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Atomic Number

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On the periodic table, the elements are identified by an atomic symbol, such as H for hydrogen and arranged in order by the number of protons. The number of protons in an element equals the atomic number which is usually listed in top on the periodic table. If we know the atomic number then we know the element.


Any given atom of each kind of element has a fixed number of protons in its atomic nucleus. The number of protons in an atom is its atomic number. The atomic number also designates the number of electrons in an atom. At present 116 elements are known. These elements have the atomic number starting form 1 to 116. The smallest atomic number 1 belongs to hydrogen and the larger 116 belongs to the heaviest unnamed element.

What is Atomic Number?

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The atomic number of an element equals the number of protons in the nucleus of an atom of that element. The atomic number identifies the element.

Atomic number = number of protons in the nucleus and the number of electrons about the nucleus in the neutral atom.

The atomic number appears above the symbol of each element in the periodic table.

Atomic Number Definition

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Define Atomic Number

The atomic number of an element is often defined as its ordinal number in the series of elements when they are arranged in order of ascending atomic weights. The symbol Z is given to atomic number.

Atomic number distinguishes atom from the atom of another element. All atom of each element has its own characteristic number. The atomic number is equal to the number of protons.

Since neutral atoms of an element have the same number of electrons and protons the atomic number is also equal to the number of electrons.

Periodic Table Atomic Number

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The periodic table lists all the elements in eighteen vertical columns, or groups and seven horizontal rows or periods. The elements are arranged so that their atomic numbers increases from left to right through the period.

Atomic number for some elements are listed below.

S.No
Elements
Atomic Number
1
Carbon atomic number 6
2
Oxygen atomic number
8
3
Nitrogen atomic number
7
4
Gold atomic number
79
5
Sodium atomic number
11
6
Iron atomic number
26
7
Potassium atomic number
19
8
Calcium atomic number
20
9
Hydrogen atomic number
1
10
Copper atomic number
29
11
Chlorine atomic number 17
12
Lead atomic number
82
13
Helium atomic number
2
14
Uranium atomic number
92
15
Lithium atomic number 3
16
Sulfur atomic number
16
17
Magnesium atomic number
12
18
Aluminum atomic number 13
19
Zinc atomic number 30
20
Phosphorus atomic number
15
21
Mercury atomic number 80
22
Silicon atomic number
14
23 Neon atomic number 10
24
Boron atomic number
5
25
Iodine atomic number
53
26
Atomic number of lead
82
27
Tin atomic number 50

Atomic Number and Mass Number

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The chemical properties of an element are determined by the number of electrons surrounding the nucleus in an unionized atom which in turn equal to the number of protons in the nucleus. This number is the elements atomic number.

Thus,

Z = Atomic number = Number of protons in the nucleus = Number of electrons outside

Nuclei with a given number of protons need not have the same number of neutrons although for the most part the spread in the number of neutrons for a given element is rather narrow. The total number of nucleons in a nucleus is the mass number of the nucleus and is denoted by A.

A = Mass number = N + Z

Where N denotes the number of neutrons in the nucleus.

Atomic Weight

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Atomic weight is the mass of an atom compared to the mass of the carbon 12 atom. Atomic weight usually refers to the average atomic weight of a given element with all its naturally occurring isotopes.
Atomic weight is sometimes referred to as atomic mass. Where atomic weight refers to the average atomic weight of an element, atomic mass is the term typically used to describe the weight of an individual isotope.

The relative atomic mass is also known as atomic weight. The term atomic weight is slowly being phrased as relative atomic mass. Atomic weight is measured in atomic mass unit (amu). Atomic mass unit is defined as $\frac{1}{12}$ of the mass of the C12 isotopes.

1 amu = 1.66054 $\times$ 10-24 gm
1 g = 6.02214 $\times$ 1023 amu

Atomic Mass Number

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The atomic mass number is the sum of the number of protons and neutrons in the nucleus of an atom and is usually denoted by the symbol A. The atomic mass number of an atom is equal to the atomic number plus the number of neutrons usually denoted by the symbol N.

Therefore the equation for atomic mass number is given by

A = Z + N

Chemists use atomic mass table to compare the masses of different atoms. The mean atomic mass number M of all elements is thus defined as the mass of M grams of the element contains exactly Avogadro number of atoms.
The atomic mass unit (u) is defined as exactly $\frac{1}{12}$ of the mass of a C12 atom. One measures the mass of an individual atom called the isotopic mass and expressed in atomic mass unit by comparing it with that of C12 atom.

The atomic mass for a naturally occurring element is a weighed average of the masses of its stable isotopes, taking into account the relative abundance of the isotopes.
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Atomic Number vs Atomic Mass

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Atomic number of an atom is equal to the number of protons. This number is always an integer. Atomic mass number is the sum of protons and neutrons in the nucleus. Hence atomic number and atomic mass number are always whole numbers.

S.No
Atomic Number
Atomic Mass
1 Atomic number represents the total number of protons in the nucleus of an atom or the total number of electrons in its orbits. Atomic mass number represents the sum of the nucleons in the nucleus of an atom (the sum of the number of protons and neutrons).
2
Its symbol is Z. Its symbol is A.
3
Usually the atomic number of an element is less than its atomic mass number except in case of hydrogen where both are = 1. Usually the atomic mass number of an element is greater than its atomic number except hydrogen. (A = Z = 1)
4
All the atoms of the same element always have the same atomic number. Hence it is the characteristic of the element. Atoms of the same element can have different atomic mass numbers due to differences in the number of neutrons present.

Atomic Mass Calculation

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The atomic mass of any element is calculated according to the equation mentioned below.

Atomic mass = $\sum$ [fraction of isotopes $\times$ mass of isotopes]

Atomic mass = [fraction of isotope 1 $\times$ mass of isotope 1] + [fraction of isotope 2 $\times$ mass of isotope 2] + ....

Solved Examples

Question 1: The three naturally occurring isotopes of magnesium with masses 23.99, 24.99 and 25.98 amu and natural abundances 78.99, 10.00 and 11.01%. Calculate the atomic mass of magnesium.
Solution:
 
Mg atomic mass = 0.7899(23.99) + 0.100(24.99) + 0.110(25.98)
Mg atomic mass = 18.949 + 2.499 + 2.857
Mg atomic mass = 24.305

 

Question 2: Calculate the number of sodium atoms present in a sample that has a mass of 1172.49 amu.
Solution:
 
This problem is solved using average atomic mass for sodium of 22.99 amu

1 Na = 22.99 amu

Which gives the conversion factor

$1172.49\ amu \times \frac{1\ Na\ atom}{22.99\ amu}$ = 51.00 Na atoms