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.

To determine the mass in grams of of nickel, we first need to convert atoms to moles. To do this we need to know that . Then we will convert moles to grams using the atomic mass of nickel, which can be found on the periodic table. The atomic mass of  is . 

Now we have all of the information we need to perform the conversion:

The conversion of grams to atoms is a two-step process. First, we need to convert grams to moles using the molecular mass of sulfur. The molecular mass of sulfur can be found on the periodic table and is equal to 

Then we will convert moles to atoms using our knowledge that . Using all this information we will perform the following conversion:

In a chemical reaction, the limiting reactant determines how much product can be created. Given two reactants in different quantities, the limiting reactant is defined as the reactants that cannot fully convert the given amount of the other reactant to product.

The mass, number of moles, volume, and partial pressure of the reactants can help to identify the limiting reagent, but cannot be used without a given chemical equation with molar ratios. For example, imagine that fifty moles of reactant A are needed to react two moles of reactant B. You are given 49 moles of reactant A and 2 moles of reactant B. Even though you have more mass, volume, and moles of reactant A, it is still the limiting reactant because it cannot fully convert reactant B to the product.