Atomic Structure & The Periodic Table

Chemistry Revision Notes Atomic Structure & the Periodic Table Protons, Neutrons & Electrons Elements are made of tiny particles of matter called atoms Each atom is made of subatomic particles called protons, neutrons and electrons Their size is so tiny that we cant really compare their masses in conventional units such as kilograms or grams, so a unit called the relative atomic mass is used One relative atomic mass unit is equal to the mass of a carbon-12 atom. All other elements are measured relative to the mass of a carbon-12 atom and since these are ratios, the relative atomic mass has no units Hydrogen for example has a relative atomic mass of 1, meaning that 12 atoms of hydrogen would have exactly the same mass as 1 atom of carbon The relative mass and charge of the subatomic particles are shown below: Defining Proton Number The atomic number (or proton number) is the number of protons in the nucleus of an atom. The symbol for this number is Z It is also the number of electrons present in an atom and determines the position of the element on the Periodic Table Defining Nucleon Number Nucleon number (or mass number)