Chlorine (Cl)
Chlorine is element 17, a halogen in group 17, period 3. A neutral atom has 17 protons, 18 neutrons and 17 electrons, 7 of them valence electrons.
Halogen- Atomic number
- 17
- Atomic mass
- 35.45 u
- Valence electrons
- 7
- Protons · neutrons
- 17 · 18
- Electrons per shell
- 2, 8, 7
- At room temperature
- gas
Bohr model of chlorine
The Bohr model draws chlorine as a nucleus of 17 protons and 18 neutrons, with its 17 electrons on 3 circular shells: 2 in shell 1, 8 in shell 2, 7 in shell 3. Press Build the atom to watch the shells fill from the inside out. The outer ring (the M 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 chlorine
1s² 2s² 2p⁶ 3s² 3p⁵
Short form (noble gas notation): [Ne] 3s² 3p⁵. The [Ne] stands for the electrons of neon, the noble gas before it.
How many valence electrons does chlorine have?
Chlorine has 7 valence electrons.
Chlorine is in group 17, and for groups 13–18 you subtract 10 from the group number: 17 − 10 = 7. It tends to gain 1 electron to complete an octet, forming a −1 ion.
Protons, neutrons and electrons
| Protons | 17 | equal to the atomic number |
|---|---|---|
| Electrons | 17 | equal to the protons in a neutral atom |
| Neutrons | 18 | mass number 35 − 17 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.
Chlorine properties
| Symbol | Cl |
|---|---|
| Category | Halogen |
| Group · period · block | 17 · 3 · p |
| Atomic mass | 35.45 u |
| State at 25 °C | gas |
| Melting point | 171.7 K (-101 °C, -151 °F) |
| Boiling point | 239.1 K (-34 °C, -29.3 °F) |
| Density | 0.0032 g/cm³ |
| Electronegativity (Pauling) | 3.16 |
| Atomic radius | 175 pm |
| First ionization energy | 12.968 eV |
| Electron affinity | 3.617 eV |
| Oxidation states | +7, +5, +1, −1 |
| Common ion charge | −1 |
| Discovered | 1774 by Carl Wilhelm Scheele |
Where chlorine sits on the periodic table
Group 17: Fluorine, Bromine, Iodine, Astatine, Tennessine.
Questions about chlorine
How many valence electrons does chlorine have?
Chlorine has 7 valence electrons.
How many protons, neutrons and electrons does chlorine have?
A neutral chlorine atom has 17 protons and 17 electrons. Its most common form has about 18 neutrons (mass number 35 − 17 protons).
What is the electron configuration of chlorine?
1s² 2s² 2p⁶ 3s² 3p⁵, written in short form as [Ne] 3s² 3p⁵.
What does the Bohr model of chlorine look like?
A nucleus of 17 protons and 18 neutrons, circled by 3 electron shells holding 2, 8, 7 electrons.
What is the atomic number of chlorine?
17. The atomic number is the number of protons in the nucleus, and it is what makes an atom chlorine.