I was reading in my textbook Chemistry Part I, Textbook for Class XI by NCERT, ed. January 2021 that:
The atomic size generally decreases a period across as illustrated in Fig. 3.4 (a) for the elements in the second period. It is because within the period the outer electrons are in the same valence shell and the effective nuclear charge increases as the atomic number increases resulting in the increased attraction of the electrons to the nucleus.
I understood what they've tried to say, but when I looked at Fig. 3.4 (a) that they provided:
I noticed that the rate of change for the atomic radius becomes less drastic as I move across the period from left to right, albeit there's an anomaly: the slope of the graph between C and N is less steep than the slope of the graph between N and O.
What explains this?
To reiterate, I understand how the atomic radius changes across a period, and I also understand why it does so. What I don't understand, however, is why the rate of change of atomic radius changes? Why is the graph not linear?