Most of these sorts of problems begin with knowing the information on the Periodic Table. It is usually in the back of the book, but if it is not, click this picture:
Every element has a name, an atomic number, a symbol, and an average mass. The average mass is useless for now. Let's focus on the other three.
The name usually comes from tradition, although there are many rules for naming elements.
The atomic number is the number of protons, also in uncharged or neutral atoms it is the number of electrons.
The symbol is a shorthand abbreviation for the atom. It is used rather than write out the chemical names over and over.
Curveball 1: Synonyms
The atomic number is the number of protons is the atomic number. In a neutral atom this number is also the number of electrons. In the molybdenum example above all of these numbers are 42.
Curveball 2: Atomic weight aka atomic mass
Atomic weight = number of protons + number of neutrons
Protons and neutrons each have a mass of one and electrons a mass of zero. While this isn't perfectly true it is true enough for non-nuclear physics purposes. To find the atomic weight add the protons and neutrons together.
The number you end up with is usually anywhere between 2x and 3x the atomic number (except for very heavy elements), so its easy to know if you're right.
Sometimes the atomic weight is given in the problem and the number of neutrons is blank. Thanks to algebra we can rearrange the above equation to:
Number of neutrons = atomic weight - number of protons
Curveball 3: What is its name?
The naming system for these goes NAME-MASSNUMBER.
For example Molybdenum-97, Uranium-235, Lead-210.
http://phet.colorado.edu/en/simulation/build-an-atom
Drag the protons, neutrons, and electrons from the bins on the lower left to the empty space. The Periodic Table will light up with the atom you are making.
On the right there are some plus symbols. Click the symbols plus and the symbol, with protons in lower left, mass number in upper left, and charge in the upper right will appear. The other symbols display the atomic weight and charge.
If the atom is unstable it is because it has too many neutrons or too few. Fix it and it will become stable.
Once you think you've got it go on to the game portion of the simulation.
Atoms don't usually change in normal situations. If you have an atom of one type it is very likely to stay that type of atom. They can be changed by primarily nuclear processes which occur if an element has an unstable nucleus. Electrons are the subatomic particle that is most easily changed. These are what the changes mean.
A change in protons changes the element identity.
A change in neutrons changes the element's atomic weight, but does not change the identity. Isotopes (same protons, different neutrons) have all the same properties and behaviors of the atom type save one, atomic mass. Isotopes of a few elements possess special properties useful in magnetic resonance imaging. Heavier isotopes react a little slower.
A change in electrons can make an atom have a charge. These are ions. More protons than electrons is a positive charge, more electrons than protons is a negative charge.
Protons change Elements
Neutrons change Isotopes
Electrons change Ions
