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Atomic Mass and Atomic Number

Last Updated : 25 Feb, 2024
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Atomic Mass and Atomic Number are somewhat similar concepts in chemistry. Atomic Number tells us about the number of electrons or protons, whereas Atomic Mass tells us about the number of neutrons and protons combined. These two parameters are important in learning about atoms and their various properties. This article explores these two concepts briefly and discusses the key differences in detail.

What is Atomic Number?

The atomic number, symbolized as ‘Z,’ is the number of protons in an atom It holds a paramount position in atomic properties. With the help of atomic numbers, we can identify atoms of various elements. For example, if we want to answer the question, ‘What is an atom with atomic number 8?’ we can easily state that atomic number 8 belongs to oxygen without any second thought. Atomic numbers play an important role in determining the placement of an atom in the periodic table.

Atomic Number Definition

Atomic Number is the whole number which defines the number of protons electrons for any element.

Atomic Number is often denoted by the symbol “Z.” Each element on the periodic table is characterized by its unique atomic number, which distinguishes one element from another.

Atomic Number Formula

Atomic Number is critical number represents the count of protons in the atom’s nucleus as well as the number of electron revolving around the nucleus in a neutral atom. Thus, we can write a formula for atomic number i.e.,

Atomic Number (Z) = Number of Proton/Electron in Element

Hydrogen (H):

Atomic Number for Hydrogen is 1.

In hydrogen (H), the atomic number (Z) is equal to the number of protons and electrons. So, hydrogen has 1 proton and 1 electron, and its atomic number is 1.

Examples of Atomic Number

The following table contains examples of atomic numbers for various elements.

Element

Atomic Number (Z)

Hydrogen

1

Helium

2

Carbon

6

Oxygen

8

Sodium

11

Chlorine

17

Iron

26

Gold

79

Uranium

92

What is Atomic Mass?

The mass number, typically denoted as “A,” constitutes another pivotal property of an atom. It signifies the collective total of protons and neutrons housed within the nucleus of an atom. While protons and neutrons contribute to an atom’s overall mass, electrons possess significantly lower mass and therefore make negligible contributions to the mass number. So mass number provides us insights about mass of an atom

Atomic Mass Definition

Atomic Mass is an non-integer number which defines the mass of protons and neutrons for any element.

Atomic Mass is represented by the letter “A” and elements in mendeleeves periodic table are arranged based on Atomic Mass as well.

Atomic Mass Formula

As mass of neutron and proton is roughly 1 unit, thus formula for approximate atomic mass is given as follows:

Atmoic Mass or Mass Number (A) = Atomic Number + Number of Neutrons

Note: This formula only hold true for less heavy elements.

Let’s consider some examples to learn the use of this formula.

Carbon-12 (C):

Atomic Number (Z) = 6 (since carbon has 6 protons)

Number of Neutrons (N) = 12 – 6 = 6

Atomic Mass (A) = Z + N = 6 + 6 = 12

So, for carbon-12 (12C), the atomic mass (mass number) is 12.

Oxygen-16 (O):

Atomic Number (Z) = 8 (since oxygen has 8 protons)

Number of Neutrons (N) = 16 – 8 = 8

Atomic Mass (A) = Z + N = 8 + 8 = 16

Thus, for oxygen-16 (16O), the atomic mass (mass number) is 16.

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Examples of Atomic Mass

There are 118 elements known in the modern-day periodic table. From that, we have selected some examples for atomic mass, which are provided in the following table.

Element

Atomic Mass (Approx.)

Hydrogen

1.008 u

Helium

4.0026 u

Carbon

12.011 u

Oxygen

15.999 u

Sodium

22.990 u

Chlorine

35.453 u

Iron

55.845 u

Gold

196.967 u

Uranium

238.050 u

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Atomic Mass vs Atomic Number

There are various differences between atomic mass and atomic number, some of those difference between Atomic and Mass Number are listed in the following table:

Property Atomic Mass Atomic Number
Definition The total mass of an atom, including protons, neutrons, and electrons. The number of protons in the nucleus of an atom.
Symbol Typically denoted by the letter “A.” Typically denoted by the letter “Z.”
Value It is usually not a whole number because it includes the masses of protons, neutrons, and electrons, each with fractional masses. Always a whole number since it represents the count of protons in an atom’s nucleus.
Relative Scale Atomic mass varies from element to element and even between isotopes of the same element. Atomic number is unique to each element and defines its identity on the periodic table.
Example Carbon-12 has an atomic mass of approximately 12.01 u due to its mixture of isotopes. Carbon-12 has an atomic number of 6 because it has 6 protons.
Importance Atomic mass is useful for calculating the mass of a sample of atoms or molecules and for determining molar mass. Atomic number is crucial for identifying elements and organizing them in the periodic table.
Units Atomic mass is typically measured in atomic mass units (u) or grams per mole (g/mol). Atomic number is dimensionless; it’s just a whole number count.
Location on PT Found on the periodic table beneath the element’s symbol or as a decimal value in the element’s box. The atomic number is the whole number typically placed above or within the element’s symbol on the periodic table.

Elements Table with Atomic Mass and Atomic Number

The following table provides the information about Atomic Mass and Number of first 20 elements.

Atomic Number

Element name

Mass Number

1

Hydrogen

1.008

2

Helium

4.0026

3

Beryllium

6.94

4

Boron

9.0122

5

Carbon

10.81

6

Nitrogen

14.01

7

Oxygen

16.00

8

Fluorine

19.00

10

Neon

20.18

11

Sodium

22.99

12

Magnesium

24.31

13

Aluminium

26.98

14

Silicon

28.09

15

Phosphorus

30.97

16

Sulphur

32.07

17

Chlorine

35.45

18

Argon

39.95

19

Potassium

39.10

20

Calcium

40.08

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Atomic Mass and Atomic Number – FAQs

1. What is Atomic Mass and Atomic Number?

Atomic mass as we are getting from the name, is the total mass of an atom and is given by the total number of protons and neutrons we are not counting electrons because their mass is negligible.

Atomic number is equal to the number of protons or the number of electrons in any element. The position of an atom in the periodic table is determined by its atomic number.

2. What are Some Differences between Atomic Mass and Atomic Number?

The primary difference between them is that atomic number is the number the protons in an atom while mass number is the sum of protons and neutrons. That is why in the above table of an element’s mass and atomic number the mass number is more than the atomic number

3. Can two Elements share the Same Atomic Number?

No, each element possesses an individual atomic number, acting as a unique identifier. But in the case of isotopes, all the isotopes have a same number of electrons. Thus all isotopes have a same atomic number.

4. Can an Element Change its Atomic Number?

Yes, through nuclear processes like transmutation, elements can modify their atomic numbers, ultimately leading to the formation of new elements.

5. What are the First 20 Elements and their Atomic Mass?

Atomic Number

Element name

Atomic Mass

1

Hydrogen

1.008

2

Helium

4.0026

3

Beryllium

6.94

4

Boron

9.0122

5

Carbon

10.81

6

Nitrogen

14.01

7

Oxygen

16.00

8

Fluorine

19.00

10

Neon

20.18

11

Sodium

22.99

12

Magnesium

24.31

13

Aluminium

26.98

14

Silicon

28.09

15

Phosphorus

30.97

16

Sulphur

32.07

17

Chlorine

35.45

18

Argon

39.95

19

Potassium

39.10

20

Calcium

40.08



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