How can you find the number of protons and electrons and neutrons in an atom or ion?

Number of protons = Atomic number
Number of electrons = Atomic number minus the charge (if it is ion)
Number of neutrons = Mass number minus Atomic number

• Number of Protons:

Look at the periodic table. Each element is assigned a number called the atomic number -- it is usually written in large on the periodic table. That number tells you the number of protons in the nucleus of each element. The atomic number is always the same as the number of protons. The number of protons in the nucleus uniquely identifies an element.

• Number of Electrons:

The number of electrons in an element can change. For a neutral atom (net charge of zero), the number of protons is exactly equal to the number of electrons. So the number of electrons is also the same as the atomic number.

However, it is possible to remove electrons and not change the identity of an element. These are called ions. The charge on the ion tells you the number of electrons -- the number of electrons is the atomic number minus the charge on the ion if it's an ion. If the charge is positive, subtract that number from the atomic number to get the number of electrons (with a positively charged ion, you will then have less electrons than protons).

If the charge is negative, subtract the charge (but note you are subtracting a negative number, which is the same as adding the magnitude of the charge to the atomic number (with a negatively charged ion, you with then have more electrons than protons).

• Number of Neutrons:

The number of neutrons in an element can also vary, and if two atoms of the same element have different numbers of neutrons, then they are called isotopes. To figure out how many neutrons are in the nucleus, you must know the mass number. Note that you cannot determine the number of neutrons for an element, only for one isotope of that element -- so you have to know which isotope you are being asked about.
Usually, a specific isotope is written like this: silicon-30 or carbon-12 or aluminum-26. It can also be written like this though, when using the symbol for the element instead of its full name: ³⁰Si or ¹²C or ²⁶Al. The number is called the mass number, and the mass number is equal to the sum of the number of protons plus the number of neutrons.

So if you know the mass number and you know the atomic number (if you know the name of the element, that tells you the atomic number because each element has only one atomic number), to find the number of neutrons, subtract the mass number from the atomic number.

Here are some examples:

• The element hydrogen (H) has atomic number of 1, so it has one proton in the nucleus, and one electron. The hydrogen ion (H⁺) has zero electrons, and the hydrogen ion (H^-) has 2 electrons. Most hydrogen atoms don't have any neutrons (these are written ¹H), but there exists an isotope called deuterium (also ²H) which has one neutron, and another isotope called tritium (also ³H) which has 2 neutrons.
  

• The element fluorine (F) has atomic number 9, so it has nine protons. The neutral atom also has 9 electrons, but F is commonly found as a negative ion F^-, which has 10 electrons. The most common isotope of fluorine has 10 neutrons, and so it has a mass number of 19, and is written Fluorine-19 or ¹⁹F. There also exists another isotope, fluorine-18 or ¹⁸F, which only has 9 neutrons, but this isotope is not stable.