattering”

Experiment:

-positively charged alpha particles (helium nuclei) were shot through a thin gold foil.

-most alpha particles passed through the foil, or were deflected through moderate angles.

-a few were reflected at extreme angles, or even came shooting right back to the source!

Rutherford’s Scattering Experiment

Rutherford’s Scattering Experiment

Rutherford’s Scattering Experiment

NIELS BOHR

● In 1913, this Danish scientist suggested

that electrons “orbit” the nucleus.

● In Bohr’s model, electrons are placed in

different energy levels based on their

distance from the nucleus. (lowest

energy level is closest to nucleus)

Summary of atomic structure:

Bohr’s Atomic Model

● What prevents an electron from falling into the

nucleus?

● The electrons in a particular path have a fixed

energy level

● The energy levels are like rungs of a ladder:

-lowest rung of ladder = lowest energy level

-one can climb up or down a ladder by going from rung

to rung = an electron can “jump” from one energy level

to another (by gaining or losing energy)

● The Bohr Atom was a “solar system” or

“planetary” model.

Bohr’s Atomic Model

● In the process of changing energy levels the electron

gains or loses energy by absorbing or emitting a

quantum of electromagnetic energy.

● the higher an electron is on the energy “ladder”, the

farther it is from the nucleus

● in the atom, the steps become closer together as an

electron climbs higher

● similarly, the higher energy level occupied by an

electron, the easier the electron escapes from the

atom…WHY?

Only certain orbits are allowed:

An electron

jumping down

gives off energy

An electron jumping up

absorbs energy

Only certain orbits are allowed:

Orbits – Energy Levels

n = 1

n = 2

n = 3

n = 4

n = 5

Orbits – Energy Levels

n = 1

n = 2

n = 3

n = 4

n = 5

Bohr’s Atomic Model

Defects of Bohr’s Model:

1) Contradicted known physics.

2) Couldn’t explain various intensities of the line

spectra.

3) Couldn’t explain WHY only certain orbits were

allowed.

4) Only worked for hydrogen.

In review…

• History of the Atomic Model VIDEO

• Bohr Model of the Atom VIDEO

Physicists were mystified, but intrigued by

Bohr’s theory of the atom.

Why are the energies of the hydrogen

electron quantized?

Why is the electron restricted to

orbiting at certain fixed distances?

Erwin Schrodinger (1887-1961)

tiV

xm

2

22

2

In 1926 Schrodinger

proposed an equation

describing the location

and energy of an electron

in a hydrogen atom.

This is very

difficult

stuff!

Solving the Schrodinger Equation gives

the probability of finding the electron at

a given place around the nucleus.

Quantum Mechanical Model:

The solution also gives rise to

energy level sub levels.

Quantum Mechanical Model:

● The quantum mechanical model of the atom

predicts energy levels for electrons; it is

concerned with the probability, or likelihood of

finding an electron in a certain location.

● In quantum theory, each electron is assigned

a set of 4 quantum numbers

**analogy: like the mailing address of an

electron!

● The first number is called the principal quantum

number (n). It describes the main energy level the

electron is on.

● The second quantum number (l) describes the sub

level, or orbital type, the electron is in. (s, p, d, f)

Quantum Mechanical Model:

Playing Chopin with Boxing Gloves

“Trying to capture the physicists’ precise

mathematical description of the quantum

world with our crude words and mental

images is like playing Chopin with a

boxing glove on one hand and a catcher’s

mitt on the other.”(1996). Johnson, George. On skinning Schrodinger’s Cat. New York

Times.

Electron Configurations

● Regions where electrons are likely to be

found are called orbitals. EACH ORBITAL

CAN HOLD UP TO 2 ELECTRONS!

S Orbitals

Energy level Sublevel Orbitals

n=1 1s 1 (1s)

n=2 2s 1 (2s)

2p 3 (2p)

n=3 3s 1 (3s)

3p 3 (3p)

3d 5 (3d)

Sublevel # Orbitals Max # elec.

s 1 2

p 3 6

d 5 10

f 7 14

Quantum Mechanical Model!

• VIDEO

Electron Configuration!

• VIDEO