Question

what are radial nodes

Answer 1

Radial and Angular Nodes. There are two types of nodes, angular and radial nodes. An angular node is a flat plane, . The ℓ quantum number determines the number of angular nodes in an orbital. A radial node is a circular ring that occurs as the principle quantum number increases.

Answer 2

A radial node is a sphere (rather than an angular node which is a flat plane) that occurs when the radial wavefunction for an atomic orbital is equal to zero or changes sign.

Radial nodes, as one could guess, are determined radially. Using the radial probability density function, places without electrons, or radial nodes, can be found. 

A radial node will occur where the radial wavefunction,
R(r)
R(r)
equals zero or changes sign. At a node the probability of finding an electron is zero; which means that we will never find an electron at a node.