Joan and mike's teacher show them a picture of an atom and ask them to identify it using the periodic table. joan says the atom is carbon, but mike says it is lithium. who is right and why?

Mike would be right, since there are only 3 electrons in the shell, if shows that there are 3 protons. Lithium is the only element with 3 protons

Mass is the amount of matter in an object.  weight is the measurement of the pull of gravity on an object.  say for example, you travel to the moon which has a different gravitational field compared to the gravitational field of earth. (gravitational fields: earth = 1, moon = 0.165)  now when you are on the moon, the pull of gravity on you is different than on the earth. and as weight is the measurement of the pull of gravity on an object, that means yes, your weight can and will change on the moon.  however, on the moon, on earth or even on jupiter, the amount of mass inside of you remains the same. meaning your physical mass remains constant.  in conclusion it is possible for your weight to change as your mass remains constant.