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Atomic number

The number of protons in the nucleus of an atom determines an element's atomic number. In other words, each element has a unique number that identifies how many protons are in one atom of that element. For example, all hydrogen atoms, and only hydrogen atoms, contain one proton and have an atomic number of 1.

All carbon atoms, and only carbon atoms, contain six protons and have an atomic number of 6. Oxygen atoms contain 8 protons and have an atomic number of 8. The atomic number of an element never changes, meaning that the number of protons in the nucleus of every atom in an element is always the same.

For example the atomic number is mark with red colour in given periodic table:
H  01 He  02
Li  03 Be  04 B  05 C  06 N  07 O  08 F  09 Ne  10
Na  11 Mg  12 Al  13 Si  14 P  15 S 16 Cl  17 Ar  18
K  19 Ca  20 Sc 21 Ti  22 V  23 Cr 24 Mn  25 Fe  26 Co  27 Ni  28 Cu  29 Zn  30 Ga  31 Ge  32 As  33 Se  34 Br  35 Kr  36
Rb 37 Sr 38 Y  39 Zr 40 Nb 41 Mo 42 Tc 43 Ru 44 Rh 45 Pd 46 Ag 47 Cd 48 In 49 Sn 50 Sb 51 Te 52 I 53 Xe 54
Cs 55 Ba 56 La 57-71 Hf 72 Ta  73 W 74 Re  75 Os  76 Ir 77 Pt 78 Au 79 Hg 80 TI 81 Pb 82 Bi 83 Po 84 At 85 Rn 86
Fr 87 Ra 88 Ac 89-103 Rf 104 Db 105 Sg 106 Bh 107 Hs 108 Mt 109 Ds 110 Rg 111 cn 112 Nh 113 FI 114 Mc 115 Lv 116 Ts 117 Og 118
Lanthanoid series Ce57 Pr 59 Nd 60 Pm 61 Sm 62 Eu 63 Gd 64 Tb 65 Dy 66 Ho 67 Er 68 Tm 69 Yb 70 Lu 71
Actinoid series Th 90 Pa 91 U 92 Np 93 Pu  94 Am  95 Cm  96 Bk  97 Cf  98 Es  99 Fm  100 Md  101 No  102 Lr  103
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