Number of Protons in an Atom Chemistry Tutorial

Key Concepts

Protons are found in the nucleus of an atom.

The number of protons in the nucleus of an atom is equal to its atomic number.

The atomic number of an element is given the symbol Z.

The Periodic Table lists the elements in order of increasing atomic number (Z).

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Finding the Number of Protons in an Atom Using the Periodic Table

The modern Periodic Table is made up of a series of rectangles.

Each rectangle provides us with four important pieces of information about an element:

atomic number

symbol

name

atomic weight

(a) The atomic number of the element, Z (always a whole number).

(b) The symbol of the element (1 or 2 letters, the first letter is always a capital letter).

(d) The atomic weight of the element² (not usually a whole number).

For example, the rectangle for the element hydrogen in the Periodic Table looks like the one below:

hydrogen

1.008

(a) atomic number of hydrogen is 1 (Z = 1)

(b) symbol for hydrogen is H

(d) atomic weight of hydrogen is 1.008

The atomic number (Z) of an element tells us how many protons are found in the nucleus of an atom of this element.

atomic number = Z = number of protons in the nucleus of the atom

For example, hydrogen has atomic number 1 (Z = 1), so there is 1 proton in the nucleus of an atom of hydrogen.

If we know the

atomic number (Z) of an element then we also know the number of protons in the nucleus of an atom of this element
atomic number = Z = number of protons in the nucleus of the atom

name of an element then we can use the Periodic Table to find its atomic number (Z) and therefore the number of protons in the nucleus of an atom of this element

symbol of an element then we can use the Periodic Table to find its atomic number (Z) and therefore the number of protons in the nucleus of an atom of this element

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Atomic Number 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	1 H hydrogen								2 He helium
Period 2	3 Li lithium	4 Be beryllium		5 B boron	6 C carbon	7 N nitrogen	8 O oxygen	9 F fluorine	10 Ne neon
Period 3	11 Na sodium	12 Mg magnesium		13 Al aluminium	14 Si silicon	15 P phosphorus	16 S sulfur	17 Cl chlorine	18 Ar argon
Period 4	19 K potassium	20 Ca calcium

Since the atomic number (Z) of an element tells us hown many protons are in the nucleus of the atom of that element, we can draw up a table to show the name, symbol, atomic number (Z) and number of protons in 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)
hydrogen	H	1	1
helium	He	2	2
lithium	Li	3	3
beryllium	Be	4	4
boron	B	5	5
carbon	C	6	6
nitrogen	N	7	7
oxygen	O	8	8
fluorine	F	9	9
neon	Ne	10	10
sodium	Na	11	11
magnesium	Mg	12	12
aluminium	Al	13	13
silicon	Si	14	14
phosphorus	P	15	15
sulfur	S	16	16
chlorine	Cl	17	17
argon	Ar	18	18
potassium	K	19	19
calcium	Ca	20	20

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Worked Examples: Number of Protons in the Nucleus of an Atom

Question 1: How many protons are in the nucleus of an atom of an element with an atomic number of 54 ?

Solution:

(Based on the StoPGoPS approach to problem solving.)

What is the question asking you to do?
Determine the number of protons in the nucleus of the atom

What data (information) have you been given in the question?
Extract the data from the question:

Z = atomic number = 54

What is the relationship between what you know and what you need to find out?
Z = atomic number = number of protons in the nucleus of the atom

Determine the number of protons in the nucleus of the atom
number of protons in the nucleus of the atom = Z = 54

Is your answer plausible?
(a) The number of protons must be a whole number, an integer. 54 is a whole number, so our answer is plausible.
(b) The atomic number, and hence the number of protons, cannot be greater than the number of known elements (currently 116). 54 is less than the number of known elements, so our answer is plausible.

State your solution to the problem "number of protons in nucleus of the atom":
Number of protons = 54

Question 2: Determine the number of protons in the nucleus of an atom of boron.

Solution:

(Based on the StoPGoPS approach to problem solving.)

What is the question asking you to do?
Find the number of protons in an atom of boron

What data (information) have you been given in the question?
Extract the data from the question:

Name of element: boron

What is the relationship between what you know and what you need to find out?
number of protons = atomic number of element = Z
Use the Periodic Table to find the atomic number of boron:
Z(boron) = 5

Determine the number of protons in an atom of boron
Z = atomic number = number of protons = 5

Is your answer plausible?
(a) The number of protons must be a whole number, an integer. 5 is a whole number, so our answer is plausible.
(b) The atomic number, and hence the number of protons, cannot be greater than the number of known elements (currently 116). 5 is less than the number of known elements, so our answer is plausible.

State your solution to the problem "number of protons in nucleus of the atom":
Number of protons = 5

Question 3: An element has the symbol Cl. Calculate the number of protons in the nucleus of an atom of this element.

Solution:

(Based on the StoPGoPS approach to problem solving.)

What is the question asking you to do?
Calculate the number of protons in an atom of element with symbol Cl

What data (information) have you been given in the question?
Extract the data from the question:

Symbol of the element: Cl

What is the relationship between what you know and what you need to find out?
number of protons = atomic number of element = Z
Use the Periodic Table to find the symbol Cl and hence its atomic number (Z):
Z(Cl) = 17

Calculate the number of protons in an atom of Cl
Z = atomic number = number of protons = 17

Is your answer plausible?
(a) The number of protons must be a whole number, an integer. 17 is a whole number, so our answer is plausible.
(b) The atomic number, and hence the number of protons, cannot be greater than the number of known elements (currently 116). 17 is less than the number of known elements, so our answer is plausible.

State your solution to the problem "number of protons in nucleus of the atom":
Number of protons = 17

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1. The IUPAC recognizes the commonly used alternative spellings for two elements:
The element with atomic number 13 and symbol Al may be spelt as aluminium or as aluminum.
The element with atomic number 55 and symbol Cs may be spelt as caesium or as cesium.
BUT it should be noted that these elements are present in the IUPAC periodic table as caesium and aluminium.

2. The IUPAC periodic table uses standard atomic weights and provides the lower and upper bounds of the standard atomic weight for some elements. Debate still rages about whether the term atomic mass, atomic weight, relative atomic mass, relative atomic weight, or standard atomic weight should be used. Don't worry about it, at High School you can treat all these terms as meaning the same thing.