Is there a formula or equation I can use that helps predict which orbitals gets filled first? In other words, how I can determine that 4s fills before a 3d orbital without having to memorize it? thanks!

1s
2s 2p
3s 3p 3d
4s 4p 4d 4f
5s 5p 5d 5f ...


Usually that's as far as you'll have to go.

Don't read this left to right, read diagonally, top right to bottom left.

1s...2s...2p...3s...3p...4s...3d...4p...5s...4d... etc

Orbitals will fill in that way. But I don't think there's an equation. HTH

When I had trouble with this I checked back in my genchem book and that helped. There are 2 ways to do this. 1. you can look at the periodic table and see that orbitals fill in order as to their location on the table. For example, 4s comes before 3d, so 4s fills first (try to find a figure in the book that depicts this and you will understand it when you see it).

2. There is also the (n +l) formula. 4s=(4+0)=4 3d=(3+2)=5. 4s has lower energy, thus it fills first. n=principal quantum # and l is the subshell (s,p,d or f, =0,1,2,3)

hope that helps.

ps be aware that a typical mcat question will be about electron configuration, and they may give you a transition metal where the 3d orbital fills before the 4s orbital. regardless, the main thing you have to know is how many electrons the element has. then you can easily figure out the answer.

^That's the best way. I've always used the 'n+l' formula. As long as you use that, you won't have to worry about which orbital fills first; it's all counting from there.

When I had trouble with this I checked back in my genchem book and that helped. There are 2 ways to do this. 1. you can look at the periodic table and see that orbitals fill in order as to their location on the table. For example, 4s comes before 3d, so 4s fills first (try to find a figure in the book that depicts this and you will understand it when you see it).

2. There is also the (n +l) formula. 4s=(4+0)=4 3d=(3+2)=5. 4s has lower energy, thus it fills first. n=principal quantum # and l is the subshell (s,p,d or f, =0,1,2,3)

hope that helps.

ps be aware that a typical mcat question will be about electron configuration, and they may give you a transition metal where the 3d orbital fills before the 4s orbital. regardless, the main thing you have to know is how many electrons the element has. then you can easily figure out the answer.

That's interesting. I stick with the triangle type thing shown on top since that's how I learned it in high school and college. EK also highlights the triangle one if you have those books.

When I had trouble with this I checked back in my genchem book and that helped. There are 2 ways to do this. 1. you can look at the periodic table and see that orbitals fill in order as to their location on the table. For example, 4s comes before 3d, so 4s fills first (try to find a figure in the book that depicts this and you will understand it when you see it).

2. There is also the (n +l) formula. 4s=(4+0)=4 3d=(3+2)=5. 4s has lower energy, thus it fills first. n=principal quantum # and l is the subshell (s,p,d or f, =0,1,2,3)

hope that helps.

ps be aware that a typical mcat question will be about electron configuration, and they may give you a transition metal where the 3d orbital fills before the 4s orbital. regardless, the main thing you have to know is how many electrons the element has. then you can easily figure out the answer.

number 2 works, just remember to fill lower p before higher s, because according to the equation, 2p = 3s, 3p = 4s, and so on.

thanks so much for the feedback guys