Atomic Structure and Isotopes

Overview

This lesson builds the nuclear model of the atom from scattering evidence, then uses isotope and nuclide notation to describe different nuclei. You should be able to read the notation, identify the number of protons and neutrons, and explain why isotopes are forms of the same element.

What You Need to Know

• The alpha-particle scattering experiment showed that most of the atom is empty space, with a very small, dense, positively charged nucleus.
• Large-angle deflections are rare because the nucleus is tiny compared with the atom.
• A simple nuclear atom contains protons and neutrons in the nucleus, with electrons around the nucleus.
• Proton number is the number of protons in the nucleus. It identifies the element.
• Nucleon number is the total number of protons and neutrons in the nucleus.
• Isotopes are atoms of the same element with the same proton number but different numbers of neutrons.
• In nuclide notation, the upper number is the nucleon number and the lower number is the proton number.

How to Work Through It

1. Start by sketching the alpha-scattering result and linking each observation to an inference about the atom.
2. Label a nuclear atom model with protons, neutrons, electrons, nucleus, and empty space.
3. Practise using proton number and nucleon number to calculate the number of neutrons.
4. Compare isotopes of the same element and explain what stays the same and what changes.

Check Your Understanding

• Why did most alpha particles pass straight through the foil?
• What does a large alpha-particle deflection show about the nucleus?
• What is the difference between proton number and nucleon number?
• Two atoms have the same proton number but different nucleon numbers. What are they called?

Common Mistakes

• Saying the nucleus is large because it contains almost all the mass.
• Confusing nucleon number with neutron number.
• Defining isotopes as different elements instead of forms of the same element.
• Reading nuclide notation without checking which number is proton number.

Next Steps

• Practise reading and writing nuclide notation for several elements.
• Use conservation of proton number and nucleon number in the next lesson on radioactive decay.