Question

Atomic number of radium is:
A.84
B.88
C.86
D.76

Answer 1

Radium is the sixth element in the group 2 and period 7 of the periodic table, known as an alkaline earth metal. Its atomic number can be determined by using 2, 8, 8, 18, 18… tricks starting from the first element of the group.

Complete step by step answer:
Radium was discovered by Marie Curie and Perre Curie in 1898, in the form of its chloride. It is a type of chemical element which has many radioactive isotopes. Its atomic symbol is Ra and is a silver-white alkaline earth metal. When it decays, it yields ionizing radiations in product form and causes radioluminescence. Also, it is the heaviest and the only radioactive element in group 2.
We can examine that down the group, atomic numbers can be easily calculated by using magic numbers. Magic numbers are 2, 8, 8, 18, 18, 32. On adding them consequently to the first one will get the next atomic number.
For example, in group 2, the first element is beryllium and has atomic number 4. Next we will be adding 8 to it and we get the atomic number of the next element, i.e. magnesium, 12.
Further on adding 8 to it, we get calcium of atomic number 20. Next, on adding 18 to it, we get a strontium of 38 atomic numbers. When further 18 is added to it, a barium element is found below it with 56 atomic numbers. And lastly, adding 32 to it, gives us radium, which has an atomic number of 88.

Hence, the correct option is (B).

Additional information:
Radium is the rarest element because of its high reactivity and it has short half-life. When uranium decays, it turns into radium and then into lead. It is carcinogenic when inhaled, ingested or exposed but is used in radiotherapy for cancer patients. Most of the radium compounds are ionic as radium exhibits +2 oxidation state.

Note:
Another way- The nucleus of radium has 88 protons and 138 neutrons, while 88 electrons occupy its available electron shells outside the nucleus. Atomic number is equal to the number of protons present in the nucleus. Therefore, radium has an atomic number 88.