Which of the following are correct statements? (choose all that apply) a. the basic unit of matter is the atom. b. the number of protons an atom has defines what kind of element it is. c. the atomic number is the same for all elements d. an element is defined by the number of neutrons in its nucleus.

The answers are A and B.

Atoms makes up matter, everything that occupies space and has a mass is matter. A is correct. As long as scientists discovered, there is more than 100 elements in the world.

Each element has different numbers of protons, neutrons, and electrons. They're all different in all elements and is not repeated. For example, hydrogen has 1 proton and 1 electron. No other elements has 1 proton and electron. Therefore, B is correct.

The number of protons in an atom can be called the atomic number. As already said, each atoms in elements has a different number of protons, therefore C is not correct.

In the atoms of the same kind of element, they may have different numbers of neutrons. They're called Isotopes. For example in hydrogen atoms can have no neutrons, 1 neutron, or 2 neutrons. Therefore, d is not correct.

Determine the  wavelength of light emitted when an electron  in an li2+ ion  makes a transition from an orbital with n  =  6 to an orbital with n  = 5.