All basic elements are made up of electrons, protons, and neutrons. Electrons are negatively charged particles that make up one part of an atom. The ability to determine the number of electrons an atom contains is a fundamental concept in chemistry. This can be done easily using a periodic table of elements. Among other important concepts are how to determine the number of neutrons and valence electrons (the number of electrons in the outer shell) of an element.
Method 1 of 2: Determine the Number of Electrons in Neutral Atoms
Step 1. Get a periodic table of elements
This is a color-coded table in which all known elements are organized by their atomic structure. Each element has an abbreviation of either one, two, or three letters and is listed along with its weight and atomic number.
It is easy to get periodic tables either in chemistry books or online as well
Step 2. Find the element in question on the periodic table
Elements are organized by atomic number and are divided into three main groups: metals, nonmetals, and metalloids (semi-metals). Then they are grouped by families, including alkali metals, halogens, and noble gases. Each column in the table is known as a "group" and each row is known as a "period."
- It will be easier to locate the item if you know the details (for example, the group or period it is in).
- In case you don't know anything about the element in question, you just have to look for its symbol in the table until you find it.
Step 3. Find the atomic number of an element
This number appears in the upper left corner of the box or in the center, above the element symbol. The atomic number denotes the number of protons in that particular element. Protons are the particles of an element that provide it with a positive charge. Electrons have a negative charge, so if an element is in its neutral state, it will contain the same number of protons and electrons.
For example, the atomic number of boron (B) is 5, so it has 5 protons and 5 electrons
Method 2 of 2: Determine the Number of Electrons in Negatively or Positively Charged Ions
Step 1. Identify the charge on the ion
Adding or removing electrons from an atom does not change its identity but changes its charge, in which case the atom is now an ion (for example, K+, Ca2+ or N3-). Charge is usually expressed by a superscript on the right hand side of the atom's abbreviation.
- Electrons have a negative charge, so adding an electron makes the ion more negative.
- By removing electrons, the ion becomes more positive.
- For example, N3- has a charge of -3 and Ca2+ has a charge of +2.
Step 2. Subtract the charge from the atomic number in case the ion is positive
If the charge is positive, this means that the ion has lost electrons, so if you want to determine the number of electrons left, you must subtract the charge from the atomic number. In this case, there are more protons than electrons.
- For example, because the charge of Ca2+ is +2, it has 2 fewer electrons than a neutral calcium atom. The atomic number of calcium is 20, so this ion has 18 electrons.
Step 3. Add the charge to the atomic number in case the ion is negative
If the charge is negative, this means that electrons have been added to the ion, so if you want to determine the total number of electrons present, you must add the charge to the atomic number. In this case, there are fewer protons than electrons.
- For example, because the charge of N3- is -3, it has 3 more electrons than a neutral nitrogen atom. Nitrogen's atomic number is 7, so this ion has 10 electrons.