Final answer:
To determine the number of protons in an atom of barium, look for its atomic number on the periodic table, which is 56. This means a barium atom contains 56 protons in its nucleus. The atomic number is always equal to the number of protons, regardless of the mass number or whether the atom forms an ion.
Step-by-step explanation:
The atomic number of an element, which is the number of protons in the nucleus of an atom, serves as a unique identifier for that element in the periodic table. For example, as you mentioned, boron has an atomic number of five, meaning it has five protons.
For the element barium, you can find its atomic number on the periodic table. Barium has an atomic number of 56. This means that there are 56 protons in the nucleus of a barium atom. When determining the number of neutrons in an atom, you can subtract the atomic number from the mass number. For barium, if you know its mass number (which is typically the atomic mass rounded to the nearest whole number), you would do the calculation: mass number - 56 = number of neutrons.
Remember, the atomic number is what defines the element, not the mass number. Even when an atom loses and gains electrons to form ions, such as the barium cation (Ba2+), the number of protons—and consequently the atomic number—remains unchanged.