Answer the following question given this paragraph: 

When ordered by atomic number, the discrepancies within Mendeleev’s table disappeared. Tellurium has an atomic number of 52, while iodine has an atomic number of 53. So even though tellurium does indeed have a greater atomic mass than iodine, it is properly placed before iodine in the periodic table. Mendeleev and Moseley are credited with being most responsible for the modern periodic law : When elements are arranged in order of increasing atomic number, there is a periodic repetition of their chemical and physical properties. The result is the periodic table as we know it today. Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with electrons. Elements with similar chemical properties appear at regular intervals, within the vertical columns called groups .


Q: Each new horizontal row of the periodic table corresponds to the beginning of a new period because a new principal energy level is being filled with what?

 Choices:

- protons

- photons

- neutrons

- electrons

A:
electrons
