Atomic Number Is The Number Of

Learning Outcomes

Define atomic and mass numbers.
Determine the number of protons, neutrons, and electrons in an atom.
Identify the charge and relative mass of subatomic particles.
Label the location of subatomic particles in the atom.
Determine the mass of an atom based on its subatomic particles.
Write A/Z and symbol-mass format for an atom.

The atomic number is the number of protons in the nucleus of an atom. The number of protons define the identity of an element (i.e., an element with 6 protons is a carbon atom, no matter how many neutrons may be present). The atomic number is the number that determines the identity of an atom, that is, the element to which that atom belongs. On the other hand, a mass number is a number that is used to compare. Atomic Number The atomic number or proton number is defined as the total number of protons in the nucleus and is given the symbol Z. The number of electrons in an electrically-neutral atom is the same as the atomic number. The total electrical charge of the nucleus is therefore +Ze. Atomic number is defined as the number of protons in the nucleus of an atom. Elements are identified based on the number protons in their nucleus. Any atom with 2 protons is called a Helium atom. Has an atomic number double that of sodium’s. Has the lowest atomic number of any group 15 element. Is in group 1; has a higher atomic number than Cl but a lower atomic number than Br. Has between 35 and 55 protons and is in group 17. Has the lowest atomic mass on the periodic table.

Atoms are the fundamental building blocks of all matter and are composed of protons, neutrons, and electrons. Because atoms are electrically neutral, the number of positively charged protons must be equal to the number of negatively charged electrons. Since neutrons do not affect the charge, the number of neutrons is not dependent on the number of protons and will vary even among atoms of the same element.

Atomic Number

The atomic number (represented by the letter Z)of an element is the number of protons in the nucleus of each atom of that element. An atom can be classified as a particular element based solely on its atomic number. For example, any atom with an atomic number of 8 (its nucleus contains 8 protons) is an oxygen atom, and any atom with a different number of protons would be a different element. The periodic table (see figure below) displays all of the known elements and is arranged in order of increasing atomic number. In this table, an element's atomic number is indicated above the elemental symbol. Hydrogen, at the upper left of the table, has an atomic number of 1. Every hydrogen atom has one proton in its nucleus. Next on the table is helium, whose atoms have two protons in the nucleus. Lithium atoms have three protons, beryllium atoms have four, and so on.

Since atoms are neutral, the number of electrons in an atom is equal to the number of protons. Hydrogen atoms all have one electron occupying the space outside of the nucleus. Helium, with two protons, will have two electrons. In the chemical classroom, the proton count will always be equivalent to an atom's atomic number. This value will not change unless the nucleus decays or is bombarded (nuclear physics).

Mass Number

Experimental data showed that the vast majority of the mass of an atom is concentrated in its nucleus, which is composed of protons and neutrons. The mass number (represented by the letter A)is defined as the total number of protons and neutrons in an atom. Consider the table below, which shows data from the first six elements of the periodic table.

Table (PageIndex{1}): Atoms of the First Six Elements
Name	Symbol	Atomic Number (Z)	Protons	Neutrons	Electrons	Mass Number (A) (rounded to two decimals)
hydrogen	(ce{H})	1	1	0	1	1.01
helium	(ce{He})	2	2	2	2	4.00
lithium	(ce{Li})	3	3	4	3	6.94
beryllium	(ce{Be})	4	4	5	4	9.01
boron	(ce{B})	5	5	6	5	10.18
carbon	(ce{C})	6	6	6	6	12.01

Consider the element helium. Its atomic number is 2, so it has two protons in its nucleus. Its nucleus also contains two neutrons. Since (2 + 2 = 4), we know that the mass number of the helium atom is 4. Finally, the helium atom also contains two electrons, since the number of electrons must equal the number of protons. This example may lead you to believe that atoms have the same number of protons and neutrons, but a further examination of the table above will show that this is not the case. Lithium, for example, has three protons and four neutrons, giving it a mass number of 7.

Knowing the mass number and the atomic number of an atom allows you to determine the number of neutrons present in that atom by subtraction.

