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18 Ar Argon 39.9481

Argon

NOBLE GAS
Bohr Model: Electrons arranged in energy shells around the nucleus.

Argon is a chemical element with symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most common gas in the Earth's atmosphere, at 0.934% (9,340 ppmv), making it over twice as abundant as the next most common atmospheric gas, water vapor (which averages about 4000 ppmv, but varies greatly), and 23 times as abundant as the next most common non-condensing atmospheric gas, carbon dioxide (400 ppmv), and more than 500 times as abundant as the next most common noble gas, neon (18 ppmv).

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Quick Facts

Atomic Number 18
Period 3
Group 18
Phase Gas
Appearance colorless gas exhibiting a lilac/violet glow when placed in a high voltage electric field
About Noble Gass

Noble gases are colorless, odorless, and extremely unreactive (inert) due to having full valence electron shells.

History

Discovered By Lord Rayleigh
Named By Unknown

Physical Properties

Atomic Mass 39.9481
Density 1.784 g/L
Molar Heat N/A J/(mol·K)
Phase Transition (Melting & Boiling Points)

Argon melts at 83.81 K and boils at 87.302 K.

Atomic Properties

Electron Config [Ne] 3s2 3p6
Electronegativity N/A
Electron Affinity -96 kJ/mol
Ionization Energy 1520.6 kJ/mol
Orbital Filling Diagram
This diagram visualizes electron configuration according to the Aufbau principle and Hund's rule. Boxes represent orbitals (s, p, d, f), while arrows indicate electrons with spin up or down. Electrons fill lower energy levels first and occupy orbitals singly before pairing.
Emission Spectrum Emission spectrum of Argon

Photograph

Image of Argon Source: Wikipedia

External Resources

Wikipedia PubChem Royal Society of Chemistry Periodic Videos (YouTube)

Study Guide

Atomic Number

The number of protons in the nucleus, which defines the element.

Atomic Mass

The average mass of an atom, including protons and neutrons.

Electronegativity

A measure of how strongly an atom attracts electrons in a bond.

Ionization Energy

The energy required to remove an electron from an atom.

Electron Config

The arrangement of electrons in the atom's energy shells.