AIM: Models of the

Atom

DO NOW:

HW: ON A SEPARATE SHEET OF PAPER MAKE A TIMELINE OF THE HISTORY AND DEVELOPMENT OF THE

ATOM. INCLUDE:

1. DALTON’S MODEL, J. J. THOMPSON’S MODEL, RUTHERFORD MODEL, BOHR MODEL, ELECTRON

CLOUD MODEL

2. YEAR AND SHORT DESCRIPTION

3. IMAGE OR DRAWING OF MODEL

Democritus’s Atomic Philosophy

Proposed an Atomic Theory which states

that all atoms are small, hard, indivisible

and indestructible particles made of a

single material formed into different

shapes and sizes.

He named the smallest piece of matter

“atomos,” meaning “not to be cut.”

Dalton’s Atomic Theory

In the early 1800s, the English Chemist John Dalton performed a number of experiments that eventually led to the acceptance of the idea of atoms.

All matter is made of atoms.

Atoms of an element are identical.

Each element has different atoms.

Atoms of different elements combine in constant ratios to form compounds.

Atoms are rearranged in reactions.

Dalton’s Atomic Theory

Based on Dalton’s Atomic Theory (5

postulates), most scientists in the 1800s

believed that the atom was like a tiny solid

ball that could not be broken up into parts.

Dalton was credited for the three Atomic Laws

that were proven after his time.

Dalton’s Atomic Laws

1. Law of Conservation of Mass

Matter cannot be created or destroyed in any

physical or chemical process, just transferred.

2. Law of Constant Composition

When atoms combine to form molecules, the

ratio of atoms is constant.

Example – H2O will always have 2 times as

many Hydrogen atoms as Oxygen.

Dalton’s Atomic Laws

3. Law of Multiple Proportions – if two elements can

combine to form more than one compound, then the

ratio of the second element combined with a certain

mass of the first element is always a ratio of small whole

numbers.

Formula

Ratio of N:O

Example:

CO vs. CO2

J. J. Thomson’s Plum Pudding

Model

Used cathode rays to prove that

Dalton’s Solid-ball model could be

broken into smaller particles

Thomson is credited with discovering

electrons

J. J. Thomson’s Plum Pudding

A cathode ray is a tube that has a

piece of metal, called an electrode, at

each end. Each electrode is connected to a power

source (battery).

When the power is turned on, the electrodes

become charged and produce a stream of charged

particles. They travel from cathode, across the tube to

the anode.

J. J. Thomson’s Plum Pudding

Model

Stated that the atom is neutral

In 1897, he proposed the Plum Pudding

Model which states that atoms mostly

consist of positively charged material with

negatively charged particles (electrons)

located throughout the positive material

Rutherford Model

Rutherford’s experiment Involved firing a stream of tiny positively charged particles (alpha particles) at a thin sheet of gold foil (2000 atoms thick)

Most of the positively charged particles passed right through the gold atoms in the sheet of gold foil without changing course at all.

Some of the positively charged particles, however,

did bounce away from the gold sheet as if they had hit

something solid. He knew that positive charges repel

positive charges.

Rutherford Model

Gold Foil Experiment and suggested the following characteristics of the atom:

It consists of a small core, or nucleus, that contains most of the mass of the atom

This nucleus is made up of particles called protons, which have a positive charge

The protons are surrounded by negatively charged electrons, but most of the atom is actually empty space

Bohr Model In 1913, the Danish scientist Niels Bohr proposed an

improvement. In his model, he placed each electron in

a specific energy level.

According to Bohr’s atomic model, electrons move in

definite orbits around the nucleus, much like planets

circle the sun. These orbits, or energy levels, are located

at certain distances from the nucleus.

Additionally, the electrons can jump from a path in one

level to a path in another level (depending on their

energy)

Bohr’s ModelHe proposed the following

Protons and neutrons are in the

nucleus

Electrons can only be certain

distances from the nucleus

The electrons orbit the nucleus at fixed energy levels

The electrons must absorb or emit a fixed amount of

energy to travel between these energy levels

Electron Cloud Model (Wave

Mechanical Model)

In 1926, Erwin Schrodinger further explained the nature of electrons in an atom by stating that the

exact location of an electron cannot be stated; therefore, it is more accurate to view the electrons in regions called electron clouds;

electron clouds are places where the electrons are likely to be found

Electron Cloud Model (Wave

Mechanical Model) Depending on the electron’s energy they are locked into a

certain area in the cloud.

Electrons with the lowest energy are found in the energy level closest to the nucleus

Electrons with the highest energy are found in the outermost

energy levels, farther from the nucleus.

Today’s atomic model is based on the principles of wavemechanics.

Summary

Who is the father of atomic theory?

Dalton

What was the first model of the atom?

Dalton’s Tiny Ball Model

What are Dalton’s 3 Laws?

Law of Conservation of Mass, Law of Constant

Composition, Law of Multiple Porportion

Summary

How were Thomson’s and Dalton’s model

different?

Dalton’s model was 1 sphere that cannot be

divided, Thomson had the plum pudding where

electrons are randomly spread throughout a

positively charged sphere.

What did Thomson find out?

Atoms have electrons, they have a negative

charge

Summary

What were Rutherford’s conclusions from the

Gold Foil Experiment?

Atom has a positively charged nucleus

electrons are outside,

atoms are mostly empty

Nucleus contains most of the mass.

Summary Bohr Model of the Atom

Electrons go around the nucleus in concentric

circular orbits

Electron Cloud Model

The electron does not move in a definite path.

An electron is in an orbital which is the most

probable location where the electron is

The most dense part of the cloud is the most

probable place to find the electron