As Atomic Number

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Define atomic number. Atomic number synonyms, atomic number pronunciation, atomic number translation, English dictionary definition of atomic number.

Atomic

Learning Objectives

  1. Arsenic is a naturally occurring allotropic pnictogen and metalloid trace element with atomic symbol As, atomic number 33, and atomic weight 74.92 that is found in water, air, food, and soil, and has a role as a micronutrient.
  2. Atomic number or proton number is defined as the total number of protons in the nucleus. The number of electrons in an electrically-neutral atom is the same as the atomic number.
  3. Atomic radii decrease, however, as one moves from left to right, across the Periodic Table. Although more electrons are being added to atoms, they are at similar distances to the nucleus; and the increasing nuclear charge 'pulls' the electron clouds inwards, making the atomic radii smaller.
  • Define and differentiate between the atomic number and the mass number of an element.
  • Explain how isotopes differ from one another.

Now that we know how atoms are generally constructed, what do atoms of any particular element look like? How many protons, neutrons, and electrons are in a specific kind of atom? First, if an atom is electrically neutral overall, then the number of protons equals the number of electrons. Because these particles have the same but opposite charges, equal numbers cancel out, producing a neutral atom.

Atomic Number

In the 1910s, experiments with x-rays led to this useful conclusion: the magnitude of the positive charge in the nucleus of every atom of a particular element is the same. In other words, all atoms of the same element have the same number of protons. Furthermore, different elements have a different number of protons in their nuclei, so the number of protons in the nucleus of an atom is characteristic of a particular element. This discovery was so important to our understanding of atoms that the number of protons in the nucleus of an atom is called the atomic number (Z).

As atomic number rises the density of electrons

For example, hydrogen has the atomic number 1; all hydrogen atoms have 1 proton in their nuclei. Helium has the atomic number 2; all helium atoms have 2 protons in their nuclei. There is no such thing as a hydrogen atom with 2 protons in its nucleus; a nucleus with 2 protons would be a helium atom. The atomic number defines an element. Table (PageIndex{1}) lists some common elements and their atomic numbers. Based on its atomic number, you can determine the number of protons in the nucleus of an atom. The largest atoms have over 100 protons in their nuclei.

Table (PageIndex{1}): Some Common Elements and Their Atomic Numbers
ElementAtomic NumberElementAtomic Nmbers
aluminum (Al)13magnesium (Mg)12
americium (Am)95manganese (Mn)25
argon (Ar)18mercury (Hg)80
barium (Ba)56neon (Ne)10
beryllium (Be)4nickel (Ni)28
bromine (Br)35nitrogen (N)7
calcium (Ca)20oxygen (O)8
carbon (C)6phosphorus (P)15
chlorine (Cl)17platinum (Pt)78
chromium (Cr)24potassium (K)19
cesium (Cs)55radon (Rn)86
fluorine (F)9silver (Ag)47
gallium (Ga)31sodium (Na)11
gold (Au)79strontium (Sr)38
helium (He)2sulfur (S)16
hydrogen (H)1titanium (Ti)22
iron (Fe)26tungsten (W)74
iodine (I)53uranium (U)92
lead (Pb)82zinc (Zn)30
lithium (Li)3zirconium (Zr)40

Example (PageIndex{1})

What is the number of protons in the nucleus of each element?

  1. aluminum
  2. iron
  3. carbon
Answer a

According to Table 2.4.1, aluminum has an atomic number of 13. Therefore, every aluminum atom has 13 protons in its nucleus.

Answer b

Iron has an atomic number of 26. Therefore, every iron atom has 26 protons in its nucleus.

Answer c

Carbon has an atomic number of 6. Therefore, every carbon atom has 6 protons in its nucleus.

Exercise (PageIndex{1})

What is the number of protons in the nucleus of each element? Use Table 2.4.1.

  1. sodium
  2. oxygen
  3. chlorine
Answer a

Sodium has 11 protons in its nucleus.

Answer b

Oxygen has 8 protons in its nucleus.

