What are the two rows placed at the bottom of the periodic table?

The two rows present at the bottom of the periodic table together form the Inner Transition elements of the periodic table. The inner transition elements in the periodic table are a part of the transition metals, which range from the third to the twelfth group of the periodic table. The two rows placed at the bottom of the periodic table are the two series of elements, the Lanthanide series and the Actinide series.


Lanthanide series
The elements in the upper row form the Lanthanide series of elements, since the first element in the row is Lanthanum. Lanthanum is placed in the third group in the sixth column of the periodic table. It is the first element of the Lanthanide series, and the rest of the elements are present in the first row at the bottom of the periodic table.

Actinide series
The elements in the lower row form the Actinide series of elements. They are named so, because the first element of this series is Actinium. This element is placed in the third group in the seventh row of the periodic table. The rest of the elements of the Actinide series are placed in the second row at the bottom of the periodic table.

Why are the two rows placed at the bottom of the periodic table?
The two rows of elements are placed at the bottom of the periodic table in order to not distort th shape of the periodic table. These elements are more than what can be placed in a single row in the periodic table. If they would be placed in the periodic table, this would cause the shape of the periodic table to become highly distorted. This is because all the elements in each of the two rows belong to one particular group - the elements of the first row, that is the Lanthanide series elements, belong to the third group of the periodic table, and so do the elements of Actinide series.

Important points about the elements in the two rows at the bottom of the periodic table.

• These elements are combined known as the Rare earth metals along with the two more element present in their group - Scandium and Yttrium.
• These elements are all radioactive in nature. See here : "What are radioactive elements?"
• These elements all belong to the third group of the periodic table.
• These elements are called the Inner Transition elements, or the Inner Transition metals.

What are radioactive elements on the periodic table?

Definition of radioactive elements:
Radioactive elements are those elements whose atoms have an unstable nucleus as a result of which they emit certian electromagnetic radiations like Alpha, Beta and Gamma radiations.

Explanation of radioactive elements:

Radio active elements emit radiations like Alpha, Beta and Gamma radiations.

Alpha, Beta and Gamma radiations are a type of electromagnetic waves, like visible light. The atoms of radioactive elements in their natural state emit these radiations.

The nucleus of the atoms of radioactive elements is unstable

The nucleus in any atom contains protons and neutrons. The number of neutrons is generally equal to the number of protons for any element. But there are elements in which the number of neutrons per nucleus is greater than the number of protons. This causes the nucleus of those elements to become unstable. It tries to lose the extra mass so that the number of neutrons becomes equal to the number of protons.
For losing the extra mass, the nucleus of the element starts emitting its subatomic particles, that is, neutrons, protons, and electrons in the form of electromagnetic radiations known as Gamma, Alpha and Beta radiations.

Position of radioactive elements in the periodic table
Radioactive elements have an unstable nucleus. The number of neutrons in the nuclei of radioactive elements is larger than the number of protons. This occurs generally in elements that have a large atomic mass. The elements having larger atomic masses are present in the lower rows of the periodic table. In general, all elements after Lead (Pb) in the periodic table have an unstable nucleus, and they all have high atomic masses.

The two rows at the bottom of the periodic table consist of radioactive elements. These are known as the rare earth metals. These are heavy metals since they have high atomic masses.
The first row elements are known as the Lanthanide series of elements, since their first element is Lanthanum. The second row of elements is known as the Actinide series of elements, since their first element is Actinium. These are all radioactive elements (rather metals).

Why are the two rows placed at the bottom of the periodic table? These are the elements that should come in the sixth and seventh rows of the periodic table. But representing them in their respectable rows would highly distort the shape of the periodic table. Therefore these elements are put in tow rows at the bottom of the periodic table, to maintain the shape of the periodic table.