What Is Arsenic?
Arsenic is a chemical element with the symbol As and atomic number 33, classified as a metalloid with both metallic and nonmetallic traits.
A toxic metalloid. Called the 'king of poisons'. Today used in semiconductor production.
As a metalloid in Period 4 and Group 15 (Nitrogen Group / Pnictogens), arsenic's placement on the table reflects its electron configuration of [Ar] 3d¹⁰ 4s² 4p³, which governs its bonding behavior and the type of compounds it typically forms.
Position on the Periodic Table
Arsenic is located in Period 4, the first row to include the transition metals.
Within that row, arsenic sits in Group 15 (Nitrogen Group / Pnictogens), alongside elements that share a similar number of valence electrons and, by extension, similar chemical behavior.
This position is not arbitrary — the periodic table was deliberately organized so that an element's row and column reveal its expected reactivity, bonding pattern, and physical state at a glance.
Physical & Chemical Properties
Arsenic has an atomic mass of 74.922 u and a density of 5.727 g/cm³. Its melting point is 817°C and its boiling point is 614°C.
As a metalloid, arsenic conducts electricity better than a typical nonmetal but worse than a true metal, a property known as semiconductivity.
Electron Configuration Explained
Arsenic's electron configuration is [Ar] 3d¹⁰ 4s² 4p³, describing how its electrons are distributed across shells and subshells around the nucleus.
Reading the configuration in order shows electrons filling shells of increasing energy, following the standard Aufbau principle that explains how atomic structure builds up across the periodic table.
The outermost (valence) electrons shown in this configuration are what ultimately determine arsenic's chemical reactivity and the type of bonds it forms with other elements.
History & Discovery
Arsenic has been known since ancient times, predating the modern, systematic study of chemical elements by thousands of years.
Its early use is traced through archaeological evidence — tools, artifacts, and written records — rather than a single documented discovery event, since ancient civilizations had no formal concept of a 'chemical element.'
What Is Arsenic Used For?
Arsenic's specific properties make it useful in several applications, including:
- Semiconductors
- Pesticides
- Wood preservatives
- Glass manufacturing
- Bronze alloys
These uses reflect arsenic's underlying classification as a metalloid, since the same properties that define its category in chemistry also determine where it becomes practically useful.
Fun Facts About Arsenic
- Arsenic was tasteless and odorless — the poison of choice for medieval assassins.
- Arsenic is element number 33 on the periodic table, in Period 4 and Group 15.
- Its standard atomic mass is 74.922 u, calculated from the natural abundance of its isotopes.
Frequently Asked Questions
What is Arsenic's atomic number?
Arsenic has atomic number 33.
What is Arsenic's chemical symbol?
Arsenic's symbol is As.
What group and period is Arsenic in?
Arsenic is in Period 4, Group 15 (Nitrogen Group / Pnictogens).
What is Arsenic's atomic mass?
Arsenic's standard atomic mass is 74.922 u.