1 2. Light has particle properties: Light is a form of - - PDF document

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Modern atomic theory

Models and the atom Model =

• a theory or explanation for a phenomenon
• Cannot prove a model
• Change as experiments dictate

History of the atomic model

Dalton’s atomic theory = Plum Pudding Model = Rutherford Nuclear model = atoms are indivisible spheres atoms consist of (+) and (-) evenly mixed atoms have small, dense, + nucleus

CRT experiment Gold foil experiment

didn’t explain the arrangement of e- Studies on atomic spectra and EM radiation led to modern atomic models (quantum theory)

Quantum Theory:

E (of light and e-) is quantized = composed of discrete bundles called quanta or photons

What was the major shortcoming with Rutherford’s Nuclear model?

• 1. Light has wave properties:

Wave = a way that E is transferred

Nature of Light

Characteristics of Waves = (λ) distance from peak to peak = (f) number of cycles/second Wavelength Frequency Speed C B D A

1 λ 1 λ

2

γ rays x rays UV radio

Micro wave IR

Visible light Highest E Lowest E Frequency directly proportional to energy λ indirectly proportional to energy longest λ Highest freq. Freq and λ are indirectly related

Light is a form of Electromagnetic Radiation

Photoelectric effect

• 2. Light has particle properties:
• Evidence that EM energy is quantized
• Irradiating a metal surface with light causes e- to be ejected
• Frequency of light must be above some threshold

V

light e-

explanation: 1 photon of certain E causes

emission of a single e-

metal Vacuum tube

• EM energy has properties of both:
• Waves
• Particles photons with discrete E
• Electrons also have both particle and

wave properties Spectra Analysis

1) Continuous spectrum =

• contains all colors from red to violet
• When passed through prism, see streak of color,

not bands 2) Bright line (Atomic) spectrum =

• contains only certain discrete λ
• When passed through prism, see series of lines
• excited atoms emit discrete spectra

Line spectra of elements

What is the significance of a line spectrum?

• atoms can only give off certain E light
• e- can only possess certain amounts of E
• Gives clues to how e- are arranged

H Hg Ne

3

• if E absorbed:

+ n = 1 n = 2 n = 3

Excited state= Higher E state

+ n = 1 n = 2 n = 3

photon

• f light

Ground state photon

• f light

+ n = 1 n = 2 n = 3

Ground state= Lowest E state

Bohr Model of the Atom:

• first quantum model of atom (e- has a discrete quantity of E)
• e- arranged in concentric shells around nucleus (= orbits)

1) e- jumps from inner to an outer shell (unstable state) 2) then e- falls from outer to inner shell (more stable) 3) E released as photon of light

This emitted E can be used to identify an element.