1.1 The structure of the atom

What did Dalton propose in his atomic theory?

• Atoms are solid, indivisible spheres

• Atoms of the same element are identical

• Atoms of different elements have different masses

• Atoms combine to form compounds

What was wrong with Dalton’s atomic theory?

• Atoms can be divided (subatomic particles exist)

• Atoms of the same element are not identical (isotopes)

What did J.J. Thomson discover? (1897)

The electron.
→ Proposed the plum pudding model (electrons in a sea of positive charge).

What did Rutherford discover? (1909)

The nucleus.
→ Atom is mostly empty space with a small, dense, positively charged nucleus.

What did Bohr propose? (1913)

Electrons orbit the nucleus in fixed energy levels (shells).
→ Electrons cannot exist between these levels.

What did Chadwick discover? (1932)

The neutron.
→ Found in the nucleus with no charge.

What is the modern (quantum) model of the atom?

Electrons exist in orbitals, which are regions of probability.
→ Not fixed paths, but areas where electrons are likely found.

What are subatomic particles?

Particles that make up atoms:

• Protons

• Neutrons

• Electrons

What is the structure of an atom?

• Nucleus: contains protons + neutrons

• Electrons: found in shells/energy levels around nucleus

What is a proton?

A positively charged particle found in the nucleus.

• Relative charge: +1

• Relative mass: 1

What is a neutron?

A neutral particle found in the nucleus.

• Relative charge: 0

• Relative mass: 1

What is an electron?

A negatively charged particle found in energy levels outside the nucleus.

• Relative charge: –1

• Relative mass: 1/1836 (very small)

Where is most of the mass of an atom found?

In the nucleus (protons + neutrons).

How do subatomic particles behave in an electric field?

• Electrons → deflected toward positive plate

• Protons → deflected toward negative plate

• Neutrons → no deflection

Why are electrons deflected more than protons?

Because electrons have a much smaller mass, so they are easier to deflect.

How do particles behave in a magnetic field?

• Electrons and protons are deflected in opposite directions

• Neutrons are not deflected

Why are neutrons not affected by electric or magnetic fields?

Because they have no charge.

What are relative mass and relative charge?

Simplified values used to compare subatomic particles instead of using very small actual values.

What are leptons and quarks?

• Leptons → fundamental particles (e.g. electrons)

• Quarks → make up protons and neutrons

Key idea about atomic theory development?

Atomic models change over time as new experimental evidence is discovered.