Models of Atomic Structure

Dalton Model• Unbreakable neutrally

charged spheres.

Thomson’s Cathode Ray Experiment• Cathode rays originate

from neutrally charged atoms.

• Cathode rays have a negative charge and are deflected by electrical charges and magnetic fields.

• Cathode rays are electrons.

Cathode Ray Tube

Cathode Rays = Electrons

Cathode Rays = Electrons

Cathode Rays = Electrons

Plum Pudding Model• Negative electrons stuck

inside and on the surface of a positive pudding.

• Atom’s mass and positive charge is widely spread.

• Electrons might wander but mostly motionless unless kicked out by a lot of energy.

• Chocolate chips in ice cream.

Plum Pudding Model• Negative electrons stuck

inside and on the surface of a positive pudding.

• Atom’s mass and positive charge is widely spread.

• Electrons might wander but mostly motionless unless kicked out by a lot of energy.

• Chocolate chips in ice cream.

Rutherford’s Gold Foil Experiment – Predicted Results

Rutherford’s Gold Foil Experiment – Predicted Results

Rutherford’s Gold Foil Experiment – Actual Results

Actual Results

Rutherford’s Gold Foil Experiment• Most alpha particles

(42He) passed right

through the gold foil. • A few alpha particles

had very large deflections.

• The Plum Pudding model would predict only small deflections.

• The mass and positive charge of an atom is concentrated in a very small nucleus.

Rutherford-Bohr Model• Nucleus takes up very

little space. • Electrons jump up to a

higher energy level when they absorb energy.

• Electrons falls down to a lower energy level when they emit energy.

• Electrons orbit in regular “planet-like” energy levels around the positive nucleus.

Rutherford-Bohr Model• Nucleus takes up very

little space. • Electrons jump up to a

higher energy level when they absorb energy.

• Electrons fall down to a lower energy level when they emit energy.

• Electrons orbit in regular “planet-like” energy levels around the positive nucleus.

Quantum Model• Electron clouds not

orbits. • Electrons are not found

in fixed locations, but rather probabilities to be in a location.

s-orbitals

p-orbitals

Atomic ModelsAtomic Model Description

Dalton Solid, neutrally charged spheres.

Plum-Pudding Negatively charged electrons in a glob of positive charge. Like chocolate chips in vanilla ice cream.

Rutherford-Bohr

Negatively charged electrons orbit a positive nucleus. Like planets around the sun.

Quantum Electrons are in layers of clouds around the nucleus.

Key11NaSodium22.99

Atomic Number•Number of Protons•Number of Electrons (when atom is neutrally charged)

•Property unique to each element

Key

Average atomic mass*•Weighted Average number of Protons and Neutrons (approximately)

NaSodium22.99

11

Subatomic Particlesthe particles that make up an atom

• Protons – high mass, positive charge. Found in nucleus.

• Neutrons – high mass, no charge. Found in nucleus.

• Electrons – low mass, negative charge. Found orbiting around nucleus. (abbreviated e– )

Basic Electrical Charge Laws

+ and – : Attract(pull together)

– and – : Repel(push away)

+ and + : Repel(push away)

Like charges repel and Opposites attract

An Atom

Nucleus

1 proton = H = hydrogen

Why doesn’t the electron fall into the nucleus?• It orbits because the electron is moving really

fast around the nucleus. • Because the electron has such a low mass,

even a small amount of energy makes it move very fast.

An Atom

Nucleus

1 proton = H = hydrogen

Another AtomSize of atom

Size of nucleus

2 protons = He = helium

Cathode Rays = Electrons

NeFONCBBe

He

Li

H

Kr

ArCl

Br

XeI

SPSiMg Al

Ca

Na

K

4 e– in valence shell