Answer: s orbitals
Explanation: s orbital is the first to fill with electrons in a given principal energy level.
It has the most penetrating power out of all the 4 orbitals.
There are 4 orbitals in which the electrons are filled regularly.These are s(sharp), p(principle), d(diffused) and f(fundamental).
S orbitals are near the nucleus . The electrons are filled as per Aufbau Principle.
