The atomic number is equal to the number of protons in an atom and can be determined using the periodic table. The atomic number is located on the top left of the box containing the desired chemical symbol. The chemical symbol for calcium is Ca and the atomic number is 20.

The 42 in calcium-42 represents the mass number of calcium. The mass number equals the sum of the number of protons and the number of neutrons. Using the periodic table, the number of protons in calcium can be determined using the atomic number. The atomic number of calcium is 20. 

Using all this information the number of neutrons can be determined by subtracting the number of protons from the atomic mass number. The number of neutrons is 22.

Atomic Mass Number = Protons + Neutrons

Atomic Mass Number - Protons = Neutrons

The mass number is equal to the number of protons plus the number of neutrons in an atom. 

Protons + Neutrons = Mass Number

The number of protons is equal to the atomic number of an element. The atomic number of an element can usually be found on the top right of the element box in the periodic table. The chemical symbol for potassium is K. Using all this information the atomic number of  is 19, therefore the number of protons in potassium-40 is 19.

To determine the chemical symbol of an atom we can use the atomic number because no two elements have the same atomic number. The atomic number is equal to the number of protons, and is usually located on the top left of the element's box in the periodic table. This atom has 8 protons, therefore the atomic number is 8. Using this information it can be determined that the chemical symbol for an atom with 8 protons is O, oxygen.

The elements are arranged based on their atomic number. The atomic number can usually be found on the top left of each element's box. The atomic number increased from left to right as you move across the periodic table. 

To determine the mass in grams of  of magnesium, we need to know the molecular mass of magnesium, which can be found on the periodic table.

The molecular mass of magnesium is  . Now we have all the information we need to convert moles to grams:

In the above conversion, the moles cancel to leave the answer in grams. 

To determine the number of moles in  of potassium, we need to convert grams to moles. To convert grams to moles we need to determine the atomic mass of potassium using the periodic table.  The atomic mass of potassium is . 

Using this information we can convert grams to moles: 

To convert atoms to moles we need to know that there are  atoms in one mole. 

To determine the how many atoms are in  of aluminum, we need to also know that there are  atoms in one mole.