The increasing atomic number as we move from left to right across a period on the periodic table is primarily due to the arrangement of electrons in atoms.
The periodic table is organized based on the electronic structure of atoms, which refers to the distribution of electrons in different energy levels or shells around the atomic nucleus. Each period (or row) on the periodic table represents a new energy level being filled with electrons.
As you move from left to right across a period, the number of protons in the nucleus increases by one for each element. Since the atomic number is determined by the number of protons in an atom, the atomic number increases accordingly. Additionally, the number of electrons also increases, as atoms in a period have one more electron than the previous element.
This increase in the number of protons and electrons is due to the filling of electrons in successive energy levels or orbitals. The elements in a period fill their outermost electron shell, which is known as the valence shell. The valence shell is responsible for the chemical behavior of elements and determines their position in the periodic table.
In summary, the increasing atomic number as we move from left to right across a period on the periodic table is a result of filling successive energy levels and orbitals with electrons, leading to an increase in the number of protons and electrons in the atoms of successive elements.