Chapter 4 – Atomic Structure

4. 1-2

4.1 Defining the Atom

What is an atom?

How can we define, study, and believe in something we can’t see?

• An Atom is the smallest particle of an element that retains its identity in a chemical reaction.

We will study the atom by looking at the evolution of the understanding of the atom. Noting both the accurate and inaccurate ideas are both important.

This is Key

Democritus’s Atomic Theory (460 B.C. -370 B.C.)

Atoms are indivisible and indestructible

Dalton’s Atomic Theory

• 1. All elements are composed of tiny indivisible particles called atoms.

• 2. Atoms of the same element are identical. Atoms of one element are different from those of all other elements.

Dalton’s Atomic Theory

• 3. Atoms of different elements can physically mix or chemically combine in simple whole number ratios to form compounds

• 4. Chemical reactions occur when atoms are separated, joined, or rearranged. However, Atoms of one element can not be changed into an atom of another element.

Sizing up the Atom

How small are we talking about?

Let’s consider this copper (Cu) penny. How many atoms line up end to end in 1 cm?

Sizing up the Atom

100,000,000 copper atoms lined up end to end is about 1 cm.

There are 2.4 x 1022 Cu atoms in a single penny

There are 6 x 109 people on the planet Earth

How can we see an atom?

Scanning Tunneling Microscopes

Field Electron Emitting Microscopes

4.2 Structure of the Nuclear Atom

• Subatomic Particles– Electrons – Protons – Neutrons

Electrons

• Thomson (1856 – 1940)

Negative Plate

Positive Plate

Electrons

Thomson (1856 – 1940)

Negative Plate

Positive Plate

What does this tell us

about the atom?

Electrons

Electrons are negatively charged subatomic particles.

Properties of Subatomic Particles

Relative Relative Particle Symbol Charge Mass

Electron e- 1- 1/1840

Proton p+ 1+ 1

Neutron n0 0 1

The Atomic Nucleus

Rutherford’s Gold Foil Experiment (1911)

The Atomic Nucleus

• Rutherford’s Gold Foil Experiment (1911)

What does th

is tell u

s

about the atom?

The Atomic Nucleus

• The nucleus is a relatively tiny core of the atom– Contains Protons and Neutrons– Makes up most of the mass and density of the

atom

This means that most of the volume is occupied by the electrons