Selenium (Se)
Selenium is element 34, a nonmetal in group 16, period 4. A neutral atom has 34 protons, 45 neutrons and 34 electrons, 6 of them valence electrons.
Nonmetal- Atomic number
- 34
- Atomic mass
- 78.97 u
- Valence electrons
- 6
- Protons · neutrons
- 34 · 45
- Electrons per shell
- 2, 8, 18, 6
- At room temperature
- solid
Bohr model of selenium
The Bohr model draws selenium as a nucleus of 34 protons and 45 neutrons, with its 34 electrons on 4 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 6 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 selenium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴
Short form (noble gas notation): [Ar] 4s² 3d¹⁰ 4p⁴. The [Ar] stands for the electrons of argon, the noble gas before it.
How many valence electrons does selenium have?
Selenium has 6 valence electrons.
Selenium is in group 16, and for groups 13–18 you subtract 10 from the group number: 16 − 10 = 6. It tends to gain 2 electrons to complete an octet, forming a −2 ion.
Protons, neutrons and electrons
| Protons | 34 | equal to the atomic number |
|---|---|---|
| Electrons | 34 | equal to the protons in a neutral atom |
| Neutrons | 45 | mass number 79 − 34 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.
Selenium properties
| Symbol | Se |
|---|---|
| Category | Nonmetal |
| Group · period · block | 16 · 4 · p |
| Atomic mass | 78.97 u |
| State at 25 °C | solid |
| Melting point | 493.6 K (221 °C, 429 °F) |
| Boiling point | 958 K (685 °C, 1,260 °F) |
| Density | 4.809 g/cm³ |
| Electronegativity (Pauling) | 2.55 |
| Atomic radius | 190 pm |
| First ionization energy | 9.752 eV |
| Electron affinity | 2.021 eV |
| Oxidation states | +6, +4, −2 |
| Common ion charge | −2 |
| Discovered | 1817 by Jöns Jakob Berzelius |
Where selenium sits on the periodic table
Group 16: Oxygen, Sulfur, Tellurium, Polonium, Livermorium.
Questions about selenium
How many valence electrons does selenium have?
Selenium has 6 valence electrons.
How many protons, neutrons and electrons does selenium have?
A neutral selenium atom has 34 protons and 34 electrons. Its most common form has about 45 neutrons (mass number 79 − 34 protons).
What is the electron configuration of selenium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁴, written in short form as [Ar] 4s² 3d¹⁰ 4p⁴.
What does the Bohr model of selenium look like?
A nucleus of 34 protons and 45 neutrons, circled by 4 electron shells holding 2, 8, 18, 6 electrons.
What is the atomic number of selenium?
34. The atomic number is the number of protons in the nucleus, and it is what makes an atom selenium.