Arsenic (As): Bohr Model, Protons, Neutrons & Electrons
Arsenic is element number 33, a metalloid in group 15, period 4. Its atom has 33 protons, 42 neutrons and 33 electrons arranged in 4 shells: 2, 8, 18, 5.
- 33 protons
- 42 neutrons
- 33 electrons
Protons, neutrons and electrons in arsenic
Le atomic number of arsenic is 33, so every arsenic atom has 33 protons. A neutral atom has the same number of electrons, 33. The number of neutrons depends on the isotope: neutrons = mass number − atomic number. For arsenic-75, that is 75 − 33 = 42 neutrons.
How to draw the Bohr model of arsenic
- Draw the nucleus and write 33 p⁺ et 42 n⁰ inside it.
- Draw the first shell and put 2 electrons on it (the first shell holds at most 2).
- Draw the second shell and put 8 electrons on it.
- Draw the third shell and put 18 electrons on it.
- Draw the fourth shell and put 5 electrons on it.
- Check: 2 + 8 + 18 + 5 = 33 electrons, the same as the number of protons.
From potassium onwards the shells don’t simply fill as 2, 8, 8: the fourth shell starts before the third is full, so the real arrangement above (2, 8, 18, 5) is the one to draw.
Electron configuration
The electron configuration of arsenic is [Ar] 4s² 3d¹⁰ 4p³. Its 5 outer electrons decide how it reacts: it usually shares electrons in covalent bonds.
Key facts about arsenic
| Symbol | As |
|---|---|
| Atomic number | 33 |
| Relative atomic mass | 74.922 |
| Group · period · block | group 15 · period 4 · p-block |
| Type | Métalloïde |
| State at 25 °C | solid |
| Couches électroniques | 2, 8, 18, 5 |
| Electron configuration | [Ar] 4s² 3d¹⁰ 4p³ |
| Électrons de valence | 5 |
| Common ion | — |
| Electronegativity (Pauling) | 2.18 |
| First ionisation energy | 9.79 eV (945 kJ/mol) |
| Atomic radius (empirical) | 115 pm |
Où arsenic sits on the periodic table
Foire aux questions
How many protons, neutrons and electrons does arsenic have?
Arsenic has 33 protons and 33 electrons (atomic number 33). Its most common isotope, arsenic-75, has 75 − 33 = 42 neutrons.
What is the electron configuration of arsenic?
[Ar] 4s² 3d¹⁰ 4p³. In shells that is 2, 8, 18, 5.
How many valence electrons does arsenic have?
5. Arsenic is in group 15, and its outer shell holds 5 electrons.
Where is arsenic on the periodic table?
In group 15, period 4, in the p-block. It is a metalloid.
