Structure of the Nucleus (Leaving Cert Physics): Revision Notes
Structure of the Nucleus
Discovery of nuclear structure
By 1932, scientists had made a groundbreaking discovery about atomic structure. They found that the tiny nucleus at the centre of every atom is actually made up of at least two different types of particles: protons and neutrons. This was a major breakthrough in understanding how atoms are constructed.
This discovery revolutionised our understanding of matter and laid the foundation for nuclear physics, leading to both peaceful applications like nuclear medicine and more controversial developments like nuclear weapons.
Properties of subatomic particles
The nucleus contains protons and neutrons, whilst electrons orbit around the nucleus. Each of these particles has very different properties:
| Particle | Symbol | Location | Charge (Coulombs) | Relative Charge | Mass (kg) | Relative Mass |
|---|---|---|---|---|---|---|
| Proton | p | Nucleus | +1.6 × 10⁻¹⁹ | +1 | 1.67 × 10⁻²⁷ | 1 |
| Neutron | n | Nucleus | 0 | 0 | 1.68 × 10⁻²⁷ | 1 |
| Electron | e⁻ | Orbiting nucleus | -1.6 × 10⁻¹⁹ | -1 | 9.1 × 10⁻³¹ | 1/2000 |
Key observations from this table:
- Protons and neutrons have nearly identical masses, both about 2000 times heavier than electrons
- Protons carry a positive charge, neutrons are neutral, and electrons are negative
- The nucleus contains almost all the atom's mass but occupies a tiny space
Atomic number (Z)
The atomic number (Z) of an element tells us exactly how many protons are present in the nucleus of that element's atoms. This number is absolutely crucial because it determines what element we're dealing with.
For example, every uranium atom always has exactly 92 protons in its nucleus. If an atom has a different number of protons, it simply isn't uranium - it's a completely different element. The atomic number is like the element's unique fingerprint.
You can find all elements listed in order of increasing atomic number on the periodic table, which organises elements based on this fundamental property.
Mass number (A)
The mass number (A) represents the total count of both protons and neutrons in the nucleus of an atom. Since protons and neutrons make up virtually all of an atom's mass, the mass number gives us the best indication of how heavy the atom is.
The relationship between these quantities is straightforward:
Nuclear notation
When scientists write symbols for specific atoms, they use a special notation that includes both the atomic number and mass number. This notation places the mass number at the top and the atomic number at the bottom, both positioned to the left of the element symbol.
Nuclear Notation Examples:
- represents regular hydrogen with 1 proton and 0 neutrons
- represents helium with 2 protons and 2 neutrons
- represents uranium with 92 protons and 143 neutrons
In this notation: where A is the mass number, Z is the atomic number, and X is the element symbol.
Calculating the number of neutrons
Once you know both the mass number and atomic number for an atom, you can easily work out how many neutrons it contains using this simple formula:
This formula works because the mass number counts all the nuclear particles (protons + neutrons), whilst the atomic number only counts the protons. The difference must therefore be the neutrons.
Isotopes
Isotopes are atoms of the same element that have identical numbers of protons but different numbers of neutrons. This means they have the same atomic number but different mass numbers.
A perfect example is hydrogen, which exists in nature as two main isotopes:
- About 99.98% of hydrogen atoms are the common form with just 1 proton and 0 neutrons
- About 0.02% are deuterium atoms with 1 proton and 1 neutron
Even though these hydrogen isotopes have different masses, they're both still hydrogen because they both have exactly one proton. All elements have multiple isotopes, though some occur naturally in very tiny quantities. Many additional isotopes can be created artificially in nuclear reactors for research and medical purposes.
Nucleons
Scientists use the term nucleons as a collective name for both protons and neutrons - the particles that live in the nucleus. This is a handy term because these two particles behave quite similarly in many nuclear processes.
Since the mass number (A) represents the total count of both protons and neutrons, it's sometimes also called the nucleon number. This alternative name emphasises that we're counting all the particles in the nucleus.
Key Points to Remember:
- The nucleus contains protons (positive) and neutrons (neutral), whilst electrons orbit outside
- Atomic number (Z) = number of protons and determines the element's identity
- Mass number (A) = total number of protons and neutrons in the nucleus
- Number of neutrons = A - Z (mass number minus atomic number)
- Isotopes are atoms with the same number of protons but different numbers of neutrons
- Nucleons is the collective term for protons and neutrons together