[text{Number of neutrons} = text{ rounded mass number} - text{atomic number}]

Atoms of the element chromium (left( ce{Cr} right)) have an atomic number of 24 and a mass number of 52. How many neutrons are in the nucleus of a chromium atom? To determine this, you would subtract as shown:

[52 - 24 = 28 : text{neutrons in a chromium atom}]

The composition of any atom can be illustrated with a shorthand notation called A/Z format. Both the atomic number and mass are written to the left of the chemical symbol. The 'A' value is written as a superscript while the 'Z' value is written as a subscript. For an example of this notation, look to the chromium atom shown below:

[ce{^{52}_{24}Cr}]

Another way to refer to a specific atom is to write the mass number of the atom after the name, separated by a hyphen. Symbol-mass format for the above atom would be written as Cr-52. In this notation, the atomic number is not included. You will need to refer to a periodic table for proton values.

Example (PageIndex{1})

Calculate each of the three subatomic particles and give specific group or period names for each atom.

mercury
platinum
bromine

Solutions

Hg (transition metal)- has 80 electrons, 80 protons, and 121 neutrons
Pt (transition metal)- has 78 electrons, 78 protons, and 117 neutrons
Br (halogen)- has 35 electrons, 35 protons, and 45 neutrons

Example (PageIndex{2})

Write both A/Z and symbol-mass formats for the atoms in Example (PageIndex{1}).

Solutions

(ce{^{201}_{80}Hg}) and Hg-201
(ce{^{195}_{78}Pt}) and Pt-195
(ce{^{80}_{35}Br}) and Br-80

Example (PageIndex{3})

Identify the elements based on the statements below.

Which element has 25 protons?
Which element has 0 neutrons?
Which element has 83 electrons?

Solutions

Atomic Number Chart

a. manganese

b. hydrogen

c. bismuth

Need More Practice?

Turn to section 3.E of this OER and answer questions #1-#2, #4, and #8.

Contributors

CK-12 Foundation by Sharon Bewick, Richard Parsons, Therese Forsythe, Shonna Robinson, and Jean Dupon.
Allison Soult, Ph.D. (Department of Chemistry, University of Kentucky)

We remember from our school chemistry course that every element has its own specific atomic number. It is the same as the number of protons that the atom of each element has, so sometimes atomic number is called proton number. It is always the whole number and it ranges from 1 to 118, according to the number of the element in the Periodic Table. This number can be really important and something essential to know, in relation to a certain chemical element which is the issue of our interest at the moment.

Why is this so? Why is the atomic number so important? First of all, it is the number that makes elements different from one another as it shows the number of protons in their nuclei. Also, knowing the atomic number of an element can give us an idea about the position of the element in the Periodic Table. Atomic number of an element never changes: for example, the atomic number of oxygen is always 8, and the atomic number of Chlorine is always 18. The atomic number is marked with the symbol Z, taken from a German word zahl (or atomzahl, which is 'atomic number' in German).

This website is created for those who need to know the atomic number of a central chemical element. By using our website, you can do it in just one click and receive short and correct information on this matter. There is also some extra summary on every each chemical element which can be found at our website, including the atomic weight of each element, as well as physical and chemical properties of every element and its importance. Use this website at any time when you need to get fast and precise information about atomic or proton number of chemical elements.

List of chemical elements in periodic table with atomic number, chemical symbol and atomic weight. You can sort the elements by clicking on the table headers. Please click on the element name for complete list of element properties.