Answer c

Chlorine has 17 protons in its nucleus

How many electrons are in an atom? Previously we said that for an electrically neutral atom, the number of electrons equals the number of protons, so the total opposite charges cancel. Thus, the atomic number of an element also gives the number of electrons in an atom of that element. (Later we will find that some elements may gain or lose electrons from their atoms, so those atoms will no longer be electrically neutral. Thus we will need a way to differentiate the number of electrons for those elements.)

Example (PageIndex{2})

How many electrons are present in the atoms of each element?

  1. sulfur
  2. tungsten
  3. argon
Answer a

The atomic number of sulfur is 16. Therefore, in a neutral atom of sulfur, there are 16 electrons.

Answer b

The atomic number of tungsten is 74. Therefore, in a neutral atom of tungsten, there are 74 electrons.

Answer c

The atomic number of argon is 18. Therefore, in a neutral atom of argon, there are 18 electrons.

Exercise (PageIndex{2})

How many electrons are present in the atoms of each element?

  1. magnesium
  2. potassium
  3. iodine
Answer a

Mg has 12 electrons.

Answer b

K has 19 electrons.

Answer c

I has 53 electrons.

Isotopes

How many neutrons are in atoms of a particular element? At first it was thought that the number of neutrons in a nucleus was also characteristic of an element. However, it was found that atoms of the same element can have different numbers of neutrons. Atoms of the same element (i.e., same atomic number, Z) that have different numbers of neutrons are called isotopes. For example, 99% of the carbon atoms on Earth have 6 neutrons and 6 protons in their nuclei; about 1% of the carbon atoms have 7 neutrons in their nuclei. Naturally occurring carbon on Earth, therefore, is actually a mixture of isotopes, albeit a mixture that is 99% carbon with 6 neutrons in each nucleus.

An important series of isotopes is found with hydrogen atoms. Most hydrogen atoms have a nucleus with only a single proton. About 1 in 10,000 hydrogen nuclei, however, also has a neutron; this particular isotope is called deuterium. An extremely rare hydrogen isotope, tritium, has 1 proton and 2 neutrons in its nucleus. Figure (PageIndex{1}) compares the three isotopes of hydrogen.

The discovery of isotopes required a minor change in Dalton’s atomic theory. Dalton thought that all atoms of the same element were exactly the same.

Most elements exist as mixtures of isotopes. In fact, there are currently over 3,500 isotopes known for all the elements. When scientists discuss individual isotopes, they need an efficient way to specify the number of neutrons in any particular nucleus. The mass number (A) of an atom is the sum of the numbers of protons and neutrons in the nucleus. Given the mass number for a nucleus (and knowing the atomic number of that particular atom), you can determine the number of neutrons by subtracting the atomic number from the mass number.

A simple way of indicating the mass number of a particular isotope is to list it as a superscript on the left side of an element’s symbol. Atomic numbers are often listed as a subscript on the left side of an element’s symbol. Thus, we might see

[mathrm{^{mass: numberxrightarrow{hspace{45px}} 56}_{atomic: number xrightarrow{hspace{35px}} 26}Fe} label{Eq1}]

which indicates a particular isotope of iron. The 26 is the atomic number (which is the same for all iron atoms), while the 56 is the mass number of the isotope. To determine the number of neutrons in this isotope, we subtract 26 from 56: 56 − 26 = 30, so there are 30 neutrons in this atom.

Example (PageIndex{3})

How many protons and neutrons are in each atom?

  1. (mathrm{^{35}_{17}Cl})
  2. (mathrm{^{127}_{53}I})
Answer a

In (mathrm{^{35}_{17}Cl}) there are 17 protons, and 35 − 17 = 18 neutrons in each nucleus.

Answer b

In (mathrm{^{127}_{53}I}) there are 53 protons, and 127 − 53 = 74 neutrons in each nucleus.

Exercise (PageIndex{3})

How many protons and neutrons are in each atom?

  1. (mathrm{^{197}_{79}Au})
  2. (mathrm{^{23}_{11}Na})
Answer a

In (mathrm{^{197}_{79}Au}) there are 79 protons, and 197 − 79 = 118 neutrons in each nucleus.

