Chapter 4: Atomic Structure

Addison Wesley Chemistry

4.1 - Atoms

Lead is a soft metal. Suppose we take a one gram cube of lead and cut it into the smallest pieces we can.

Eventually we will get to a particle that cannot be cut any further and still retain its physical properties.

This would be one atom of lead

4.1 – The Atom

An atom is the smallest particle of an element that retains the properties of that element.

The ideas of atoms was first brought up by Democritus, an ancient Greek. In the 4th century, BCE.

However at the time they were not useful b/c they were not based on empirical evidence.

The great philosopher Aristotle theorized there were only four elements: Fire, Water, Air, & Earth. That theory held until the late 1700s

In the late 1700s John Dalton brought back the idea of atoms and first stated: Dalton’s Atomic Theory:

1. All elements are composed of atoms.

2. Atoms of the same elements are the same.

3. Atoms of different elements can combine together to make compounds.

4. Chemical reactions occur when atoms are separated, joined, or rearranged.

4.2 – Atomic Theories

Dalton’s Theory – The “Billiard Ball” model

Thomson’s Theory (1893) – the “raisin bun” modelFirst suggested that electrons are

subatomic particles.Electrons are embedded on a positive

sphereDiscovered electrons by using a cathode

ray

Thomson’s Model

Rutherford’s Theory (1907) – “beehive” or “nuclear” model.Performed the Gold foil experimentAtoms contain a tiny dense positive core

called the nucleus, and the first suggested the proton

Electrons occupy the empty space around the nucleus

Bohr Theory – the ‘planet’ modelThe nucleus contains both protons and

neutronsThe electrons move around in fixed energy

levels

4.2 – Electrons, Protons, & Neutrons

Most of Dalton’s atomic theory is still accepted.

Now we know that atoms are made up of subatomic particles called electrons, protons, and neutrons.

Protons – Have a positive chargeElectrons – Have a negative chargeNeutrons – Have no charge

Electrons

Electrons are negatively charged subatomic particles.

They were discovered by physicist Sir Joseph Thomson in 1897.

They were discovered by using a device called a cathode ray.

Electrons are extremely small and light about 1/2000 of a hydrogen atom

Protons

Hydrogen is the simplest element consisting of one electron and a proton.

After the discovery of the electrons scientists wondered about what was left over when hydrogen lost it’s sole electron

Since atoms were electrically neutral scientists reasoned that if the negatively charged electrons were removed from a hydrogen atom then the resulting particle should have a positive charge

Experimental evidence for this was found.

Researchers named the positively charged particles protons.

It is 1840 times heavier than an electron.A proton is what remains when a

hydrogen atom loses its’ electron

Hydrogen Atom

Neutrons

In 1932 physicist Sir James Chadwick confirmed the existence of the neutron.

Neutrons are subatomic particles with no charge

They have a mass close to that of a proton