History and Models of the Atom

Early Atomic Models

Believed universe made of invisible units called atoms

Different types of matter made of different types of atoms

John Dalton

Wrote the first Atomic Theory, 1804: Solid Sphere Model

Solid Sphere Model

1. All matter is made of tiny particles called atoms

2. Atoms of the same element are identical

3. Atoms cannot be created, destroyed, or divided

4. Atoms combine in simple whole number ratios to form compounds

5. In chemical reactions, atoms are joined, separated, and rearranged, but never changed into atoms of another element

What was Dalton Wrong About?

All elements of the same type being identical
INSTEAD OF
Different isotopes of atoms of the same element can have different numbers of neutrons.

J.J Thompson

1897: The Plum Pudding Model

1897: The Plum Pudding Model

Using available data on the atom, atoms contain negatively charged particles called electrons.

Used a cathode ray tube to determine that negatively charged particles could be removed from atoms

Other Conclusions From The Study Of The Electron:

Cathode rays have identical properties regardless of the element used to produce them.

All elements must contain identically charged electrons.

Atoms are neutral, so there must be positive particles in the atom to balance the negative charge of the electrons

Electrons have so little mass that atoms must contain other particles that account for most of the mass

Robert Millikan

1909: Oil Drop Experiment

1909: Oil Drop Experiment

Measured the charge of electrons

Charge of a single electron: 1.602 x 10-19 Coulombs.

Ernest Rutherford (1)

1911- The Nuclear Model

1911- The Nuclear Model

Discovered the positively charged nucleus

Gold Foil Experiment demonstrated that while much of the atom was empty space, there was a tiny dense core.

Gold Foil Experiment

Alpha particles shot through a thin sheet of gold foil

Most shots passed through, but a small amount bounced back

Hence: atoms must be made of mostly empty space with a dense positive core

The Gold Foil Experiment Compared:

Thompson vs. Rutherford Models

Niels Bohr

1913- The Solar System Model

1913- The Solar System Model

Electrons travel around the nucleus at specific levels like planets around the sun

Electrons at each orbital level had a specific amount of energy

When electrons move between levels energy is emitted or absorbed

Electrons can only revolve in certain orbits or at certain energy levels

• energy levels are quantized, similar to stairsteps

Ernest Rutherford (2)

1918- The Proton Model

1918- The Proton Model

Discovered the proton

Rutherford found that the hydrogen nucleus – or the proton, is a constituent of larger atomic nuclei

Erwin Schrodinger

1926- The Quantum Mechanical Model

1926- The Quantum Mechanical Model

Improved on Bohr’s findings

Exact electron location can not be predicted

Wave equation - describes the likelihood of finding an electron in a certain position

Werner Heisenberg

1927-The Electron Cloud Model

1927-The Electron Cloud Model

Found that electrons live in fuzzy probability regions or “clouds” not distinct orbits

Uncertainty Principle - mathematical way of expressing the energy levels of electrons in atoms

James Chadwick

1932- The Neutron Model

1932- The Neutron Model

Bombardment of beryllium-9 with alpha particles

This gave off high energy radiation with no charge

These rays, when directed at paraffin wax, displaced protons

Concluded that the nucleus includes neutral particles called neutrons.

Atomic Number

The # of protons in an atom

Protons are what define an atom of an element

# of protons=> type of atom

Isotopes

An atom that has the same number of protons but a different number of neutrons

1 AMU

1.67×10^{-24 grams}

1 AMU: Where does it come from?

1/12 the mass of one Carbon-12 atom

Average Atomic Mass

The mass of the periodic table

The weighted average of the atomic masses of the naturally occurring isotopes of that element

Marie Curie

Named the process by which materials give off such rays radioactivity.

The penetrating rays and particles emitted by a radioactive source are called radiation.

Nuclear Forces

STUDY FROM THE PACKET!!!

Nuclear Reactions

The nuclei of unstable isotopes, called radioisotopes, gain stability by undergoing changes, becoming different elements

Chemical Reactions

Involve electrons, not protons or neutrons

Atoms tend to attain stable electron configurations by losing or sharing electrons

How does an unstable nucleus release energy?

Emitting radiation during the process of radioactive decay

Mass Defect

When an atom is formed, there is a difference between the mass of the atom and the masses of individual protons, neutrons, and electrons that make up the atom. 

Caused by the conversion of mass to energy upon the nucleus formation

Nuclear Binding Energy

The energy released when a nucleus is formed, or the energy needed to break a nucleus

Nuclear Stability and Decay

More than 1500 nuclei are known

Only 264 are stable and don’t decay or change with time

Those nuclei are located in the band of stability

Radioactivity

The nuclei of some isotopes are unstable and when they decay they emit energy that causes electrons to be removed from atoms

What does radioactivity produce?

Ionizing Radiation

Henri Becquerel

Placed uranium salts on a photographic plate. This caused the blackening of the plate despite paper in between.

Nuclear Reaction

A reaction that affects the nucleus of an atom

• Can change one element into a different element

Why do Nuclear Reactions Occur?

Unstable nucleus wants to be stable

Unstable nuclei undergo sudden changes that change their # of protons or neutrons to find stability

Radioactive Decay

Ejecting or emitting pieces from the nucleus of an atom to become stable

Balancing Nuclear Reactions

Total mass number=Atomic mass number

Must be on both sides of the equation

Types of Radiation

Alpha

Beta

Gamma

Positron Emission

Neutron Capture

Alpha Radiation

Consists of helium nuclei that have been emitted from a radioactive source

• Emitted particles, Alpha particles contain 2 protons and 2 neutrons and have a double positive charge

Alpha Decay

Atomic # reduced by 2

Atomic Mass reduced by 4

Beta Radiation

An electron resulting from the breaking a part of a proton in an atom

Called a beta particle

Beta Decay

Atomic # increased by 1

Atomic mass staying same

Positron

A particle that has the same mass as an electron but has a positive charge and is emitted from the nucleus during radioactive decay

Positron Emission

Atomic # decreased by 1

Atomic mass stays the same

Electron Capture

Inner orbital electron is captured by the nucleus of its own atom

The inner orbital electron combines with a proton, and a neutron is formed

Electron Capture Formula

Atomic # decreased by 1

Atomic mass stays the same

Gamma Radiation

A high-energy photon emitted by a radioisotope

The high-energy photons are electromagnetic radiation.

Penetration of Particles

Alpha particles are the least penetrating

Gamma rays are the most penetrating.

Paper

Stops Alpha

Wood

Stops Alpha and Beta

Lead/Concrete

Stops Alpha, Beta, and some Gamma

Half Life

The time required for one half of the nuclei of a radioisotope same to decay to products

After Each Half Life:

Half of the existing radioactive atoms have decayed into atoms of a new element

Half Life Facts

Each radioisotope has its own half life

Half lives range from nanoseconds to billions of years

• The shorter the half life - the more energetic (unstable)

Fission

The splitting apart of an atomic nucleus into two smaller nuclei accompanied by a release of energy

When does fission happen?

When the nuclei of certain isotopes are bombarded with neutrons

Nuclear Power Plant- Chain Reaction

Some of the neutrons produced react with other fissionable atoms, producing more neutrons which react with still more fissionable atoms

How do we Control Neutrons in Fissionable Chain Reactions?

Neutron Moderation

• slows down neutrons

Neutron Absorption

• decreases the number of slow-moving neutrons