The number of electron layers and the distribution of electrons over the layers for bromine and arsenic.

In the periodic system, the serial number shows the total number of electrons, the period number – the number of energetic levels in the atom, the group number – the number of electrons at the outer energy level.
Bromine (Br) – element number 35, located in the 4th period and 7th group, its atomic structure: Br + 35 1s2 2s22p6 3s23p63d10 4s24p5.
Arsenic (As) – element number 33, located in period 4 and group 5, its atomic structure: As +33 1s2 2s22p6 3s23p63d10 4s24p3.