Electrons

Electrons

1. Electrons in atoms

   • Protons + neutrons give atoms their mass but electrons are outer part of atom + only electrons are involved in changes that happen during chemical reactions.
   • Electrons arranged in shells which correspond to different energy levels occupied by electrons.

2. Arrangements of electrons

   • 1913, Bohr suggested that as electrons could only possess energy in quanta (fixed amounts of energy) - would not exist in stable way anywhere outside nucleus unless they were fixed/ 'quantised' energy levels.
   • If an electron gained/lost energy, could move to higher/lower energy levels but no in between.
   • Shells are no. 1,2,3,4. No.s known as principal quantum numbers,n. Correspond to no.s of rows (or periods) in periodic table.
   • Atomic no. tells us no. of electrons present in atom of element. For given element, electrons are added to shells as follows:
     • Up to 2 electrons in shell 1.
     • Up to 8 electrons in shell 2.
     • Up to 18 electrons in shell 3.
     • Up to 32 electrons in shell 4.