The wavefunction of a 2s electron is given by ψ2s=14√2π(1a0)32(2−ra0)e−ra0
It has a node at r=r0. The value of r0a0 is
ψ2=132π2(1a0)3(2−ra0)2e−2ra0
When r=r0, it has a node (i.e.) ψ2=0
132π2(1a0)3(2−r0a0)2e−2r0a0=0
From this relation it is clear that any of these terms
(i.e)132π2(or)(1a0)3(or)(2−r0a0)2(or)(e−2r0a0) has to be equal to zero.
It is only possible for (2−r0a0) to be equal to 0, all the others cannot ever become zero
Hence, 2−r0a0=0(or)r0a0=2