Nuclear Decay
SPH3U
Ms. Kueh
Recall:
What is the general structure of an atom?
What determines the identity of an element?
Standard Atomic Notation:
16
8𝑂
𝐴
𝑍𝑋
𝐴 = Atomic mass = 𝑝 + 𝑛
𝑍 = Atomic number = 𝑝
𝑋 = symbol of element
Chemical Reaction - the interaction of substances to form new substances
word equation:
carbon + oxygen -> carbon dioxide + energy
chemical equation:
In a chemical reaction the elements involved do not change. All elements present in the
reactants are present in the products.
Isotopes
All oxygen atoms have _________________ protons.
Most oxygen atoms have _________________ neutrons.
______________________ - atoms of the same element that have different
numbers of neutrons.
1
1𝐻
2
1𝐻
3
1𝐻
Protium
Deuterium
Tritium
http://atomic.lindahall.org/what-is-an-isotope.html
Forces in the atom
Electrostatic Force - the force of __________________ or _________________ due to
electric charges
Strong Nuclear Force - attraction between nucleons (particles that make up the nucleus)
- helps hold together the neutrons and protons in the nucleus of an atom
Radioactive Decay
Key idea - some isotopes are instable. For each element, there is an ideal ratio of 𝑝: 𝑛.
Transmutation – changing one element to another or one isotope to another
Radioactivity (Radioactive decay) -
a process by which the nucleus of an atom spontaneously
disintegrates and forms different atoms
Nuclear Reactions – involves a change of nuclei of atoms, sometimes resulting in completely
new elements
There are 3 different types of radioactive decay:
Alpha Decay - a helium nucleus consisting of two protons and two neutrons (called an ∝
particle), is spontaneously emitted from the nucleus
An atom of plutonium-240 decays into uranium-236, and a helium-4 nucleus.
When a substance undergoes alpha decay, the mass number (A) is reduced by four and the
atomic number (Z) is reduced by two.
In a nuclear reaction the parent atom (X) is the reactant atom, and the daughter atom (Y) is
the product atom
Example 1
When Lead-204 undergoes alpha decay, it produces a stable Isotope. Determine
the element and its atomic number and mass number. Write the nuclear
reaction equation for this alpha decay.
Beta Decay - reaction that involves the emission or capture of an electron or a positron (called
beta particles)
positron - particle similar to an electron except that is has a positive charge instead of a
negative charge
There are 3 main types of beta decay:
Beta Negative Decay - an electron is emitted from the nucleus of a parent atom
When a nucleus contains too many neutrons, the strong nuclear force becomes stronger than
the electrostatic force. To maintain stability, a neutron spontaneously decays into a proton and
an electron, and the electron is ejected from the nucleus.
The equation for this reaction is:
3
3
0
1𝐻 → 2𝐻𝑒 + −1𝑒
𝐴
𝑍𝑋
→
𝐴
𝑍+1𝑌
+ −10𝑒
Beta Positive Decay - a proton changes into
a neutron and a positron (symbolized by +10𝑒)
11
6𝐶
𝐴
𝑍𝑋
→
→
11
5𝐵
+ +10𝑒
𝐴
𝑍−1𝑌
+ +10𝑒
Electron Capture - this is where an electron is absorbed by a nucleus and combines with a
proton to form a neutron.
56
28𝑁𝑖
𝐴
𝑍𝑋
+ −10𝑒 →
+ −10𝑒 →
56
27𝐶𝑜
𝐴
𝑍−1𝑌
Example 2
When bismuth-214 undergoes beta-negative decay, it produces a stable isotope.
Determine the element and its atomic number and mass number. Write the
nuclear reaction equation for this beta decay.
Gamma Decay - after a nuclear reaction such as alpha or beta decay has occurred, the daughter
nucleus is in high-energy, or excited state. As a result the nucleus spontaneously releases
energy in the form of a gamma ray to return to a lower more stable energy state.
gamma ray - a highly energetic form of electromagnetic radiation that is emitted as a photon
𝐴 ∗
𝑍𝑋
→ 𝐴𝑍𝑋 + 00𝛾
3
∗
2𝐻𝑒
→ 32𝐻𝑒 + 00𝛾
The * means the nucleus is in an excited
state.
Example 3
When dysprosium-152 undergoes gamma decay, its nucleus changes from an
excited state to a stable state. Write the nuclear reaction equation for this
gamma decay.
Characteristics of Radioactive Decay
Alpha particles, beta particles and gamma rays all pose a danger to living tissue because they
can ionize, or strip the electrons from, atoms.
Types of radiation that can ionize atoms are known as ionization radiation. When this makes
contact with living tissue, it can result in burns, tumours and other harmful effects. It is
important to protect tissues from exposure to ionizing radiation.
Alpha particles have a strong ionizing ability due to their positive charge and relatively high
mass but they travel a short distance before becoming absorbed making their danger minimal.
Gamma rays and Beta particles have a greater penetrating range in air and must be shielded
against.
Homework:
Pg 322 #3, 5, 6
Pg 329 #1 – 3, 5