Understanding the chemical behavior of Arsenic (As), atomic number 33, begins with an examination of its electron structure. Arsenic is a naturally occurring element that straddles the line between metals and nonmetals. Its tendency to form specific chemical bonds and its resulting physical properties are dictated by the arrangement of the electrons orbiting its nucleus. Analyzing the distribution of these outer electrons predicts how Arsenic will interact with other substances.
What Defines a Valence Electron
Valence electrons are specifically those found in the outermost energy level, referred to as the valence shell. These electrons are exposed to the environment and are primarily responsible for determining how an atom will form bonds with other atoms.
Electrons in the inner energy shells are called core electrons. These core electrons are tightly bound to the positively charged nucleus and do not typically participate in chemical reactions. Inner shells possess lower energy levels, meaning core electrons are significantly harder to dislodge than their outer-shell counterparts.
Because valence electrons are farthest from the nucleus, they require less energy to be removed or shared. The number of valence electrons an atom possesses governs its desire to achieve a stable, full outer shell. Atoms will gain, lose, or share these outermost electrons to reach a lower-energy, more stable configuration, driving chemical behavior.
Locating Arsenic and Determining the Count
Arsenic’s position on the periodic table provides the simplest method for determining its valence electron count. Arsenic has an atomic number of 33, meaning a neutral atom contains 33 protons and 33 electrons. Elements in the same vertical column, known as a group, share the same number of valence electrons.
Arsenic is found in Period 4 and is located in Group 15 of the periodic table. For the main group elements, the group number directly correlates with the number of valence electrons. The column is also known as the Pnictogens.
Following this rule, Arsenic possesses five valence electrons. This count applies consistently to all elements in Group 15, including Nitrogen and Phosphorus. For all main groups (Groups 1, 2, and 13–18), the last digit of the group number indicates the total number of valence electrons.
Arsenic’s location in Period 4 indicates that its outermost electrons are in the fourth principal energy level. This simple relationship between location and electron count confirms the number of five.
Arsenic’s Full Electron Configuration
To scientifically confirm the valence count, chemists use the electron configuration, which describes the arrangement of all 33 electrons. The full ground state configuration for an Arsenic atom is \(1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^3\).
A condensed form replaces the 28 inner core electrons with the symbol of the nearest noble gas, Argon (\([\text{Ar}]\)). The resulting configuration is \([\text{Ar}] 4s^2 3d^{10} 4p^3\).
Valence electrons are defined as those residing in the shell with the highest principal quantum number, \(n\). In Arsenic’s configuration, the highest number is \(n=4\), meaning the valence shell includes the \(4s\) and \(4p\) orbitals. The \(3d^{10}\) electrons are considered part of the core because they belong to the \(n=3\) shell.
Summing the electrons in the \(n=4\) shell confirms the count. There are two electrons in the \(4s\) orbital (\(4s^2\)) and three electrons in the \(4p\) orbital (\(4p^3\)), totaling \(2 + 3 = 5\) valence electrons.
How Five Valence Electrons Determine Arsenic’s Chemistry
The presence of five valence electrons dictates Arsenic’s chemical reactivity. Atoms strive to achieve the stable configuration of eight outer electrons, known as the octet rule. Since Arsenic has five, it achieves stability by either gaining three electrons or by sharing its five electrons.
The simplest path to a full octet is gaining three electrons, forming the arsenide ion (\(As^{3-}\)), which results in an oxidation state of \(-3\). This behavior is typical of nonmetals and is observed in compounds called arsenides.
Arsenic also frequently achieves stability through covalent bonding by sharing its valence electrons. By sharing its three \(p\)-orbital electrons, it forms three bonds, leading to a \(+3\) oxidation state, such as in Arsenic trioxide (\(As_2O_3\)). Furthermore, Arsenic can utilize all five of its valence electrons in sharing, which results in the maximum oxidation state of \(+5\), seen in compounds like Arsenic pentoxide (\(As_2O_5\)). This bonding flexibility contributes to Arsenic’s classification as a metalloid, an element with properties intermediate between a metal and a nonmetal.