Answer b

In (mathrm{^{23}_{11}Na}) there are 11 protons, and 23 − 11 = 12 neutrons in each nucleus.

It is not absolutely necessary to indicate the atomic number as a subscript because each element has its own unique atomic number. Many isotopes are indicated with a superscript only, such as 13C or 235U. You may also see isotopes represented in print as, for example, carbon-13 or uranium-235.

Summary

The atom consists of discrete particles that govern its chemical and physical behavior. Each atom of an element contains the same number of protons, which is the atomic number (Z). Neutral atoms have the same number of electrons and protons. Atoms of an element that contain different numbers of neutrons are called isotopes. Each isotope of a given element has the same atomic number but a different mass number (A), which is the sum of the numbers of protons and neutrons.

Almost all of the mass of an atom is from the total protons and neutrons contained within a tiny (and therefore very dense) nucleus. The majority of the volume of an atom is the surrounding space in which the electrons reside. A representation of a carbon-12 atom is shown below in Figure (PageIndex{2}).

Concept Review Exercises

  1. Why is the atomic number so important to the identity of an atom?
  2. What is the relationship between the number of protons and the number of electrons in an atom?
  3. How do isotopes of an element differ from each other?
  4. What is the mass number of an element?

Answers

Atomic Mass Same As Atomic Number

Element with 33 as atomic number
  1. The atomic number defines the identity of an element. It describes the number of protons in the nucleus.
  2. In an electrically neutral atom, the number of protons equals the number of electrons.
  3. Isotopes of an element have the same number of protons but have different numbers of neutrons in their nuclei.
  4. The mass number is the sum of the numbers of protons and neutrons in the nucleus of an atom.

Key Takeaways

  • Each element is identified by its atomic number. The atomic number provides the element's location on the periodic table
  • The isotopes of an element have different masses and are identified by their mass numbers.

Contributors and Attributions

  • Anonymous

As Atomic Number Increases Atomic Size

Number of Protons and Neutrons - An Atom determines an Element
The purest type of atom is called an element. Atoms are composed of three kinds of smaller particles, called protons, neutrons and electrons. Protons and neutrons are made up of even smaller particles called quarks. The number of protons, the number of neutrons and the number of electrons an atom has determines what the element it is.

  • Atoms are composed of 3 kinds of small particles: protons, neutrons and electrons
  • Atoms must have equal numbers of protons and electrons

The nucleus is in the middle of the atom and contains protons and neutrons. These smaller particles - the protons, neutrons and electrons - all have different properties.

Number of Protons and Neutrons

Number of Protons and Neutrons - What are Electrons?
A reminder of the definition before discovering how to calculate the numbers. Electrons are tiny, very light particles that have a negative electrical charge. Atoms must have equal numbers of protons and electrons.

Number of Protons and Neutrons - What are Protons?
Protons are much larger and heavier than electrons and have a positive charge. Atoms must have equal numbers of protons and electrons.

Number of Protons and Neutrons - What are Neutrons?
Neutrons are large and heavy like protons, however neutrons have no electrical charge.

Number of Protons and Neutrons - How to find the Atomic Number of an element
Every element has a unique Element Symbol and a unique Atomic Number which can be accessed via thePeriodic Table with Atomic Mass.

  • Definition: The atomic number is the number of protons in the atom of an element.

The periodic Table is a really important tool to Chemistry students enabling them to recognise the names of elements from their Atomic number. The chemical elements of the periodic table are listed in order of atomic number. You can also refer to the bottom of this page to an at-a-glance table of all the elements and their atomic numbers in numerical order.

Number of Protons and Neutrons - How to find the Number of Protons - Examples
The atomic number is based on the number of protons in the atom of an element. (Note: Atoms must also have equal numbers of protons and electrons.) So, if we know the atomic number of an element then we also know how many protons in an element.

