Which of the following factors contributes to the increase in ionization energy from left to right across a period?

The increased number of proton increase in ionization energy from left to right across a period.

Ionization Energy:

It is the energy that required to take out one electron from the outer shell of an isolated atom.

When we move from left to right in the periodic table, the number of proton increase but the number of shell remain constant.

This increase the strength of attraction between outer most shell's electron and nucleus.

Therefore, the increased number of proton increase in ionization energy from left to right across a period.

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an increase in the number of protons

Explanation:

As one moves from left to right in the periodic table, the protons steadily increase by one across each period. This increase in number of protons implies an increase in the size of the nuclear charge and a consequent increase in electrostatic attraction between the nucleus and the outermost electron. This increased nuclear attraction leads to an increase in ionization energy across the period.