If an element has 7 electrons how many of them will be in the 2nd energy level

There are 5 electrons present in the second energy level.

Explanation:

The electronic configuration an element with 7 electrons is give as:

Second energy level means that the second principle quantum number.

There are 2 electrons in first energy level and 5 electrons present in the second energy level.

b.

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what experimental evidence led scientists to change from the previous model to this one?

a few of the positive particles aimed at a gold foil seemed to bounce back.

the colors of light emitted from heated atoms had very specific energies.

experiments with water vapor showed that elements combine in specific proportions.

cathode rays were bent in the same way whenever a magnet was brought near them.