# Number of Electrons in an Atom Chemistry Tutorial

## Key Concepts

• Electrons are found in the space surrounding an atom's nucleus.
• An electron carries a charge of -1
• Protons are found in the nucleus of an atom and carry a charge of +1
• An atom has no overall charge:

number of negative charges = number of positive charges

• The number of electrons surrounding an atom's nucleus is equal to the number of protons in the nucleus of the atom.
number of electrons = number of protons

number of electrons = atomic number (Z)

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## Finding the Number of Electrons in an Atom Using Atomic Number (Z)

A proton is a positively charged sub-atomic particle that is found in the nucleus of an atom.
Each proton carries a charge of +1.

An electron is a negatively charged sub-atomic particle that is found in the volume of space surrounding the nucleus of an atom.
Each electron carries a charge of -1.

An atom of an element carries no overall electric charge, that is, the overall charge on an atom is 0.
Since an atom is made up of positively charged protons in the nucleus and negatively charged electrons surrounding the nucleus this means that the number of protons and the number of electrons must be the same.
For an atom of an element:

number of protons in nucleus = number of electrons surrounding nucleus

We know that the atomic number (Z) of an element tells us how many protons are in the nucleus of an atom of that element, so, the number of electrons in an atom of this element must also be equal to the atomic number (Z) of the element:

number of electrons in atom = atomic number = Z

Consider an atom of the element lithium.
Using the Periodic Table we find that lithium has the symbol Li and an atomic number (Z) of 3.
This tells us that there will be 3 protons in the nucleus of an atom of lithium.
Since each proton carries a charge of +1, we could represent the number of charged protons in the nucleus in a diagram like the one below:

 3 positively charged protons in Li nucleus: + + +

Now, each electron surrounding the nucleus of an atom of lithium carries a charge of -1, so we will need 3 electrons to "cancel out" the charge of the 3 protons in the nucleus:

 3 positively charged protons in Li nucleus: 3 negatively charged electrons surrounding the Li nucleus: + + + - - -

And so the overall, or net, charge on an atom of lithium is zero because +1 + -1 = 0:

 3 positively charged protons in Li nucleus: 3 negatively charged electrons surrounding the Li nucleus: Net Charge: + + + - - - 0 0 0

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## Number of Electrons in Atoms of the First Twenty Elements in the Periodic Table

Below is an extract from the Periodic Table showing the atomic numbers (Z), the symbols and names for the first twenty elements:

 Group 1 Group 2 Groups 3 - 12 Group 13 Group 14 Group 15 Group 16 Group 17 Group 18 Period 1 1Hhydrogen 2Hehelium Period 2 3Lilithium 4Beberyllium 5Bboron 6Ccarbon 7Nnitrogen 8Ooxygen 9Ffluorine 10Neneon Period 3 11Nasodium 12Mgmagnesium 13Alaluminium 14Sisilicon 15Pphosphorus 16Ssulfur 17Clchlorine 18Arargon Period 4 19Kpotassium 20Cacalcium

Since the atomic number (Z) of an element tells us hown many protons are in the nucleus of the atom of that element, and we know that for an atom of the element the number of electrons is equal to the number of protons, we can draw up a table to show the name, symbol, atomic number (Z), number of protons in the nucleus, and, number of electrons surrounding the nucleus of the atom as shown below:

Name of Element Symbol of Element Atomic Number of Element (Z) Number of Protons in Nucleus of an Atom (= Z) Number of Electrons Surrounding the Nucleus of an Atom (= Z)
hydrogen H 1 1 1
helium He 2 2 2
lithium Li 3 3 3
beryllium Be 4 4 4
boron B 5 5 5
carbon C 6 6 6
nitrogen N 7 7 7
oxygen O 8 8 8
fluorine F 9 9 9
neon Ne 10 10 10
sodium Na 11 11 11
magnesium Mg 12 12 12
aluminium Al 13 13 13
silicon Si 14 14 14
phosphorus P 15 15 15
sulfur S 16 16 16
chlorine Cl 17 17 17
argon Ar 18 18 18
potassium K 19 19 19
calcium Ca 20 20 20

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## Worked Examples of Finding the Number of Electrons in an Atom

Question 1: An atom of an element has a nucleus containing 72 protons.
How many electrons surround the nucleus of an atom of this element?

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

Find the number of electrons surrounding the nucleus of the atom

2. What data (information) have you been given in the question?

Extract the data from the question:

number of protons = 72
3. What is the relationship between what you know and what you need to find out?
At atom has no overall charge.

number of positive charges = number of negative charges

number of protons = number of electrons

4. Determine the number of electrons surrounding the nucleus of the atom
number of protons = number of electrons

72 = number of electrons

(a) The number of protons, and therefore the number of electrons, must be a whole number, an integer. 72 is a whole number, so our answer is plausible.

(b) For an atom to have no overall charge (0 net charge):
total positive charge + total negative charge = 0
total positive charge = 72 × +1 = +72
total negative charge = 72 × -1 = -72
+72 + -72 = 0

6. State your solution to the problem "number of electrons surround the nucleus of the atom":

Number of electrons = 72

Question 2: Determine the number of electrons in an atom of chlorine.

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

Find the number of electrons in an atom of chlorine

2. What data (information) have you been given in the question?

Extract the data from the question:

Name of element: chlorine
3. What is the relationship between what you know and what you need to find out?
number of electrons in atom = number of protons in the nucleus of the atom = atomic number (Z)

Use the Periodic Table to find the atomic number (Z) of chlorine:
Z(chlorine) = 17

4. Determine the number of electrons in an atom of chlorine
number of electrons = number of protons = atomic number (Z) = 17
(a) The number of protons, and therefore the number of electrons, must be a whole number, an integer. 17 is a whole number, so our answer is plausible.

(b) For an atom to have no overall charge (0 net charge):
total positive charge + total negative charge = 0
total positive charge = 17 × +1 = +17
total negative charge = 17 × -1 = -17
+17 + -17 = 0

6. State your solution to the problem "number of electrons in the atom":

Number of electrons = 17

Question 3: An element has the symbol Fe. Calculate the number of electrons surrounding the nucleus of an atom of this element.

Solution:

(Based on the StoPGoPS approach to problem solving.)

1. What is the question asking you to do?

Calculate the number of electrons in an atom of element with symbol Fe

2. What data (information) have you been given in the question?

Extract the data from the question:

Symbol of the element: Fe
3. What is the relationship between what you know and what you need to find out?
number of electrons = number of protons = atomic number of element = Z

Use the Periodic Table to find the symbol Fe and hence its atomic number (Z):
Z(Fe) = 26

4. Calculate the number of protons in an atom of Fe
number of electrons = number of protons = atomic number = 26
(a) The number of protons, and therefore the number of electrons, must be a whole number, an integer. 26 is a whole number, so our answer is plausible.

(b) For an atom to have no overall charge (0 net charge):
total positive charge + total negative charge = 0
total positive charge = 26 × +1 = +26
total negative charge = 26 × -1 = -26
+26 + -26 = 0