This wikiHow teaches you how to add protons, neutrons, and electrons, and what to do if ions are present.
Step
Part 1 of 2: Counting the Number of Protons, Electrons and Neutrons
Step 1. Find the periodic table of the elements
The periodic table is a table that arranges the elements based on their atomic structure. This table is color coded and has a unique 1, 2, or 3-letter abbreviation for each element. Other elemental information includes weight and atomic number.
- You can look up the periodic table online or in chemistry books.
- Usually, a periodic table will be provided during the exam.
Step 2. Find your element in the periodic table
The periodic table ranks the elements by atomic number and separates the elements into three main groups: metals, nonmetals, and metalloids (semimetals). Further classifications of elements include the alkali metals, halogens, and noble gases.
- Groups (columns) or periods (rows) can make it easier to find elements in a table.
- You can also look up the element symbol in the table if you don't know the other properties.
Step 3. Find the atomic number of the element
The atomic number is above the element symbol, in the upper-left corner of the box. The atomic number indicates the number of protons that make up a single atom of an element.
For example, boron (B) has an atomic number of 5. Thus, boron has 5 protons
Step 4. Determine the number of electrons
Protons are particles in the nucleus or nucleus of an atom that have a positive charge. Electrons are particles that have a negative charge. Thus, an element in a neutral state has the same number of protons and electrons.
- For example, boron (B) has an atomic number of 5. Thus, boron has 5 protons and 5 electrons.
- However, if an element has negative or positive ions, the number of protons and electrons will not be the same. You have to count the number. The ionic number is a small number located behind the element.
Step 5. Find the atomic mass of the element
To find the number of neutrons, you must first find the atomic mass. The atomic mass of an element (also referred to as atomic weight) is the average atomic mass of an element. The atomic mass can be found at the bottom of the element symbol.
Make sure that you round the atomic mass to the nearest whole number. For example, the atomic mass of boron is 10.811, but you can round the atomic mass to 11
Step 6. Subtract the atomic number from the atomic mass
To find the number of neutrons, you must subtract the atomic number from the atomic mass. Remember that the atomic number is the number of protons you are looking for.
For the boron example, 11 (atomic mass) – 5 (atomic number) = 6 neutrons
Part 2 of 2: Counting Electrons Based on Number of Ions
Step 1. Find the number of ions
The number of ions in an element is written in small numbers after the element. Ions are atoms that have a positive or negative charge due to the addition or removal of electrons. Although the number of protons in an atom remains the same, the number of electrons changes in an ion.
- Since electrons have a negative charge, as you lose electrons, the ions become more positive. As you add more electrons, the ion becomes more negative.
- For example, N3- has a -3 charge, while Ca2+ has a charge of +2.
- Keep in mind that you don't have to do this calculation if there isn't a small ion number behind the element.
Step 2. Subtract the charge on the ion from its atomic number
When an ion has a positive charge, the atom loses electrons. To calculate the number of electrons left, you subtract the total charge from the atomic number. In the case of positive ions, there are more protons than electrons.
For example, Ca2+ has a +2 charge so the ion loses 2 electrons from its neutral state. The atomic number of calcium is 20. Thus, the ion has 18 electrons.
Step 3. Add the charge of the ion to the atomic number for the negative ion
When an ion is negatively charged, it means the atom is gaining electrons. To calculate the total number of electrons present, you just need to add the charge of the ion to its atomic number. In the case of negative ions, the number of protons is less than the number of electrons.
