Question

If the atomic number of an element is 15 and the atomic mass is 31, how many protons, neutrons, and electrons does it have?
Can some one please explain this concept to me? I am so lost. :(

Answer 1

protons: 15
neutrons: 31-15 = 16
electrons: 15

Answer 2

The protons and the electrons are always what the atomic number is. To find how many neutrons you simply take the number of protons (or electrons) away from the mass number.

Answer 3

An atom is defined by its protons. An atom can't gain or lose protons, that's nuclear fission and fusion and when that happens it becomes a different element. Ions (charged atoms) are defined by a lack or abundance of electrons (electrons doesn't equal protons). If you have an atom with one proton and two electrons, you don't have a helium ion that is negatively charged because it lost a proton, you have a hydrogen ion that is negatively charged because it gained an electron. If you have 6 protons, it's carbon. End of story. The number of protons doesn't change. Ever. Regular atoms (not ions) will have a net charge of 0 because they have a number of electrons equal to their number of protons and the charges cancel each other out. So, unless you are given a charge, assume that the number of protons and electrons are equal. Atomic number is literally the number of protons. They use the number of protons to number the atoms because the protons don't change. Atomic number IS the number of protons.

In the same way that atoms can have different numbers of electrons to become ions, they can have different numbers of neutrons to become different isotopes. The atomic mass is the number of protons plus the number of neutrons. A single proton and neutron have the same mass, and an electron has virtually no mass, so it turns into simple addition. If you have an atomic mass of 31, it means there are 31 particles in the nucleus. If the atomic number is 15, that means 15 of the particles are protons. So, logically, the rest must be neutrons. Then you do subtraction.

Answer 4

@nanocreation76
Perfect answer!