Greetings. I am having problems understanding some things about shells. Actually, i am not even sure of if i am talking about shells (i am not sure this is the term in English, but i think it is). I have been taught that a shell is a place of space round the nuclei with a highest probability of having an electron. I also have been taught that the size of a shell gets bigger as its quantum number n gets bigger. That is, a shell in 2s is bigger than a shell in 1s. My problem became really clear when i started to study the atomic radius. Let's take two imaginary atoms, 3X:1s22s1 and 4Y:1s22s2. We know that the atom radius of Y is smaller than the atomic radius of X (it has been explained to me that the protons in Y are more effective in attracting the electrons of 2s than the protons in X). If this is true, then does this mean that the size of the shell in 2s is smaller when it is has 2 electrons than when it has only one ? I am also unable to understand how we say that a certain atom ending with a shell in p has an atomic radius, if the shells in p are not spherical, how can they have a constant radius ? Any help would be appreciated, since i feel really lost ! Thanks in advance.