Definition Of Atomic Number

Atomic Number	Chemical Symbol	Element Name	Atomic Weight (u)
1	H	Hydrogen	1.008
2	He	Helium	4.003
3	Li	Lithium	6.94
4	Be	Beryllium	9.012
5	B	Boron	10.81
6	C	Carbon	12.011
7	N	Nitrogen	14.007
8	O	Oxygen	15.999
9	F	Fluorine	18.998
10	Ne	Neon	20.18
11	Na	Sodium	22.99
12	Mg	Magnesium	24.305
13	Al	Aluminium	26.982
14	Si	Silicon	28.085
15	P	Phosphorus	30.974
16	S	Sulfur	32.06
17	Cl	Chlorine	35.45
18	Ar	Argon	39.948
19	K	Potassium	39.098
20	Ca	Calcium	40.078
21	Sc	Scandium	44.956
22	Ti	Titanium	47.867
23	V	Vanadium	50.942
24	Cr	Chromium	51.996
25	Mn	Manganese	54.938
26	Fe	Iron	55.845
27	Co	Cobalt	58.933
28	Ni	Nickel	58.693
29	Cu	Copper	63.546
30	Zn	Zinc	65.38
31	Ga	Gallium	69.723
32	Ge	Germanium	72.63
33	As	Arsenic	74.922
34	Se	Selenium	78.971
35	Br	Bromine	79.904
36	Kr	Krypton	83.798
37	Rb	Rubidium	85.468
38	Sr	Strontium	87.62
39	Y	Yttrium	88.906
40	Zr	Zirconium	91.224
41	Nb	Niobium	92.906
42	Mo	Molybdenum	95.95
43	Tc	Technetium	98
44	Ru	Ruthenium	101.07
45	Rh	Rhodium	102.906
46	Pd	Palladium	106.42
47	Ag	Silver	107.868
48	Cd	Cadmium	112.414
49	In	Indium	114.818
50	Sn	Tin	118.71
51	Sb	Antimony	121.76
52	Te	Tellurium	127.6
53	I	Iodine	126.904
54	Xe	Xenon	131.293
55	Cs	Caesium	132.905
56	Ba	Barium	137.327
57	La	Lanthanum	138.905
58	Ce	Cerium	140.116
59	Pr	Praseodymium	140.908
60	Nd	Neodymium	144.242
61	Pm	Promethium	145
62	Sm	Samarium	150.36
63	Eu	Europium	151.964
64	Gd	Gadolinium	157.25
65	Tb	Terbium	158.925
66	Dy	Dysprosium	162.5
67	Ho	Holmium	164.93
68	Er	Erbium	167.259
69	Tm	Thulium	168.934
70	Yb	Ytterbium	173.045
71	Lu	Lutetium	174.967
72	Hf	Hafnium	178.49
73	Ta	Tantalum	180.948
74	W	Tungsten	183.84
75	Re	Rhenium	186.207
76	Os	Osmium	190.23
77	Ir	Iridium	192.217
78	Pt	Platinum	195.084
79	Au	Gold	196.967
80	Hg	Mercury	200.592
81	Tl	Thallium	204.38
82	Pb	Lead	207.2
83	Bi	Bismuth	208.98
84	Po	Polonium	209
85	At	Astatine	210
86	Rn	Radon	222
87	Fr	Francium	223
88	Ra	Radium	226
89	Ac	Actinium	227
90	Th	Thorium	232.038
91	Pa	Protactinium	231.036
92	U	Uranium	238.029
93	Np	Neptunium	237
94	Pu	Plutonium	244
95	Am	Americium	243
96	Cm	Curium	247
97	Bk	Berkelium	247
98	Cf	Californium	251
99	Es	Einsteinium	252
100	Fm	Fermium	257
101	Md	Mendelevium	258
102	No	Nobelium	259
103	Lr	Lawrencium	266
104	Rf	Rutherfordium	267
105	Db	Dubnium	268
106	Sg	Seaborgium	269
107	Bh	Bohrium	270
108	Hs	Hassium	277
109	Mt	Meitnerium	278
110	Ds	Darmstadtium	281
111	Rg	Roentgenium	282
112	Cn	Copernicium	285
113	Nh	Nihonium	286
114	Fl	Flerovium	289
115	Mc	Moscovium	290
116	Lv	Livermorium	293
117	Ts	Tennessine	294
118	Og	Oganesson	294

Lists of Elements in Periodic Table

Atomic Number Is The Number Of What

You can also list the elements in various ordered properties with printable tables below.

Lists of Elements by Group Number in Periodic Table

Atomic Number Is The Number Of Protons In An Atom

» Group 1» Group 2» Group 3» Group 4» Group 5» Group 6» Group 7» Group 8» Group 9» Group 10» Group 11» Group 12» Group 13» Group 14» Group 15» Group 16» Group 17» Group 18