Step-by-step explanation:
Ionization energy is defined as the energy necessary to remove an electron from a gaseous atom or ion.
Therefore, smaller is the size of an atom or ion more energy it needs to remove an electron because more is the charge on an ion smaller will be its size.
Hence, more will be the attraction between nucleus and valence electrons of the atom. So, more difficulty is faced by the atom to lose an electron. As a result, ionization energy will increase.
Across the period, there will be decrease in size of elements of the periodic table.
Thus, we can conclude that there will be increase in first ionization energy across the periodic table.