Finding the electron dot structure of an element is the first step to understanding how elements bond and form ions. This can be done with a degree of ease for the first 20 elements following several simple steps.
These steps result in workable electron dot diagrams, which are also called Lewis Dot Diagrams. For a more detailed discussion see electron subshell filling and the electron dot diagrams based on that approach. Once the basics have been mastered as they are presented here, those two pages will add an extra level of understanding to this topic.
Steps For Making Diagrams
Number of Electrons: These dot diagrams are made for neutral atoms, meaning atoms that have the same number of electrons as protons. The number of protons (also called the Atomic Number) of an element makes it what it is. All we need to do is find the element on the Periodic Table and read its Atomic Number:
So for example, Fluorine has the symbol F and has 9 protons. That means this element in its neutral state has 9 electrons. For Aluminium, with the symbol Al, there are 13 protons and so we have 13 electrons.
The Filling Pattern: Atoms can be viewed as onions. The nucleus is a solid core, and that core is covered by shells of electrons. Just as the outside layer of an onion is the only one that goes brown, so it is only the outer shell of an atom that reacts with other atoms. It makes sense then that we need to know how many electrons fit into each onion like shell surrounding the nucleus.
The pattern is 2, 8, 8, 2 for the first four shells. It is important to realise that this only applies for the first 20 elements; after this things become more complex. We need to be able to split the number of electrons an element has into these four layers.
The trick with this is to start closest to the nucleus and work your way outwards. The first shell holds 2 electrons and then is completely filled. The second holds 8, the third holds 8 and after this there are only 2 left over for the fourth shell. Of course the fourth shell can hold more than 2 electrons but we are limiting ourselves to the first 20 elements.
So let us consider Fluorine again. It has 9 electrons. 2 fit in the first shell, leaving 7. Then all 7 can be fitted into the second shell, giving an electron dot structure of 2, 7. Silicon has 14 electrons and so has an electron arrangement of 2, 8, 4.
Making A Diagram: Once we know the numbers of electrons in each layer, all we need do is draw them onto a basic picture of an atom like that at the start of the page. I will use Silicon as the example in the following steps.
Step 1: Draw a blank atom with the required number of shells around the nuclues. In the case of Silicon that is three shells, as we know the arrangement is 2,8,4.
Step 2: Draw a big red “X” through the middle of the picture. This splits the electron shells into quarters.
Step 3: For the first shell, both electrons go in the TOP QUARTER.
Step 4: For ALL OTHER shells, start at the top quarter and place one electron in each quarter, moving around clockwise until all the electrons for that shell are used up. Then proceed to the next shell until there are no electrons remaining.
Of course it is not necessary to redraw the picture with the addition of each electron. That was done to show the process as it unfolds.
Step 5: Redraw the atom showing the outermost shell only. This is now the lewis dot or electron dot structure of that element.
The First 20 Elements
Here is the electron dot structure for each of the first 20 elements. Only the last stage of each diagram is given, and then what it looks like when redrawn only with the outer shell.
Hydrogen: 1 electron
Helium: 2 electrons. Note that the two electrons in the first shell are put in the same quarter. This shows that they form a full pair and that there is no opportunity for the atom to bond with any other elements.
Lithium: 3 electrons. On the right of the image below, only the outermost shell is shown.
Beryllium: 4 electrons
Boron: 5 electrons
Carbon: 6 electrons
Nitrogen: 7 electrons
Oxygen: 8 electrons
Fluorine: 9 electrons
Neon: 10 electrons
Sodium: 11 electrons
Magnesium: 12 electrons
Aluminium: 13 electrons
Silicon: 14 electrons
Phosphorous: 15 electrons
Sulfur: 16 electrons
Chlorine: 17 electrons
Argon: 18 electrons
Potassium: 19 electrons
Calcium: 20 electrons. Note how the electron dot structure is repeating every eight elements.
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