Electron Arrangements

 No Brain Too Small  Science  AS 90944 Acids and Bases Problem You want to draw a diagram of an atom or an ion that shows the arrangement of the electrons (electronic configuration) for the elements between atomic numbers 1 to 20. Solution Use the Periodic Table to find the atomic number of the element and the number of electron shells available to determine the number of electrons available to place on the diagram. Then place electrons into their shells following the 2.8.8 rule (see discussion below). Discussion • For atoms – the number of electrons to place is the same as the atomic number • For positive ions (+) the number of electrons to place is the (atomic number – the number of positive (+) charges on the symbol). • For negative ions (-) the number of electrons to place is the (atomic number + the number of negative charges on the symbol). • Then place electrons starting from the electron shell closest to the nucleus and place as follows: st o 1 electron shell – up to 2 electrons maximum. When this is full move onto the second electron shell. nd o 2 electron shell – up to 8 electrons maximum. When this is full move onto the third electron shell. rd o 3 electron shell – up to 8 electrons maximum * When this is full move onto the fourth electron shell. th o 4 electron shell – you will only ever need to place 1 or 2 electrons in this shell at L1 NCEA Science. • *Note – at senior Chemistry level this “rule” does not apply.

Worked Example 1: Write and draw the electron configuration for a sodium atom Na From the Periodic Table we see Na has the atomic number of 11. It is also in group 1 (meaning 1 electron in the outer electron shell) and period 3 (meaning 3 electron shells are being used). Atoms have the same number of electrons as protons in the nucleus (in this case, 11).

Shorthand form is written as Na 2.8.1 Diagram form

 No Brain Too Small  Science  AS 90944

Worked Example 2: Write and draw the electron configuration for a sodium ion Na+ The ion has a +1 electrical charge, so subtract 1 electron from the Na atom configuration. Ions form when valence (outer) shell electrons are lost or gained – so we take away the outer electron. Shorthand form is written as Na+ 2.8 (the outer shell is now full) Diagram form is

Worked Example 3: Write and draw the electron configuration for an oxide ion O2- The ion has a -2 electrical charge, so add 2 electrons to the O (oxygen) atom configuration. Ions form when valence (outer) shell electrons are lost or gained – in this case we need to add the 2 electrons to the outer (valence) electron shell. Oxygen has an atomic number of 8 and is in the 2nd period of the Periodic Table, telling us that 2 electron shells are being used to place all the electrons. Shorthand form is written as 2.8 (the outer shell is now full). Diagram form is

Important note! When you have given the electron configuration (electron arrangement) of any ion it must have a full outer shell of electrons. Ions are drawn with square brackets around them with the charge written as a superscript.

 No Brain Too Small  Science  AS 90944

Try these Complete the table to give the electron arrangement of the following atoms and ions. Use shorthand and a diagram.

 No Brain Too Small  Science  AS 90944 Answers

K 2,8,8,1 K+ 2,8,8 F 2,7

N3- 2,8 Ne 2, 8 Ca 2,8,8,2