what makes 4s orbital to have lower energy than 3d orbital

answer: the reason the 4s orbital is usually filled first is because the 3d orbital electrons feel a lot of repulsion between the other 3rd level electrons (those in 3s and 3p). the 4s orbital, while farther away, requires less energy for the electron to occupy because it is not repelled as much by the coexisting electrons.

air

in a tire you fill a tire with air

then a couple days later you check the air pressure in the tire

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