Fill electron shells
Distribute the electrons of an element over the shells, from the inside out. Go through the elements in order or pick one at random.
0 of 20 solved
Task 1/20: Hydrogen (H) has atomic number 1. Distribute its electrons over the shells.
What you need to know for this exercise
The electrons of an atom do not move freely. They move in shells around the nucleus. The shells are called K, L, M and N, or numbered from the inside out: shell 1, shell 2 and so on.
Each shell holds only a certain maximum number of electrons. For the first 20 elements the school model uses at most 2, 8, 8 and 2 electrons. The electrons fill the shells from the inside out: the next shell only starts once the one before it is full. Sodium with eleven electrons therefore has the arrangement 2-8-1, and calcium with twenty electrons has 2-8-8-2.
The electrons in the outermost shell are called valence electrons. They decide how an atom reacts. In the periodic table the period equals the number of occupied shells, and the group tells you the number of valence electrons: for groups 1 and 2 it is the group number, for groups 13 to 18 it is the last digit. Helium is the exception: it has only two valence electrons, but sits with the noble gases because its only shell is full.
How to practise: use the buttons to put electrons into the shells and check your result. The feedback tells you which shell is wrong, but not the correct number.
The shell model is a simplification. From Z = 21, starting with scandium, you need the orbital model, and the simple 2-8-8-2 rule is no longer enough.