Explain why there is Sometimes a delay in the
visible reaction when a metal
such as aluminium or magnesium
reacts with a dilute acid.

particle   p is an electron and particle q is a neutron

answer: atomic mass was originally used to arrange elements on the periodic table, but is no longer used to organize the modern version.

explanation:

originally atomic mass was used to arrange elements on the periodic table from left to right in a row. each column then shows similar properties of elements of a group.

in modern periodic table, the atoms are arranged in increasing atomic number. as atomic number represents the number of protons in an atom which is unique for every atom.