  • Example 1 - Numbers of Protons in Gold: The element Gold (Symbol Au) has the Atomic Number of 79. Any atom that contains exactly 79 protons in its nucleus is an atom of gold. The number of protons in atom of gold is therefore 79.
  • Example 2 - Numbers of Protons in Silver: The element Silver (Symbol Ag) has the Atomic Number of 47. Any atom that contains exactly 47 protons in its nucleus is an atom of silver. The number of protons in atom of silver is therefore 47.
  • Example 3 - Numbers of Protons in Neon: The element Neon (Symbol Ne) has the Atomic Number of 10. Any atom that contains exactly 10 protons in its nucleus is an atom of gold. The number of protons in atom of gold is therefore 10.

Adding or removing protons from the nucleus of an atom creates a different element. For example, removing one proton from an atom of Gold creates an atom of Platinum (Platinum has the Atomic number 78 therefore the number of protons =78.) Adding one proton from an atom of Gold creates an atom of Mercury (Mercury has the Atomic number 80 therefore the number of protons =80.). Refer to the table below and work out the number of protons in various elements.

Number of Protons and Neutrons - How to find the Atomic Mass number
Calculating the number of neutrons in an atom or element is a little more complicated. For this calculation you need to establish the atomic weight from which you will be able to establish the Atomic Mass Number. The atomic weight or mass is, basically, a measurement of the total number of particles in an atom's nucleus. The atomic mass number is established by rounding the atomic weight to the nearest whole number. The Periodic Table with Atomic Mass will give you the atomic weight, or atomic mass, of the elements.

Atomic Mass Number = Atomic Weight of Element, rounded to nearest whole number

So, if we know the number of protons and neutrons in an atom (element) we can determine the Atomic Mass Number

  • Example 1 - Atomic Mass of Gold: The element Gold (Symbol Au) has the Atomic Number of 79. The number of protons in atom of gold is therefore 79. Gold has the Atomic Mass weight of 196.97. Round to the nearest whole number. The atomic mass number of gold is therefore 197.
  • Example 2 - Atomic Mass of Silver: The element Silver (Symbol Ag) has the Atomic Number of 47. The number of protons in atom of silver is therefore 47. Silver has the Atomic Mass weight of 107.87. Round to the nearest whole number. The atomic mass number of silver is therefore 108.
  • Example 3 - Atomic Mass of Neon: The element Neon (Symbol Ne) has the Atomic Number of 10. The number of protons in atom of neon is therefore 10. Neon has the Atomic Mass weight of 20.18. Round to the nearest whole number. The atomic mass number of neon is therefore 20.

As Atomic Number Increases Valence Electrons

Number of Protons and Neutrons - How to find the Number of Neutrons
We can identify the number of neutrons in an atom or element by using the information we already have. To find the numbers of neutrons, subtract the atomic mass number from the atomic number.

Number of Neutrons = Mass Number - Atomic Number

Element With 33 As Atomic Number

  • Example 1 - The element Gold (Symbol Au). The atomic mass number of gold is 197 and has the Atomic Number of 79. The Numbers of Neutrons in Gold is 197 - 79 = 118
  • Example 2 - The element Silver (Symbol Ag). Number of Neutrons in Silver: The atomic mass no. of silver is 108 and has the Atomic Number of 47. The Numbers of Neutrons in Neon is 108 - 47 = 61
  • Example 3 - The element Neon (Symbol Ne). The atomic mass number of neon is 20 and has the Atomic no. of 10. Numbers of Neutrons in Neon is 20 - 10 = 10

Adding or removing protons from the nucleus of an atom creates a different element but adding or removing neutrons from the nucleus of an atom does not create a different element. Compare the numbers of neutrons and protons in the chart below.

