Metals tend to lose electron(s) and become cations (positively charged ions). Recall, this pattern for the formation of anions and cations: For anions, the charge tells how many extra electrons there are compared to the number of protons. If it is a cation, then the positive indicates how many more protons it has compared to the number of electrons. The first thing you need to remember here is the charge of the ion is a result of an imbalance between the number of protons and electrons. Therefore, you can determine the number of protons if the number of electrons is given.įor example, identify the element of an atom with 54 electrons.Īll we need to do is look up the element with the atomic number 54, and that is Xe. P = 35 – 18 = 17, and therefore, the element is Clįor a neutral atom, the number of protons and the number of electrons are equal. We know that the mass number (A) = number of protons + the number of neutrons, and therefore, the number of protons is equal to: In this case, the mass number must be given.įor example, how many protons does the element with a mass number 35 if it has 18 neutrons? Suppose you are not given a periodic table, and the questions asks to determine the number of protons from the number of neutrons. Pd is always going to have 46 protons as the atomic number is the identity of the element. Therefore, Ca has 20 protons.ĭo not get confused if the element is given with some other data such as the atomic mass or the average atomic mass.įor example, how many protons does the following isotope of palladium have?Īgain, we don’t care what isotope it is, what mass, and how many neutrons it has. In an easy example, you may be asked to simply determine the number of protons for the given element.įor example, how many protons does Ca have?įor this, you just need to look up the atomic number of Ca, which we find, from the periodic table, that it is 20. Now, remember, isotopes are atoms with the same number of protons but different numbers of neutrons and therefore, the difference in mass numbers comes from the different number of protons. If you are given data stating that the atomic mass of potassium is, for example, 39, then this is referred to that specific isotope. This is the average atomic mass of all the isotopesof potassium and that is why it is not a whole number.

Next, on the button, we have the average atomic mass of potassium which is equal to 39.098.

There cannot be another element with the atomic number 19. So, potassium has 19 protons, and this is true for all its isotopes. This is the number usually given on the top of the symbol, and, in this case, it is 19. The first, you want to identify is the atomic number of the element because this is unique for each element. For example, if we look up potassium, K in the periodic table, we will see the following numbers:

Before we start with some examples of calculating the number of protons, neutrons, and electrons, let’s recall what information we get about a particular element from the periodic table.

Mn element protons