Chapter 4: Atomic Structure

4.1: Studying Atoms 4.2: The Structure of an Atom 4.3: Modern Atomic Theory

4.1: Studying Atoms

Greeks Dalton Thomson Rutherford

Atomic Model

Ancient Greek Models of Atoms Democritus believed that all matter

consisted of extremely small particles that could not be divided.

Ancient Greek Models of Atoms

Democritus called these particles atoms from the Greek word atomos, which means “uncut” or “indivisible.”

He thought there were different types of atoms with specific sets of properties

Aristotle Thought that all substances were built up

from only four elements.

Dalton’s Atomic Theory

Proposed the theory that all matter is made up of individual particles called atoms, which cannot be divided.

Compounds contain atoms of more than on element.

In a particular compound, atoms of different elements always combine in the same way.

Dalton’s Experiment

The ratio of the masses of the elements in the compound is always the same. A 100 gram sample of magnesium

combines with 65.8 grams of oxygen. A 10 gram sample of magnesium

combines with 6.58 grams of oxygen.

Dalton’s Model The elements are pictured as solid spheres.

Thomson’s Experiment

Thomson’s Experiment

Thomson’s experiments provided the first evidence that atoms are made of even smaller particles.

Thomson’s Model (plum pudding)

In an atom, the negative charges were evenly scattered throughout an atom filled with a positively charged mass of matter.

Rutherford’s Experiment

Rutherford’s Gold Foil Experiment

Rutherford’s Model All of an atom’s positive charge is

concentrated in its nucleus. The nucleus is a dense, positively charged

mass located in the center of the atom.

Pretending to be Alpha Particles Mv.

4.2:Structure of an Atom Protons, electrons, and neutrons are subatomic

particles. Because these particles are even smaller than an

atom, they are called subatomic particles. These three types of particles are arranged in an

atom as shown below.

What makes one element different from another if?

All protons are the same. All electrons are the same. All neutrons are the same.

Protons

The number of protons distinguishes an atom of one element from the atom of another element.

All atoms of the same element will have the same number of protons, and atoms of different elements will have different numbers of protons.

Properties of Subatomic Particles

Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom.

Atomic Number The number of protons in an atom

of that element. Because the number of protons in an

atom remains the same during physical and chemical changes, we can refer to each element by the number of protons its atoms contain. This unique number is called the atomic number.

Atomic Number

Because atoms have an overall neutral charge, atoms have an equal number of protons & electrons.

# protons = # electrons

Mass Number the sum of the protons and neutrons in the

nucleus of that atom.

Isotopes atoms of the same element that have

different numbers of neutrons and different mass numbers.

Isotopes

4.3 Modern Atomic Theory

"D'oh!"

Bohr’s Atomic Model Focused on the Electrons

1803: John Dalton Atoms are tiny,

indestructible particles, with no internal structure.

1897: JJ Thomson Discovers the

electron He pictures

electrons embedded in a sphere of positive electric charge.

1911: Ernest Rutherford Atoms have a

dense, positively charged nucleus.

Electrons move randomly around nucleus.

Gold foil experiment

1913: Niels Bohr Electrons move in

spherical orbits at fixed distances from the nucleus.

1926: Electron Cloud Model A visual model of

the probable locations of electron in an atom.

1932: James Chadwick Confirms the

existence of neutrons in the nucleus.

Neutrons have no charge.