Atomic Structure

Atomic Structure

1. Nucleus and shells

   • An atom has a central nucleus. This is surrounded by electrons arranged in shells.
   • The masses of subatomic particles are very tiny.
     • Instead of writing their actual masses in kilograms, we often use their relative masses. The relative mass of a proton is 1, and a particle with a relative mass smaller than 1 has less mass.
   • The mass of an electron is very small compared to a proton or a neutron. Since the nucleus contains protons and neutrons, most of the mass of an atom is concentrated in its nucleus.
   • Protons and electrons have electrical charges that are equal and opposite.

2. Atomic number + Mass number

   • The number of protons in an atom of an element is its atomic number
   • An atom contains equal numbers of protons and electrons.
     • Since protons and electrons have equal and opposite charges
       • atoms have no overall electrical charge.
   • number of protons = atomic number
   • number of electrons = atomic number
   • number of neutrons = mass number - atomic number

3. Isotopes

   • Atoms of the same element must have the same number of protons
     • can have different numbers of neutrons
       • Atoms of the same element with different numbers of neutrons are called isotopes
   • An isotope is named after the element and the mass number of its atoms. For example, carbon-12 is an isotope of carbon with a mass number of 12.
   • The relative atomic mass of an element is a weighted average of the masses of the atoms of the isotopes. It takes account of the abundance of each of the isotopes of the element.

4. Calculating relative atomic mass

   • Ar = Total mass of atoms / Total number of atoms