human
Why is there a gap between the elements helium and lithium on the Periodic Table of Atoms?
assistant
The gap between helium and lithium is because there are no stable elements between them. Helium is the second lightest element, and lithium is the third lightest element. Helium is a noble gas, which means that it is not reactive. Lithium is a reactive metal, which means that it reacts with other elements.