Actinides are a series of elements that have atomic numbers ranging from 89 to 103. They are located in the periodic table below the lanthanide series and are part of the f-block elements. These elements are known for their radioactive properties and many of them have important applications in nuclear energy and medicine.

Explanation:

1. Actinide Series:
The actinide series is composed of 15 elements, starting from actinium (atomic number 89) to lawrencium (atomic number 103). These elements are named after actinium, the first element in the series. They are characterized by the filling of 5f orbitals in their electron configurations.

2. Atomic Numbers:
Atomic numbers represent the number of protons in an atom's nucleus. In the case of actinides, their atomic numbers range from 89 to 103. This means that each element in the actinide series has a unique number of protons, which determines its position in the periodic table.

3. f-Block Elements:
The actinides, along with the lanthanides, form the f-block of the periodic table. This block is located at the bottom of the periodic table, separated from the main body of elements. The f-block elements have their outermost electrons in f orbitals, which gives them unique electronic configurations and properties.

4. Radioactivity:
One of the defining characteristics of actinides is their radioactivity. Many actinides are unstable and undergo radioactive decay, emitting radiation in the process. This property makes them useful in various applications, such as nuclear reactors, where they can be used as fuel or in the production of isotopes for medical purposes.

5. Importance:
Actinides have significant importance in nuclear energy production, scientific research, and medical applications. For example, uranium and plutonium are commonly used as fuel in nuclear reactors, while americium is used in smoke detectors. Additionally, actinides like radium and radon have medical uses in cancer treatment and diagnostic imaging.

In conclusion, actinides are elements with atomic numbers ranging from 89 to 103. They are part of the f-block in the periodic table and are known for their radioactive properties and various applications in nuclear energy and medicine.