Atomic Number? Definition, Examples & Classification in Periodic Table
We explain what the atomic number is and its relation to the atomic mass. In addition, the atomic number of each chemical element.
What is the Atomic Number?
In both physics and chemistry , the atomic number is the total number of protons that make up the atomic nucleus of a given chemical element.
It is usually denoted by the letter Z (from the German word zahl , “number”) and placed as a subscript to the left of the chemical symbol of the element in question, just below the mass number (A). As follows:
^{23 }_{11} Na (element: sodium, atomic number: 11, and mass number: 23).
All atoms are composed of subatomic particles : some deposited in its nucleus ( protons and neutrons ) and others rotating around it ( electrons ). The former have a positive charge (protons) or neutral (neutrons) and the latter have a negative charge (electrons).
Since atoms in nature are electrically neutral, the amount of positive and negative particles is equivalent, so if an atom has Z = 11, it will have not only eleven protons, but the same number of electrons around it.
In addition, the atomic number allows to organize the known elements in the Periodic Table , going from the smallest to the largest number of protons in the nucleus. Thus, for example, hydrogen (H) has only one proton (Z = 1), while oganeson (Og) has one hundred eighteen (Z = 118). Thus, light elements can be distinguished from heavy elements.
Examples of atomic number
These are the atomic numbers of the complete Periodic Table:
Hydrogen (H): Z = 1  Helium (He): Z = 2 
Lithium (Li): Z = 3  Beryllium (Be): Z = 4 
Boron (B): Z = 5  Carbon (C): Z = 6 
Nitrogen (N): Z = 7  Oxygen (O): Z = 8 
Fluorine (F): Z = 9  Neon (Ne): Z = 10 
Sodium (Na): Z = 11  Magnesium (Mg): Z = 12 
Aluminum (Al): Z = 13  Silicon (Si): Z = 14 
Phosphorus (P): Z = 15  Sulfur (S): Z = 16 
Chlorine (Cl): Z = 17  Argon (Ar): Z = 18 
Potassium (K): Z = 19  Calcium (Ca): Z = 20 
Scandium (Sc): Z = 21  Titanium (Ti): Z = 22 
Vanadium (V): Z = 23  Chromium (Cr): Z = 24 
Manganese (Mn): Z = 25  Iron (Fe): Z = 26 
Cobalt (Co): Z = 27  Nickel (Ni): Z = 28 
Copper (Cu): Z = 29  Zinc (Zn): Z = 30 
Gallium (Ga): Z = 31  Germanium (Ge): Z = 32 
Arsenic (As): Z = 33  Selenium (Se): Z = 34 
Bromine (Br): Z = 35  Krypton (Kr): Z = 36 
Rubidium (Rb): Z = 37  Strontium (Sr): Z = 38 
Yttrium (Y): Z = 39  Zirconium (Zr): Z = 40 
Niobium (Ni): Z = 41  Molybdenum (Mb): Z = 42 
Technetium (Tc): Z = 43  Ruthenium (Ru): Z = 44 
Rhodium (Rh): Z = 45  Palladium (Pd): Z = 46 
Silver (Ag): Z = 47  Cadmium (Cd): Z = 48 
Indium (In): Z = 49  Tin (Sn): Z = 50 
Antimony (Sb): Z = 51  Tellurium (Te): Z = 52 
Iodine (I): Z = 53  Xenon (Xe): Z = 54 
Cesium (Cs): Z = 55  Barium (Ba): Z = 56 
Lanthanum (La): Z = 57  Cerium (Ce): Z = 58 
Praseodymium (Pr): Z = 59  Neodymium (Nd): Z = 60 
Promethium (Pr): Z = 61  Samarium (Sm): Z = 62 
Europium (Eu): Z = 63  Gadolinium (Gd): Z = 64 
Terbium (Tb): Z = 65  Dysprosium (Dy): Z = 66 
Holmium (Ho): Z = 67  Erbium (Er): Z = 68 
Thulium (Tm): Z = 69  Ytterbium (Yb): Z = 70 
Lutetium (Lu): Z = 71  Hafnium (Hf): Z = 72 
Tantalum (Ta): Z = 73  Tungsten (W): Z = 74 
Rhenium (Re): Z = 75  Osmium (Os): Z = 76 
Iridium (Ir): Z = 77  Platinum (Pt): Z = 78 
Gold (Au): Z = 79  Mercury (Hg): Z = 80 
Thallium (Tl): Z = 81  Lead (Pb): Z = 82 
Bismuth (Bi): Z = 83  Polonium (Po): Z = 84 
Astatine (At): Z = 85  Radon (Rn): Z = 86 
Francium (Fr): Z = 87  Radium (Ra): Z = 88 
Actinium (Ac): Z = 89  Thorium (Th): Z = 90 
Protactinium (Pa): Z = 91  Uranium (U): Z = 92 
Neptunium (Np): Z = 93  Plutonium (Pu): Z = 94 
Americium (Am): Z = 95  Curium (Cm): Z = 96 
Berkelium (Bk): Z = 97  Californium (Cf): Z = 98 
Einsteinium (Es): Z = 99  Fermium (Fm): Z = 100 
Mendelevium (Md): Z = 101  Nobelium (No): Z = 102 
Lawrencium (Lr): Z = 103  Rutherfordium (Rf): Z = 104 
Dubnium (Db): Z = 105  Seaborgium (Sg): Z = 106 
Bohrium (Bh): Z = 107  Hassium (Hs): Z = 108 
Meitnerium (Mt): Z = 109  Darmstadtium (Ds): Z = 110 

Copernicium (Cn): Z = 112 
Nihonium (Nh): Z = 113  Flerovium (Fl): Z = 114 
Moscovium (Mc): Z = 115  Livermorium (Lv): Z = 116 
Tennessine (Ts): Z = 117  Oganesson (Og): Z = 118 
Mass number and atomic mass
The mass number is the sum of protons and neutrons . It is denoted by the letter A (from the German Atomgewicht ) as a superscript to the left of the chemical symbol (for example: ^{23} Na).
It is usually about twice the atomic number, since the neutrons provide stability to the atomic nucleus, thus overcoming the natural repulsion between positively charged protons. Unlike the atomic number, the mass number varies with each isotope.
The mass number can be calculated according to the formula:
Mass number (A) = Atomic number (Z) + number of neutrons (N).
The mass number should not be confused with atomic mass. Atomic mass is measured in units or Da . This unit is calculated from the carbon atom and each u is one twelfth of its mass. The periodic table shows the atomic mass of the most stable isotope.