The half -life period of a radioactive element A is same as mean life of another radioactive element B . Initially both of them have same number of atoms
Answers
Group 8A (or VIIIA) of the periodic table are the noble gases or inert gases: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The name comes from the fact that these elements are virtually unreactive towards other elements or compounds. They are found in trace amounts in the atmosphere (in fact, 1% of the atmosphere is argon); helium is also found in natural gas deposits. In their elemental form at room temperature, the Group 8A elements are all colorless, odorless, monatomic gases.
The Group 8A elements have a full octet of eight valence electrons in their highest-energy orbitals (ns2np6), so these elements have very little tendency to gain or lose electrons to form ions, or share electrons with other elements in covalent bonds. They can be forced to form compounds with other elements, but require special conditions to do so. Xenon, krypton compounds are known (this was first accomplished by Neil Bartlett in 1962), including XeF2, XeF4, XeF6, XeOF2, XeOF4, XeO2F2, XeO3F2, XeO2F4, XeO3, XeO4, KrF2, RnF2, among others.
Helium (He, Z=2).
Helium is a colorless, odorless, unreactive gas which liquefies at -268.97°C (4.18 K). The name "helium" is derived from the Greek word for the Sun, helios. Helium was originally discovered on the Sun in 1868 by Pierre Janssen: his analysis of the light emitted from the Sun's corona during a solar eclipse showed that there were lines in the Sun's spectrum that were produced by a previously unknown element. When helium was found on Earth in the late 1890's (in association with uranium ores), it was found to have the same emission spectrum as the element previously found in the Sun, indicating that it was indeed the same element. The concentration of helium in the Earth's crust is about 8 ppb, making it the 71st most abundant element; it is found in the atmosphere at a concentration of 5 ppm (by volume). It is found in some minerals, where it is produced by radioactive elements that decay by alpha particle emission (see below).
Helium is the second most abundant element in the universe (23% by mass); hydrogen and helium together make up 99% of the "normal" matter of the universe. (Of course, there's also "dark matter" and "dark energy" to worry about, but that's another story.) Hydrogen, helium, and trace amounts of lithium were produced at the beginning of the Universe in the Big Bang, and became concentrated into stars by the force of gravity. The fusion of hydrogen atoms in stars to produce helium produces huge amounts of energy; the energy in sunlight is captured by plants in photosynthesis, and drives most of the chemistry of living organisms.
The most common isotope of helium, helium-4, consists of two protons and two neutrons in the nucleus, surrounded by two electrons. An alpha particle is a type of radiation emitted by some radioactive nuclei, which consists of two protons and two neutrons, having a 2+ charge; when an alpha particle is emitted by a radioactive element, it quickly picks up two electrons from its environment, becoming a helium atom. Helium is found along with some radioactive minerals that decay by alpha particle emission, however the concentration of helium in these minerals is very low. The main source of helium is from natural gas, where it is separated out from the other components by fractional distillation.