Arsenic (As)
Arsenic is element 33, a metalloid in group 15, period 4. A neutral atom has 33 protons, 42 neutrons and 33 electrons, 5 of them valence electrons.
Metalloid- Atomic number
- 33
- Atomic mass
- 74.922 u
- Valence electrons
- 5
- Protons · neutrons
- 33 · 42
- Electrons per shell
- 2, 8, 18, 5
- At room temperature
- solid
Bohr model of arsenic
The Bohr model draws arsenic as a nucleus of 33 protons and 42 neutrons, with its 33 electrons on 4 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 5 in shell 4. Press Build the atom to watch the shells fill from the inside out. The outer ring (the N shell) is dashed: the electrons there are the ones that take part in chemistry.
Real electrons don't travel in neat circles. They live in fuzzy orbitals, shown below, but the Bohr picture is the quickest way to count shells and valence electrons.
Electron configuration of arsenic
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³
Short form (noble gas notation): [Ar] 4s² 3d¹⁰ 4p³. The [Ar] stands for the electrons of argon, the noble gas before it.
How many valence electrons does arsenic have?
Arsenic has 5 valence electrons.
Arsenic is in group 15, and for groups 13–18 you subtract 10 from the group number: 15 − 10 = 5. It tends to share electrons in covalent bonds.
Protons, neutrons and electrons
| Protons | 33 | equal to the atomic number |
|---|---|---|
| Electrons | 33 | equal to the protons in a neutral atom |
| Neutrons | 42 | mass number 75 − 33 protons |
Atoms of the same element can have different numbers of neutrons (isotopes). The count above uses the rounded atomic mass, the way it's taught in class.
Arsenic properties
| Symbol | As |
|---|---|
| Category | Metalloid |
| Group · period · block | 15 · 4 · p |
| Atomic mass | 74.922 u |
| State at 25 °C | solid |
| Melting point | 1,090 K (817 °C, 1,500 °F) |
| Boiling point | 887 K (614 °C, 1,140 °F) |
| Density | 5.776 g/cm³ |
| Electronegativity (Pauling) | 2.18 |
| Atomic radius | 185 pm |
| First ionization energy | 9.815 eV |
| Electron affinity | 0.81 eV |
| Oxidation states | +5, +3, −3 |
| Common ion charge | +5, +3 |
| Discovered | Known since ancient times |
Where arsenic sits on the periodic table
Group 15: Nitrogen, Phosphorus, Antimony, Bismuth, Moscovium.
Questions about arsenic
How many valence electrons does arsenic have?
Arsenic has 5 valence electrons.
How many protons, neutrons and electrons does arsenic have?
A neutral arsenic atom has 33 protons and 33 electrons. Its most common form has about 42 neutrons (mass number 75 − 33 protons).
What is the electron configuration of arsenic?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p³, written in short form as [Ar] 4s² 3d¹⁰ 4p³.
What does the Bohr model of arsenic look like?
A nucleus of 33 protons and 42 neutrons, circled by 4 electron shells holding 2, 8, 18, 5 electrons.
What is the atomic number of arsenic?
33. The atomic number is the number of protons in the nucleus, and it is what makes an atom arsenic.