Question

Which one contains a filled orbital of electrons in the ground state of helium?
(A) 1s
(B) 2s
(C) 3s
(D) 3p
(E) 3d

Answer 1

Hint: Helium is the second element of the periodic table. Helium atoms can be represented by He. The atomic number of He is 2 and the electronic configuration of the helium atom is $$1s^2$$.

Complete step by step solution:
Helium is a chemical element having the symbol He and atomic number 2. It is colourless, tasteless, non-toxic, inert, monoatomic gas. It is the first member of noble gases in the periodic table. Helium is the second lightest and second most abundant element in the universe while hydrogen is first. The boiling and freezing points of helium are lower than those of any other known substance.
Helium atom consists of two electrons in its shell and electronic configuration of the atom is shown by $$1s^2$$ and have completely filled electron shell i.e. 1s it will not easily donate or accept electron as it is acquired noble gas configuration.
Hence we can say that 1s shell contains a filled orbital of electrons in the ground state of helium.

Thus, (A) 1s is the right answer.

Note: Helium gas is present in the category of noble gases. Noble gases are those which have fully filled the outermost shell of electrons. Helium is the first member of noble gas with 1s fully filled atomic shell, another one is neon (10) in which 2p shell is fully filled.