Exploring the Atom: Beyond Indivisibility
2.2 Atomic Number and Mass Number
Definition of Fundamental Particles
Before we dive deeper, let’s confirm the basic building blocks of matter:
Click to learn about the fundamental particles:
All elements contain these particles, but in different numbers, making each element unique.
Atomic Number (Z)
The number of protons present in the atoms of an element is always fixed. This is called the atomic number (Z).
Quick Check: Since atoms are electrically neutral, how many electrons does an atom have compared to its protons?
Since atoms are electrically neutral, they have an equal number of electrons to protons.
- The atomic number is unique to each element and identifies the element.
- In the periodic table, elements are arranged in increasing order of atomic numbers.
Interactive: Element Identifier
Enter the number of protons and see which element it is!
Mass Number (A)
The mass number (A), or nucleon number, is the total number of protons and neutrons in an atom.
Remember: The mass of an electron is negligible, so it is not included in the mass number. Just like atomic number, the mass number can also help identify an atom (specifically, an isotope).
Interactive: Calculate Neutrons (N = A – Z)
Find out how many neutrons are in an atom:
Examples:
- Oxygen Atom: Atomic number (Z) = 8, Mass number (A) = 16. Symbolized as ₈¹⁶O. (Neutrons = 16 – 8 = 8)
- Carbon Atom: Symbolized as ₆¹²C. Contains 6 protons and 6 neutrons. (Neutrons = 12 – 6 = 6)
- Chlorine Atom: Atomic number (Z) = 17, Mass number (A) = 35. Neutrons (N) = 35 – 17 = 18.
2.3 Isotopes and Their Masses
Definition of Isotopes
All the atoms of an element must necessarily have the same atomic number (same number of protons), but their mass number may vary depending upon the number of neutrons present in the nucleus.
Define: Based on the above, how would you define “isotopes”?
Atoms of the same element having different number of neutrons in their nuclei are called isotopes.
Examples of Isotopes
Let’s look at carbon and hydrogen isotopes:
Carbon Isotopes Visualizer:
Notice how the number of neutrons changes, but protons (and thus the element) stay the same!
6 Protons, 6 Neutrons
6 Protons, 7 Neutrons
6 Protons, 8 Neutrons
Hydrogen Isotope Builder:
Build the hydrogen isotopes by dragging neutrons into the nucleus. Hydrogen (₁¹H) already has 1 proton!
Isotope: Hydrogen (₁¹H)
Interesting Fact: ₁¹H is the only atom which does not have a neutron.
Chemical and Physical Properties of Isotopes
Since the chemical properties of elements are determined by the number of electrons (which is determined by protons), all isotopes of an element will show almost the same chemical behavior. However, their physical properties (like mass) may be different.
Exercise:
Why do isotopes of an element show same chemical properties while their physical properties are different?
Isotopes of an element have the same number of protons and thus the same number of electrons. Since chemical properties are determined by electron configuration (how electrons interact and form bonds), their chemical behaviors are similar.
However, isotopes have different numbers of neutrons, leading to different mass numbers. Physical properties like density, melting point, boiling point, and diffusion rates are affected by mass, which is why their physical properties differ.
For example, ₁²H (Deuterium) has twice the mass of ₁¹H, and ₁³H (Tritium) has thrice the mass of ₁¹H. Similarly, the masses of the three different isotopes of carbon are different.
Radioactive Isotopes
Unstable Nuclei and Radiation
While most isotopes are stable, some have unstable nuclei. These unstable isotopes emit excess energy in the form of radiation. This process is called radioactivity, and the isotope which emits energy is called a radioactive isotope.
When an unstable nucleus emits radiation, it’s called radioactivity. These isotopes are radioactive!
Example: Tritium (₁³H) is a radioactive isotope. The other two hydrogen isotopes (₁¹H, ₁²H) are stable and do not emit radiation.
Radioactive Decay Process
When a radioactive element emits radiation, it is transformed into another chemical element. This process is called radioactive decay. The new element may be stable or radioactive, and may also emit radiation.
Thorium is unstable and further disintegrates to give ₉₁²³⁴Pa (Protactinium).
Applications of Radioactive Isotopes
Radioactive isotopes have vital applications in many fields:
Use in Medical Imaging
Doctors use radioactive isotopes to diagnose diseases by injecting a small amount of radioactive fluid into the patient.
- Technetium-99m is used for diagnostic imaging across human organs like brain, lungs, etc.
- A special camera is used to track how the radioactive fluid moves inside the body, helping to identify issues.
Radiocarbon Dating
Radiocarbon dating is a method for finding out the age of a historical object containing organic material with the help of radioactive isotope of carbon ¹⁴C.
The method involves:
- Measuring the proportion of ¹⁴C in a sample from a dead plant or animal, like a piece of wood or bone.
- This provides information about when the organism died.
- The older the sample is, the less ¹⁴C is detected.
Applications in other fields
Radioactive isotopes are used in many fields:
- To test the strength of metals and concrete mixtures.
- To generate cheap nuclear power.
- To find oil fields.
- In medicine, they are used to diagnose and treat many medical conditions, including cancer and thyroid disorders.
Exercise:
Why does a radioactive isotope emit radiation?
A radioactive isotope emits radiation because its nucleus is unstable. This instability arises from an imbalanced ratio of protons to neutrons, or simply having too many nucleons, leading the nucleus to spontaneously decay to a more stable state by releasing excess energy and particles.
Give an example of a radioactive isotope which disintegrates to give a stable atom.
One common example is Carbon-14 (¹⁴C), which decays to Nitrogen-14 (¹⁴N), a stable atom, by emitting a beta particle. This is the basis of radiocarbon dating.
Another example from the text: Uranium-238 (²³⁸U) undergoes a series of decays, eventually leading to stable Lead-206 (²⁰⁶Pb).
Ionization of Atoms by a Radioactive Source
Radiation emitted from a radioactive source can cause ionization of atoms. This means it can remove an electron from an atom.
Think: What kind of energy must ionizing radiation have to remove an electron from an atom?
This ionizing radiation must have enough energy to remove the tightly bound electron from the orbit of an atom.
Interactive: Ionization Example
See how Sodium (Na) becomes an ion (Na⁺) and Chlorine (Cl) accepts an electron to become an ion (Cl⁻).