sentence: Exceptions are rare, but they do happen and are difficult to predict.
sentences_before: Around the nucleus some electrons are further out than others. These are called 'electron shells'. In most atoms the first shell has two electrons, and all after that have eight.
sentences_after: The further away the electron is from the nucleus, the weaker the pull of the nucleus on it. This is why bigger atoms, with more electrons, react more easily with other atoms.
concept_name: exception
headings: Atom Structure and parts Electrons
categories: Chemistry Nuclear physics