Question

The atomic number of natural radioactive element is:
(A) Greater than 82
(B) Less than 82
(C) Not defined
(D) At least 92

Answer 1

We know that an unstable nucleus spontaneously emits particles and energy in a process known as radioactive decay. The term radioactivity refers to the particles emitted. When enough particles and energy have been emitted to create a new, stable nucleus (often the nucleus of an entirely different element), radioactivity ceases. The most common types of radiation are called alpha, beta, and gamma radiation, but there are several other varieties of radioactive decay. Radioactive decay is the spontaneous breakdown of an atomic nucleus resulting in the release of energy and matter from the nucleus. Remember that a radioisotope has unstable nuclei that does not have enough binding energy to hold the nucleus together. Using this we can solve this question.

Complete step by step answer
It should be known to us that the atomic number or proton number (symbol Z) of a chemical element is the number of protons found in the nucleus of every atom of that element. The atomic number uniquely identifies a chemical element. It is identical to the charge number of the nucleus. The number of protons determines an element's atomic number and is used to distinguish one element from another. Together, the number of protons and the number of neutrons determine an element's mass number.
The two additional series at the bottom of the periodic table are known as lanthanide and actinide series. The elements present in the actinide series are radioactive elements. Actinide series starts from the element thorium having the atomic number 90. Hence, elements having the atomic number greater than 82 are radioactive.

So, the correct answer is option A.

Note: We can write that Lanthanides are the rare earth elements of the modern periodic table i.e. the elements with atomic numbers from 58 to 71 following the element Lanthanum. They are called rare earth metals since the occurrence of these elements is very small. They are called lanthanides because the elements in the series are chemically similar to lanthanum. Both lanthanum and lutetium have been labelled as group 3 elements, because they have a single valence electron in the 5d shell.
On the other hand, Actinides are the 15 elements with atomic numbers from 89 to 103. They are named after the first element in the series, actinium. The actinides are most known for the elements uranium and plutonium which are used in nuclear reactors and nuclear bombs. The actinides are valuable primarily because they are radioactive. These elements can be used as energy sources for applications as varied as cardiac pacemakers and generation of electrical energy for instruments on the moon. Uranium and plutonium have been employed in nuclear weapons and in nuclear power plants.