Chromium (Cr)
Chromium is element 24, a transition metal in group 6, period 4. A neutral atom has 24 protons, 28 neutrons and 24 electrons, 1 of them valence electrons.
Transition metal- Atomic number
- 24
- Atomic mass
- 51.996 u
- Valence electrons
- 1*
- Protons · neutrons
- 24 · 28
- Electrons per shell
- 2, 8, 13, 1
- At room temperature
- solid
Bohr model of chromium
The Bohr model draws chromium as a nucleus of 24 protons and 28 neutrons, with its 24 electrons on 4 circular shells: 2 in shell 1, 8 in shell 2, 13 in shell 3, 1 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 chromium
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵
Short form (noble gas notation): [Ar] 4s¹ 3d⁵. The [Ar] stands for the electrons of argon, the noble gas before it.
Chromium breaks the usual filling order. The Aufbau rule predicts 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d⁴, but experiments show the configuration above. Moving one electron from the s subshell gives a half-full or full d subshell, which is more stable.
How many valence electrons does chromium have?
Chromium has 1 valence electron.
Chromium is a transition metal, so its valence electrons are less clear-cut than for main-group elements. It has 1 electron in its outermost shell (shell 4), and electrons in its inner d subshell can also take part in bonding.
Protons, neutrons and electrons
| Protons | 24 | equal to the atomic number |
|---|---|---|
| Electrons | 24 | equal to the protons in a neutral atom |
| Neutrons | 28 | mass number 52 − 24 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.
Chromium properties
| Symbol | Cr |
|---|---|
| Category | Transition metal |
| Group · period · block | 6 · 4 · d |
| Atomic mass | 51.996 u |
| State at 25 °C | solid |
| Melting point | 2,180 K (1,910 °C, 3,460 °F) |
| Boiling point | 2,944 K (2,670 °C, 4,840 °F) |
| Density | 7.15 g/cm³ |
| Electronegativity (Pauling) | 1.66 |
| Atomic radius | 189 pm |
| First ionization energy | 6.767 eV |
| Electron affinity | 0.666 eV |
| Oxidation states | +6, +3, +2 |
| Common ion charge | +6, +3 |
| Discovered | 1797 by Louis Nicolas Vauquelin |
Where chromium sits on the periodic table
Group 6: Molybdenum, Tungsten, Seaborgium.
Questions about chromium
How many valence electrons does chromium have?
Chromium has 1 valence electron in its outer shell; inner d or f electrons can also bond.
How many protons, neutrons and electrons does chromium have?
A neutral chromium atom has 24 protons and 24 electrons. Its most common form has about 28 neutrons (mass number 52 − 24 protons).
What is the electron configuration of chromium?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s¹ 3d⁵, written in short form as [Ar] 4s¹ 3d⁵.
What does the Bohr model of chromium look like?
A nucleus of 24 protons and 28 neutrons, circled by 4 electron shells holding 2, 8, 13, 1 electrons.
What is the atomic number of chromium?
24. The atomic number is the number of protons in the nucleus, and it is what makes an atom chromium.