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Atomic Number

02nd Jul 2019 @ 15 min read

Physical Chemistry

An atom comprises electrons, protons, and neutrons. Electrons revolve around the nucleus of an atom just like the Earth around the Sun. Protons and neutrons reside in the centre of an atom called nucleus. Electrons are negatively charged while protons are positively charged. Neutrons are neutral i.e., no charge. In an unbounded stable atom, the number of protons equals the number of electrons. This makes the net charge of an atom zero. These number of protons are unique for a given element, and the number is called the atomic number or proton number.

Definition of Atomic Number

The Gold Book of the International Union of Pure and Applied Chemistry (IUPAC) defines an atomic number as “the number of protons in an atomic nucleus.”

Symbol

The symbol used for the atomic number is Z.

Representation

The atomic number is subscripted to the left of the symbol of an element. Consider an example of calcium, which has the atomic number of 20 (Z = 20). So, number 20 is subscripted to the symbol Ca i.e., 20Ca.

Note: While the atomic number is subscripted, the mass number of an element is superscripted (see below figure).

Atomic Number vs Mass Number

As discussed, the atomic number is the number of protons presents in the nucleus while the mass number is the number of nucleons (protons and neutrons) presents in the nucleus of an atom. This both numbers are different by the definition itself. The below table highlights some of the key differences between the two numbers.

 Atomic Number Mass Number The atomic number is the number of protons present in the nucleus of an atom. The mass number is the number of nucleons (protons and neutrons) present in the nucleus of an atom. It is denoted as Z. It is denoted as A. It is always fixed for a given element, for example, the atomic number of an oxygen atom is 16. This is true for any oxygen atom in the universe. It can be different for a given element, for example, an oxygen atom can have a mass number of 16, 17, or 18. Two different elements always have a different atomic number as in 4020Ca and 4018Ar. Two different elements can have the same mass number as in 4020Ca and 4018Ar. It is always a whole number. It is also always a whole number. It is always smaller than the mass number (except in the case of 11H where both are equal). In fact, the atomic number is a subset of mass number. It is always larger than the atomic number (except in the case of 11H where both are equal).

Relation with Mass Number and Neutron Number

The atomic number (Z) is related to the mass number (A) and the neutron number (N) by the following equation.

Atomic Number and Atomic Weight

The atomic weight or the relative atomic mass is the ratio of the average atomic mass of an atom to some reference standard. If the reference standard is considered 1 u, the atomic weight is the ratio of the average atomic mass to one unified mass unit (1 u). In other words, it is the numeric value of the average atomic mass. It is a dimensionless quantity. For lighter elements, the atomic weight almost twice the atomic number (see below figure). But for heavier elements, the atomic weight exceeds the twice of the atomic number. This is due to an increase in the neutron number compare to the proton number.

Atomic Number and Chemical Properties

The chemical properties of an atom are determined by the number of electrons particularly the number of electrons present in the outermost shell. For an uncharged, unbounded atom, the number of electrons always equals the number of protons. Thus, the atomic number decides the chemical nature of an atom.

Atomic Number and Isotopes

Isotopes are the atoms of the same elements, but with a different mass number. The atomic number can be used to distinguished the isotopes of an element. This is because isotopes of a given element have the same number of protons; they only differ in the number of neutrons. Instead, the mass number, which incorporates the neutron number, is used to identify the isotopes. Consider an example of oxygen. The common isotopes of oxygen are 16
8
O
, 17
8
O
, and 18
8
O
. All these isotopes have the atomic number of 8; they only differ in the mass number.

List of Elements by Atomic Number

The below table lists all 118 elements by atomic number.

 Atomic Number Element Symbol 1 Hydrogen H 2 Helium He 3 Lithium Li 4 Beryllium Be 5 Boron B 6 Carbon C 7 Nitrogen N 8 Oxygen O 9 Fluorine F 10 Neon Ne 11 Sodium Na 12 Magnesium Mg 13 Aluminium Al 14 Silicon Si 15 Phosphorus P 16 Sulphur S 17 Chlorine Cl 18 Argon Ar 19 Potassium K 20 Calcium Ca 21 Scandium Sc 22 Titanium Ti 23 Vanadium V 24 Chromium Cr 25 Manganese Mn 26 Iron Fe 27 Cobalt Co 28 Nickel Ni 29 Copper Cu 30 Zinc Zn 31 Gallium Ga 32 Germanium Ge 33 Arsenic As 34 Selenium Se 35 Bromine Br 36 Krypton Kr 37 Rubidium Rb 38 Strontium Sr 39 Yttrium Y 40 Zirconium Zr 41 Niobium Nb 42 Molybdenum Mo 43 Technetium Tc 44 Ruthenium Ru 45 Rhodium Rh 46 Palladium Pd 47 Silver Ag 48 Cadmium Cd 49 Indium In 50 Tin Sn 51 Antimony Sb 52 Tellurium Te 53 Iodine I 54 Xenon Xe 55 Caesium Cs 56 Barium Ba 57 Lanthanum La 58 Cerium Ce 59 Praseodymium Pr 60 Neodymium Nd 61 Promethium Pm 62 Samarium Sm 63 Europium Eu 64 Gadolinium Gd 65 Terbium Tb 66 Dysprosium Dy 67 Holmium Ho 68 Erbium Er 69 Thulium Tm 70 Ytterbium Yb 71 Lutetium Lu 72 Hafnium Hf 73 Tantalum Ta 74 Tungsten W 75 Rhenium Re 76 Osmium Os 77 Iridium Ir 78 Platinum Pt 79 Gold Au 80 Mercury Hg 81 Thallium Tl 82 Lead Pb 83 Bismuth Bi 84 Polonium Po 85 Astatine At 86 Radon Rn 87 Francium Fr 88 Radium Ra 89 Actinium Ac 90 Thorium Th 91 Protactinium Pa 92 Uranium U 93 Neptunium Np 94 Plutonium Pu 95 Americium Am 96 Curium Cm 97 Berkelium Bk 98 Californium Cf 99 Einsteinium Es 100 Fermium Fm 101 Mendelevium Md 102 Nobelium No 103 Lawrencium Lr 104 Rutherfordium Rf 105 Dubnium Db 106 Seaborgium Sg 107 Bohrium Bh 108 Hassium Hs 109 Meitnerium Mt 110 Darmstadtium Ds 111 Roentgenium Rg 112 Copernicium Cn 113 Nihonium Nh 114 Flerovium Fl 115 Moscovium Mc 116 Livermorium Lv 117 Tennessine Ts 118 Oganesson Og

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