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Course: High school chemistry > Unit 1
Lesson 4: The periodic table and Lewis diagramsDetermine valence electrons using the periodic table
Learn how to determine the number of valence electrons for an element using the periodic table.
An atom's valence electrons are the electrons in its outermost shell. In the chlorine model below, the valence electrons are shown in .
The number of valence electrons determines most of an atom's chemical behaviors. So, it's important to be able to identify how many valence electrons atoms of different elements have.
Fortunately, we can use the periodic table to quickly determine the number of valence electrons for main group elements.
Valence electrons on the periodic table
Bohr models for the first three periods of the periodic table are shown below. The valence electrons are shown in .
Do you notice any patterns in the number of valence electrons?
The number of valence electrons increases going left to right across a period. But, with the exception of , the number of valence electrons remains the same going down a group.
For example, , , and are in group 1 and all have one valence electron. Both and are in group 15 and have five valence electrons.
This is why elements in the same group have similar chemical properties—they have the same number of valence electrons!
This pattern allows us to quickly identify the number of valence electrons for main group elements on the periodic table, even if we don't have Bohr models to reference. The diagram below shows the number of valence electrons (VE) for the main group elements.
To summarize, we can use an element's group on the periodic table to determine how many valence electrons its atoms have:
Main group | 1 | 2 | 13 | 14 | 15 | 16 | 17 | 18 |
---|---|---|---|---|---|---|---|---|
Valence electrons | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8* |
*except helium, which has 2
Want to join the conversation?
- Why doesn't helium have 8 electrons instead of 2?(6 votes)
- Since helium is the second element in the table, a neutral helium atom can't have 8 electrons as it only has two. The other elements have an atomic number greater than 8.(19 votes)
- so how many valence electrons does group 3-12 have? or do they have none at all?(5 votes)
- It varies. Also, they have a weird habit of being able to have more than one oxidation state. Iron (26) is in the 8th group and usually has a charge of +2 or +3.(7 votes)
- How many electrons can fill each shell?(3 votes)
- first shell is 2, second shell is 8, third shell is 18, and then 32.(5 votes)
- At12:29, what about the 2 8 8 method?(5 votes)
- How many electrons can fill each shell?(3 votes)
- 2, 8, 18, 32, 32, 18, 2. IT follows the rule of 2n^2, where n is equal to the position of the shell.(4 votes)
- What about the elements at the bottom who can't really fit in so they left it out? How much valence electrons do they have?(2 votes)
- They have three to four valence electrons, according to my research.(3 votes)
- Why is Helium so special and why doesn't this apply to transition metals?(3 votes)
- Hydrogen and helium are in the first period (row). Those elements only have one shell. The first shell only needs two electrons to get filled.(1 vote)
- This is the "1A,2A,3A..." scale right?(2 votes)
- What about the transition metals(2 votes)
- "The number of valence electrons for transition metals does NOT follow the same pattern as main group elements. This is due to how electrons fill in subshells of each shell.
If you're interested in learning more about this, explore electron configurations in AP Chemistry."(1 vote)
- Are the electrons always in the same spot in the rows or can it be different?(1 vote)