Numerical list of Atomic Numbers of Elements

As Atomic Number Increases Atomic Radius

Name of Element Symbol of ElementNumber of Protons
/ Electrons
Mass NumberAtomic NumberNumber of Neutrons
Hydrogen
Helium
Lithium
Beryllium
Boron
Carbon
Nitrogen
Oxygen
Fluorine
Neon
Sodium
Magnesium
Aluminium
Silicon
Phosphorus
Sulfur
Chlorine
Argon
Potassium
Calcium
Scandium
Titanium
Vanadium
Chromium
Manganese
Iron
Cobalt
Nickel
Copper
Zinc
Gallium
Germanium
Arsenic
Selenium
Bromine
Krypton
Rubidium
Strontium
Yttrium
Zirconium
Niobium
Molybdenum
Technetium
Ruthenium
Rhodium
Palladium
Silver
Cadmium
Indium
Tin
Antimony
Tellurium
Iodine
Xenon
Cesium
Barium
Lanthanum
Cerium
Praseodymium
Neodymium
Promethium
Samarium
Europium
Gadolinium
Terbium
Dysprosium
Holmium
Erbium
Thulium
Ytterbium
Lutetium
Hafnium
Tantalum
Tungsten
Rhenium
Osmium
Iridium
Platinum
Gold
Mercury
Thallium
Lead
Bismuth
Polonium
Astatine
Radon
Francium
Radium
Actinium
Thorium
Protactinium
Uranium
Neptunium
Plutonium
Americium
Curium
Berkelium
Californium
Einsteinium
Fermium
Mendelevium
Nobelium
Lawrencium
Rutherfordium
Dubnium
Seaborgium
Bohrium
Hassium
Meitnerium
Darmstadtium
Roentgenium
Ununbium
Ununtrium
Ununquadium
Ununpentium
Ununhexium
Ununseptium
Ununoctium
H
He
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Sc
Ti
V
Cr
Mn
Fe
Co
Ni
Cu
Zn
Ga
Ge
As
Se
Br
Kr
Rb
Sr
Y
Zr
Nb
Mo
Tc
Ru
Rh
Pd
Ag
Cd
In
Sn
Sb
Te
I
Xe
Cs
Ba
La
Ce
Pr
Nd
Pm
Sm
Eu
Gd
Tb
Dy
Ho
Er
Tm
Yb
Lu
Hf
Ta
W
Re
Os
Ir
Pt
Au
Hg
Tl
Pb
Bi
Po
At
Rn
Fr
Ra
Ac
Th
Pa
U
Np
Pu
Am
Cm
Bk
Cf
Es
Fm
Md
No
Lr
Rf
Db
Sg
Bh
Hs
Mt
Ds
Rg
Uub
Uut
Uuq
Uup
Uuh
Uus
Uuo
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
19
19
20
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22
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26
27
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31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
1
4
7
9
11
12
14
16
19
20
23
24
27
28
31
32
35
40
30
40
45
48
51
52
55
56
58
58
64
65
70
73
75
79
80
84
85
88
89
91
93
96
98
101
103
106
108
112
115
119
122
128
127
131
133
137
139
140
141
144
145
150
152
157
159
163
165
167
169
173
175
178
181
184
186
190
192
195
197
201
204
207
209
209
210
222
223
226
227
232
231
238
237
244
243
247
247
251
252
257
258
259
262
261
268
263
264
269
268
272
273
277
286
289
288
292
292
293
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
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26
27
28
29
30
31
32
33
34
35
36
37
38
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40
41
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46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
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72
73
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78
79
80
81
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84
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86
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91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
0
2
4
5
6
6
7
8
10
10
12
12
14
14
16
16
18
22
21
20
24
26
28
28
30
30
31
30
35
35
39
41
42
45
45
48
48
50
50
51
52
54
55
57
58
60
61
64
66
69
71
76
74
77
78
81
82
82
82
84
84
88
89
93
94
97
98
99
100
103
104
106
108
110
111
114
115
117
118
121
123
125
126
125
125
136
136
138
138
142
140
146
144
150
148
151
150
153
153
157
157
157
159
157
N/A
157
157
161
159
162
162
165
173
175
173
176
175
175
Name of Element (Alphabetical list)Symbol of ElementNumber of Protons
Electrons
Mass NumberAtomic NumberNumber of Neutrons

Atomic Weight Of As

Numerical list of Atomic Numbers of Elements