The given electron configuration is 1s2, 2s2, 2p6, 3s23p3. To determine the atomic number of the element below this one in the periodic table, we need to understand the pattern of electron configurations and how they relate to the atomic number.

- Electron Configuration:
The electron configuration of an element represents the arrangement of electrons in its energy levels or shells. The first energy level (closest to the nucleus) is labeled as 1s, the second as 2s and 2p, the third as 3s and 3p, and so on. Each energy level has a specific number of subshells (s, p, d, f), and each subshell can hold a specific number of electrons.

- Filling Order of Electrons:
Electrons fill the energy levels and subshells in a specific order according to the Aufbau principle, which states that electrons occupy the lowest energy level and subshell available before filling higher ones. The order of filling is as follows:
1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.

- Given Electron Configuration:
The given electron configuration is 1s2, 2s2, 2p6, 3s23p3. This indicates that the element has a total of 16 electrons. Breaking down the configuration, we have:
- 1s2: 2 electrons in the 1s subshell.
- 2s2: 2 electrons in the 2s subshell.
- 2p6: 6 electrons in the 2p subshell.
- 3s23p3: 6 electrons in the 3s subshell and 3 electrons in the 3p subshell.

- Atomic Number of the Element Below:
To determine the atomic number of the element below this one, we need to consider the filling order of electrons. Since the given configuration has filled up to the 3p subshell, the element belongs to the 3rd period of the periodic table. The element directly below it in the periodic table will be in the same group (column) but in the next period.

- Group:
The group number of an element corresponds to the number of valence electrons it has. Valence electrons are the electrons in the outermost energy level. In the given configuration, the element has 5 valence electrons (3s23p3). Therefore, it belongs to group 15 (or group VA) of the periodic table.

- Period:
The period number of an element corresponds to the highest energy level (or shell) that is completely filled with electrons. In the given configuration, the highest energy level completely filled is the 3rd energy level. Therefore, the element belongs to the 3rd period of the periodic table.

- Conclusion:
Based on the above analysis, the element with the electron configuration 1s2, 2s2, 2p6, 3s23p3 belongs to the 3rd period and group 15 of the periodic table. The atomic number of the element directly