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Helium – Atomic Number – Atomic Mass – Density of Helium

Atomic Number of Helium

Helium is a chemical element with atomic number 2 which means there are 2 protons and 2 electrons in the atomic structure. The chemical symbol for Helium is He.

Since the number of electrons is responsible for the chemical behavior of atoms, the atomic number identifies the various chemical elements.

How does the atomic number determine the chemical behavior of atoms?

Atomic Mass of Helium

Atomic mass of Helium is 4.0026 u. 

Proton Number - Atomic Number - Density of Helium
Note that each element may contain more isotopes. Therefore this resulting atomic mass is calculated from naturally-occurring isotopes and their abundance.

The unit of measure for mass is the atomic mass unit (amu). One atomic mass unit is equal to 1.66 x 10-24 grams. One unified atomic mass unit is approximately the mass of one nucleon (either a single proton or neutron) and is numerically equivalent to 1 g/mol.

For 12C, the atomic mass is exactly 12u, since the atomic mass unit is defined from it. For other isotopes, the isotopic mass usually differs and is usually within 0.1 u of the mass number. For example, 63Cu (29 protons and 34 neutrons) has a mass number of 63, and an isotopic mass in its nuclear ground state is 62.91367 u.

There are two reasons for the difference between mass number and isotopic mass, known as the mass defect:

  1. The neutron is slightly heavier than the proton. This increases the mass of nuclei with more neutrons than protons relative to the atomic mass unit scale based on 12C with equal numbers of protons and neutrons.
  2. The nuclear binding energy varies between nuclei. A nucleus with greater binding energy has lower total energy, and therefore a lower mass according to Einstein’s mass-energy equivalence relation E = mc2. For 63Cu, the atomic mass is less than 63, so this must be the dominant factor.

The atomic mass number determines especially the atomic mass of atoms. The mass number is different for each different isotope of a chemical element.

How does the atomic mass determine the density of materials?

Density of Helium

Density of Helium is 0.1785g/cm3.
Density - Gas - Liquid - Solid

Typical densities of various substances at atmospheric pressure.

Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:

ρ = m/V

In other words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).

See also: What is Density

See also: Densest Materials of the Earth

density - chemical elements

Helium – Properties Summary

Element Helium
Atomic Number 2
Symbol He
Element Category Noble Gas
Phase at STP Gas
Atomic Mass [amu] 4.0026
Density at STP [g/cm3] 0.1785
Electron Configuration 1s2
Possible Oxidation States 0
Electron Affinity [kJ/mol]
Electronegativity [Pauling scale]
1st Ionization Energy [eV] 24.5874
Year of Discovery 1895
Discoverer Ramsey, Sir William & Cleve, Per Teodor
Thermal properties
Melting Point [Celsius scale] -272.2
Boiling Point [Celsius scale] -268.9
Thermal Conductivity [W/m K] 0.1513
Specific Heat [J/g K] 5.193
Heat of Fusion [kJ/mol]
Heat of Vaporization [kJ/mol] 0.0845

 

Helium in Periodic Table

Hydro­gen1H He­lium2He
Lith­ium3Li Beryl­lium4Be Boron5B Carbon6C Nitro­gen7N Oxy­gen8O Fluor­ine9F Neon10Ne
So­dium11Na Magne­sium12Mg Alumin­ium13Al Sili­con14Si Phos­phorus15P Sulfur16S Chlor­ine17Cl Argon18Ar
Potas­sium19K Cal­cium20Ca Scan­dium21Sc Tita­nium22Ti Vana­dium23V Chrom­ium24Cr Manga­nese25Mn Iron26Fe Cobalt27Co Nickel28Ni Copper29Cu Zinc30Zn Gallium31Ga Germa­nium32Ge Arsenic33As Sele­nium34Se Bromine35Br Kryp­ton36Kr
Rubid­ium37Rb Stront­ium38Sr Yttrium39Y Zirco­nium40Zr Nio­bium41Nb Molyb­denum42Mo Tech­netium43Tc Ruthe­nium44Ru Rho­dium45Rh Pallad­ium46Pd Silver47Ag Cad­mium48Cd Indium49In Tin50Sn Anti­mony51Sb Tellur­ium52Te Iodine53I Xenon54Xe
Cae­sium55Cs Ba­rium56Ba Lan­thanum57La 1 asterisk Haf­nium72Hf Tanta­lum73Ta Tung­sten74W Rhe­nium75Re Os­mium76Os Iridium77Ir Plat­inum78Pt Gold79Au Mer­cury80Hg Thallium81Tl Lead82Pb Bis­muth83Bi Polo­nium84Po Asta­tine85At Radon86Rn
Fran­cium87Fr Ra­dium88Ra Actin­ium89Ac 1 asterisk Ruther­fordium104Rf Dub­nium105Db Sea­borgium106Sg Bohr­ium107Bh Has­sium108Hs Meit­nerium109Mt Darm­stadtium110Ds Roent­genium111Rg Coper­nicium112Cn Nihon­ium113Nh Flerov­ium114Fl Moscov­ium115Mc Liver­morium116Lv Tenness­ine117Ts Oga­nesson118Og
1 asterisk Cerium58Ce Praseo­dymium59Pr Neo­dymium60Nd Prome­thium61Pm Sama­rium62Sm Europ­ium63Eu Gadolin­ium64Gd Ter­bium65Tb Dyspro­sium66Dy Hol­mium67Ho Erbium68Er Thulium69Tm Ytter­bium70Yb Lute­tium71Lu
1 asterisk Thor­ium90Th Protac­tinium91Pa Ura­nium92U Neptu­nium93Np Pluto­nium94Pu Ameri­cium95Am Curium96Cm Berkel­ium97Bk Califor­nium98Cf Einstei­nium99Es Fer­mium100Fm Mende­levium101Md Nobel­ium102No Lawren­cium103Lr