 # 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).

(c) The name of the element1.

(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:

 1 H hydrogen 1.008
(a) atomic number of hydrogen is 1 (Z = 1)

(b) symbol for hydrogen is H

(c) name of the element is hydrogen

(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 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, 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.)

1. What is the question asking you to do?

Determine the number of protons in the nucleus of the atom

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

Extract the data from the question:

Z = atomic number = 54
3. 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
4. Determine the number of protons in the nucleus of the atom
number of protons in the nucleus of the atom = Z = 54
(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.
6. 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.)

1. What is the question asking you to do?

Find the number of protons in an atom of boron

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

Extract the data from the question:

Name of element: boron
3. 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

4. Determine the number of protons in an atom of boron
Z = atomic number = number of protons = 5
(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.
6. 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.)

1. What is the question asking you to do?

Calculate the number of protons in an atom of element with symbol Cl

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

Extract the data from the question:

Symbol of the element: Cl
3. 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

4. Calculate the number of protons in an atom of Cl
Z = atomic number = number of protons = 17
(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.
6. 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.