Theory of Atomic Structure

Greeks – Democritus, Leucippus Over 2000 years ago All matter is composed of tiny particles These particles are so small that they cannot be

broken down (indestructible) Named these particles atoms (from the Greek

work for indestructible) Common Greek theory was that all matter

consisted of four "elements" - earth, air, fire, and water

John Dalton 1803 - Proposed an "atomic theory" with spherical

solid atoms based upon measurable properties of mass All elements are composed of atoms, which are

indivisible and indestructible particles All atoms of the same element are exactly alike; in

particular they all have the same mass Joining atoms of two or more elements forms

compounds. In any compound, the atoms are joined in a definite whole number ratio (H2O, 2 to 1 ratio)

Dalton Did account for the following laws:

Law of conservation of mass (a chemical change is simply a rearrangement of atoms)

Law of definite proportions (atoms are combined in definite ratios)

Law of multiple proportions (elements can combine in different ratios to form new compounds – H2O, H2O2

Did not account for isotopes

J.J Thomson

Through the use of cathode rays, Thompson was able to discover a negatively charged particle with almost no mass

Discovered electrons

“Plum Pudding” Model (1890) Negative charges were

scattered throughout the positive atom

Ernest Rutherford Discovered that atoms have

a positive, dense nucleus surrounded by negative “empty” space (the electrons are in the “empty” space)

Gold Foil Experiment – 1909 See the experiment

Gold Foil Experiment What was used

Alpha (α) particles – positively charged Same as Helium nuclei (4

2He2+)

Gold Foil – only a few atoms thick

What was done Shot a beam of α particles through the

gold foil

Gold Foil Experiment What happened?

The majority of the particles went straight through the foil and exposed the film behind it

A few particles were deflected back from the foil and scattered

Gold Foil Experiment What does this mean?

Since the α particles are (+) charged, they must have been pulled through the foil by a strong (-) charge

The atom was mostly negatively charged “empty” space

Since some α particles bounced back, they must have hit a positive part of the foil

The atom must have a small positive part located in the center

Gold Foil Experiment Atomic Theory (Conclusion)

An atom consists of a very small, positively charged nucleus and the rest is negatively charged empty space. This empty space must contain the electrons.

Lets see the experiment again!

Bohr Model – Neils Bohr 1919 Electrons resemble our solar

system Electrons revolve (orbit)

around the nucleus Principle energy levels (PEL)

= rings around the nucleus

Principle Energy Levels The only regions in which electrons can be

found Each level can only contain a specific

number of electrons (2 in the 1st, 8 in the 2nd) Electrons farther away from the nucleus

have higher energy Number of PEL’s is given by the period

number (found on the periodic table)

James Chadwick 1932 – discovered neutrons Using alpha particles he discovered a

neutral atomic particle with a mass close to a proton.

Valence Electrons – outer electrons Outermost electrons give an atom its

properties These are the electrons that are involved in

reactions Last number in the electron configuration Kernel electrons – inner electrons

Electron Dot Symbols / Lewis Dot Diagrams

Dots represent valence electrons

Examples:1. K

2. Ca

3. S

4. Ar

5. Ca2+

6. S2-

Warm-up Questions1. Who is credited with discovering the nucleus?

a. Thompson

b. Rutherford

c. Bohr

d. Dalton

2. Which subatomic particle was discovered first?

3. Draw the Bohr Diagram for 39K+.

Electron Configurations Distribution of electrons Given in the lower left hand corner of the

periodic table (below the atomic number)

Rules1. 1st PEL can hold 2 electrons2. 2nd PEL can hold 8 electrons3. 3rd PEL can hold 18 electrons4. 4th PEL can hold 32 electrons5. No more than 8 electrons are in the outermost

principle energy level This equal 8 valence electrons, the maximum amount of

valence electrons possible

6. The electrons are added one at a time to the unfilled principle energy levels

Examples Ca: 2-8-8-2

What does this mean? 4 principle energy levels contain electrons 1st PEL contains 2e-, 2nd and 3rd PEL contain

8e-, and the 4th PEL contains 2e-

1st and 2nd are filled, 3rd and 4th are not completely full

Ca has 2 valence electrons, and 18 kernel electrons

Examples:

1. C

2. Li

3. F

Detailed Electron Configurations Within each principle energy levels there

are sublevels Each s sublevel contains 2 electrons Each p sublevel contains 6 electrons Each d sublevel contains 10 electrons Each f sublevel contains 14 electrons

PEL Sublevels # of electrons

1 s 2

2 s, p 2 + 6 = 8

3 s, p, d 2 + 6 + 10 = 18

4 s, p, d, f 2 + 6 + 10 + 14 = 32

Example Calcium:

1s22s22p63s23p64s2

What does this mean??? The number in front of the sublevel indicates the

PEL (4s = 4th PEL, s sublevel) The number of electrons is given by the

superscript (4s2 = 4th PEL, s sublevel, 2 e-)

Rules Fill the lowest energy sublevels first

See energy chart and diagonal method

No more than 8 electrons can be in the outermost principle energy level

4d5s

4p3d

4s

3p3s

2p2s

1s

5p

6s

5d4f

6p

6d5f

7s

7p

Incr

easi

ng E

nerg

y

Low Energy

High Energy

Energies of

Sublevels

Diagonal Method

Examples

1. C

2. Al

3. Sc

4. Fe

Ground State Electrons are in the lowest available energy

levels This is how the electron configuration is

written on the periodic table Normal configuration Stable

Excited State When an electron gains energy, the

electron is at a higher energy state (excited state)

Electrons absorb energy and move to new higher energy levels

UNSTABLE

Excited State When an electron returns from the excited

state to the ground state, energy is released The energy is often emitted in the form of

light The wavelength (color) of the light can be

used to identify the element because each element absorbs and releases a specific amount of energy

Quantum Theory Electrons can only absorb (or release) specific

amounts of energy called quanta This energy corresponds to the differences

between energy levels Energy is always absorbed (or released) in

definite amounts rather than in a continuous flow

A quantum of radiation is called a photon

Flame Tests Every element absorbs and hence emits

different amounts of energy (different colors)

Can be used for identification

Spectra/Spectroscope

• Spectra– The photons of light that are given off can be

broken down into lines (spectral lines)– Every element has a unique spectra, so it can be

used for identification

• Spectroscope– An instrument that breaks light into colored bands – Emission Spectrum

Modern Orbital Theory (Wave Mechanical/Electron Cloud Model)

Electrons do not have a specific path or location – they have probabilities of being located there

Regions of high probabilities are called orbitals

Electrons are located in orbitals