The answer should be the third option "number of protons and neutrons." The atomic mass of a element is equal to the amount of protons, and neutrons added together in that element. So for example if we are trying to find the atomic mass of hydrogen you would add the total amount of protons and neutrons together to find it's mass although it doesn't equal the exact amount of it's mass but it's close.
Hope this helps.
The Atlas. This is the topmost vertebra, and with the axis, it forms a joint that connects the skull to the spine. These two parts of the body (The Atlas and The Axis) are special, and different from normal vertebra, because they are made to allow a greater range of motion and movement in the head. I hope this helps! Also, google is always a helpful tool to use as well. :)
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The two gasses that make up 99% of the earth's atmosphere are nitrogen and oxygen.
Hope that helped! =)
