Chapter 2. Atomic Structure: The Nuclear Model

Atom

Smallest particle of an element that still has the properties of that element

Dalton’s Atomic Theory

1. All matter is made up of very small particles called atoms.

2. All atoms are indivisible. They cannot be broken down into simpler particles

What did William Crookes discover and investigate?

Cathode rays, investigated what happens when electricity is passed through gases

What is J.J. Thomson credited with?

The discovery of the electron

Cathode rays

Streams of negatively charged electrons

What did Robert Millikan measure with what experiment?

The size of the charge on the electron with his oil drop experiment.

What is the plum pudding model and who made it?

J.J. Thomson proposed a simple model of the structure of the atom, his assumptions were:

• An atom is like a sphere of positive charge

• electrons are embedded in the sphere at random

What did Ernest Rutherford do to contribute towards our understanding of the structure of an atom?

Performed an experiment where a thin piece of gold foil was bombarded with alpha particles alongside a fluorescent screen to detect alpha particles. He also discovered the proton when he found small positively charged particles given off in similar experiments.

Observations and conclusions of Rutherford’s experiment

• Most alpha particles pass straight through the gold foil → Most of the atom is empty space

• Some alpha particles are deflected at large angles → The alpha particles are repelled when they pass near the small positive nucleus

• A small number of alpha particles are reflected back along their own paths → A small number of alpha particles collide head on with the nucleus

Assumptions of Rutherford’s nuclear model

1. Atoms contain a small, dense positive core called the nucleus, most of the mass of the atom is concentrated here.

2. Positive particles called protons are located in the nucleus

3. The atom is mostly empty space

4. Electrons are scattered around in the space surrounding the nucleus called the electron cloud. It is proposed that these electrons move around the nucleus in some unknown motion.

Limitations of Rutherford's nuclear model

1. Since like charges repel, why does the nucleus not fly apart due to the repulsion between protons in the nucleus?

2. Since opposite charges attract, why don’t electrons spiral into the nucleus?

What did James Chadwick discover?

The neutron

Assumptions of the updated nuclear model of the atom

1. Atoms consist of a small, dense positive core called the nucleus, most of the mass of the atom is concentrated here.

2. Positive particles called protons and neutral particles called neutrons are located in the nucleus.

3. The atom is mostly empty space.

4. Electrons are scattered around in the space surrounding the nucleus. this space is called the electron cloud. It’s proposed by Rutherford that these electrons are moving around the nucleus but the motion is not described.

What is the major limitation of the updated nuclear model of the atom?

Since opposite charges attract, how do the electrons not lose energy in their motion around the nucleus and spiral into the nucleus?

Relative mass of the electron?

1/1838