Arranging Electrons

Electron Arrangement - The Rules:

1. Represent the Nucleus with the Element Symbol
2. Find the Element in the Periodic Table - and then find the Row (or Period) that this element is on (Potassium is on Period 4)
3. The number of Circles (shells) surrounding the nucleus is the same as the Row number - Potassium is on Row 4 and so has 4 circles - Draw the circles - label these 1 to 4 with the 1st circle being the inner most circle and the fourth circle being the farthest out
4. You can now 'Read' the periodic table from left to right - start at the top left (at Hydrogen) like you would if you were reading a book - move left to right along Row 1 - when you encounter a new element place a X on Circle 1
   • When reading left to right on row 1 you should encounter Hydrogen and Helium - this would mean you would have two crosses on the inner most circle if you have done this correctly
5. Now read along Row 2 - From Lithium to Neon - Add the number of elements you encountered as crosses on Circle 2
   • If you have done this correctly you should have 8 Crosses on Circle 2
6. Repeat this for Row 3 - From Sodium to Argon - Add the number of elements you encountered as crosses on Circle 3
7. Row 4 is the final circle for Potassium - for the final Row - Read left to right UNTIL YOU ENCOUNTER YOUR ELEMENT - you then include your element as the FINAL CROSS on your Diagram
   • If you have done this correctly The outermost circle should only have 1 cross on it
8. Finally - represent the number of electrons in each shell with a number to write this structure in shorthand
   • Example - Potassium should have 2 crosses in circle 1, 8 crosses in circle 2, 8 crosses in circle 3 and 1 cross in circle 4
   • This would be 2,8,8,1

See below for Electron Arrangements of the First 20 elements and a video summarising this

You should be able to spot some repeating patterns if you can draw the first 20 elements electron arrangements:

• All elements found in the first column (called Group 1) have 1 electron on the outer shell (Outer shell electrons are called Valence Electrons)
• In fact - the group number = the number of electrons in the outer shell
  • EG - Oxygen is in Group 6 and has 6 Valence Electrons in it's outer shell
• The elements found in Group 0 (or 8) have Full Shells of Electrons

Changing Numbers of Electrons - Ionic Theory

Earlier we learned that you cannot change the number of protons of an element as this would change the element

However Atoms can SHARE, LOSE OR GAIN ELECTRONS in order to become more STABLE and this explains why atoms join together to form molecules and compounds

All atoms are more stable when they have FULL OUTER SHELLS - this is called the STABLE OCTET THEORY.

So atoms of elements that do not have full numbers of electrons in their outer shells are not as stable as they could be - these atoms can either SHARE electrons with another atom OR LOSE/ GAIN ELECTRONS to make their outer shell complete.

In this theory - the Maximum number of electrons in a full shell is:

• 2 in Row 1 (circle 1)
• 8 in Rows 2 and 3 (circles 2 and 3)

How do we decide if atoms Lose or Gain electrons?

Let's take Potassium as an example:

• Potassium has one electron in it's outer shell
• Is it easier for Potassium to Lose 1 electron or Gain 7 electrons?
• The answer is to Lose one electron
• So Potassium Loses it's outer shell electron - leaving a Full third shell
• Potassium would go from:
• K K +1 ion
• P = 19 P = 19
• E = 19 E = 18
• Charge on atom . = +19-19=0 =+19-18 = +1
• This is shown below in the diagram:

Here you can see that the Potassium atom has lost it's outer electron to become a Positively Charged Particle - all Charged particles are called IONS - Positive Ions are called CATIONS

The Positive charge is represented by the + sign in the top right hand corner and surrounded by Square brackets

ALL METALS LOSE THEIR OUTER SHELLS TO FORM POSITIVE IONS

But what about elements that gain electrons?

Let's take Chlorine as an example:

• Chlorine has Seven electrons in it's outer shell
• Is it easier for Chlorine to Gain 1 electron or Lose 7 electrons?
• The answer is to Gain one electron
• So Chlorine Gains an electron in the outer shell - resulting in a Full third shell
• Chlorine would go from:
• Cl Cl -1 ion
• P = 17 P = 17
• E = 17 E = 18
• Charge on atom . = +17-17=0 =+17-18 = -1
• This is shown below in the diagram:

Here you can see that the Chlorine atom has gained one electron to become a Negatively Charged Particle - all Charged particles are called IONS - Negative Ions are called ANIONS

The Negative charge is represented by the - sign in the top right hand corner and surrounded by Square brackets

ALL NON-METALS GAIN ELECTRONS IN THEIR OUTER SHELLS TO FORM NEGATIVE IONS

Finally - As the periodic table can tell us how many electrons are found in the valence shell we can use this knowledge to understand how many electrons an atom can gain or lose - as this will be the same number every single time (for now) we can use the Group number to work out IONIC CHARGE

• For Group 1, 2 and 3 elements it is easier for these atoms to LOSE electrons and so they have +1 charges (for group 1) +2 charges (for group 2) and +3 charges (for group 3) - THESE ARE ALL METALS
• For Group 5, 6 and 7 elements it is easier for these atoms to GAIN electrons and so they have -1 charges (for group 7) -2 charges (for group 6) and -3 charges (for group 5) - THESE ARE ALL NON-METALS

For a summary of Ionic Theory please view the below video