The electron arrangement of an atom gives the number of electrons in each energy level. Now recall, as the value of n increases, then both the size and energy level of an atomic orbital will also increase. And we're going to say as we increase the energy levels, the number of electrons within a given orbital will also increase. So, for example, if we have electrons in shells n25, n5 is a higher energy level so we'd expect it to have more electrons than an energy level of n2. Now the energy level, shell numbers, of an atom can be tied to the period or rows of the periodic table. So these are things that we have examined before when it comes to the atom itself, but now we're going to apply them to electron arrangements. So now click on the next video and let's take a look at an example question.
Electron Arrangements - Online Tutor, Practice Problems & Exam Prep
Electron Arrangement gives the number of electrons in each energy level (n).
Electron Arrangements Concept 1
Video transcript
Electron Arrangements Example 1
Video transcript
So here we have to complete the electron arrangements for the following elements of the periodic table. Alright. So we're going to start out with hydrogen, which has an atomic number of 1, which means it has only 1 electron. So its electron arrangement is simply just 1. Helium is 2 because it has 2 electrons because its atomic number is 2. Alright. So now what's going to start happening is we're going to start adding more and more electrons. Remember, in the first shell, we can hold a maximum of 2 electrons, and that's because 2 times n squared. In the second shell, we can theoretically hold up to 8 electrons. So now we're at lithium. Lithium has an atomic number of 3. The first two electrons are in the first shell, this space here. So now we're talking about electrons in the second shell, so dash, how many is that? 1. And it's 1 because again its total atomic number is 3, which means it has in total 3 electrons. We've accounted for the first 2 in the first shell, and then this third one is in the second shell. Then we move over, we go from lithium, then beryllium, beryllium will be 2<sub>1</sub>2<sub>2</sub>. Now let's go to, boron here. So boron here would be 2<sub>1</sub>3<sub>2</sub>, right? Because its atomic number is 5, so it can have 5 total electrons. 2 are in the first shell because the first shell can only hold a maximum of 2. The remaining 3 that we need are in the second shell. Then we have carbon, nitrogen, oxygen. Let's look at fluorine here. Ford's atomic number is 9. That means it has 9 total electrons. 2 of them are in the first shell and then the other 7 are in the second shell. Let's keep going. Alright. So for sodium, sodium has an atomic number of 11 on the periodic table. The first 2 are in the first shell, the next 8 are in the second shell. We need one more electron and it will be here in the 3rd shell. Let's keep going. Skip over to aluminum. Aluminum has an atomic number of 13. So we have 2 electrons in the first, 8 electrons in the second shell, and 3 in the 3rd shell.
All Right. When it comes to depicting the number of electrons in each of these shells, we're going to say that the second shell can have up to 8. And when we're doing electron arrangements, we're going to say, yes, theoretically, that the 3rd shell can hold up to 18 electrons, but here, in purposes of an electron arrangement, we're going to say we go up to 8. So potassium has an atomic number of 19. So it has 2 electrons in the first shell, 8 in the second, 8 in the third, and we need one more to get to 19, so it has one in the 4th shell. So here we'd have our electron arrangements of different elements on the periodic table. And what's important to know is that electron arrangements are simple as long as we keep them that way. We're going to say here that elements beyond an atomic number of 20 can have partially filled orbitals and are beyond the scope of this course. So really only need to know up to calcium. All the remaining elements, you don't need to worry about. Right? So keep this in mind. When it comes to electron arrangements, the maximum we can hold in the first shell is 2, in the second shell 8, in the 3rd shell 8, and in the 4th shell 2. Those are the maximum numbers that we can have electrons in each one of those 4 shells.
Write the electron arrangement for the following element:Calcium (Z = 20)
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