Electron arrangement

• Most atoms have an incomplete outer shell of electrons
• Stable atoms


  Have a full outer shell of electrons
• Atoms with incomplete outer shells will react with other atoms to gain or lose electrons and achieve a full outer shell
• Arranging electrons in an atom

  1. Electrons are arranged in shells around the nucleus

  2. First shell can hold 2 electrons

  3. Second shell can hold up to 8 electrons

  4. Electrons are added in a clockwise pattern to fill the shells
• Sodium has 11 protons and 11 electrons
• Sodium's outer shell only has 1 electron, making it unstable
• Noble gases


  Have completely full outer shells and do not react
• Determining electron arrangement of an atom

  1. Find the atomic number

  2. Draw the shells around the nucleus

  3. Fill the shells with electrons following the pattern
• Argon has 18 protons and 18 electrons
• Argon's electron arrangement is 2,8,8
• Calcium has 20 protons and 20 electrons
• Calcium's electron arrangement is 2,8,8,2
• Calcium loses 2 electrons to become a stable calcium ion with a 2+ charge
• Fluorine has 9 protons and 9 electrons
• Fluorine's electron arrangement is 2,7
• Fluorine gains 1 electron to become a stable fluoride ion with a 1- charge