Chlorine
Chlorine is a chemical element with symbol Cl and atomic number 17. It also has a relative atomic mass of 35.5. Chlorine is in the halogen group (17) and is the second lightest halogen following fluorine.
At a glance
- Atomic number
- 17
- Atomic mass
- 35.45 u
- Phase
- Gas
- Density
- 3.2 g/L
- Melting point
- 171.6 K
- Boiling point
- 239.11 K
- First ionization energy
- 1,251.2 kJ/mol
- Electronegativity
- 3.16
- Electron affinity
- 348.575 kJ/mol
- Molar heat capacity
- Unavailable
- Period
- 3
- Appearance
- pale yellow-green gas
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Chlorine’s orbitals use a hydrogen-like approximation. Every orbital of each visible subshell is shown: in Shapes as the surface holding half its probability, with light and dark shades for opposite signs of the wavefunction; in Cloud as dots where its electrons are likely to be. Sizes are stepped by shell, like the Shell model.
10 is the [Ne] core total across two shells.
Preparing the orbital view…
17 protons18 neutrons
Chlorine-35: its most common form in nature, or its longest-lived if it has none. The nucleus is drawn far too big: it’s about 100,000 times smaller than the atom.
Turn up the heat
A step of 1 K is the same size as 1 °C, so the two scales are always 273.15 apart. Fahrenheit is different: its degrees are smaller and it starts elsewhere, so water freezes at 273.15 K, 0 °C and 32 °F.25 °C · 77 °FGas
At this temperature, chlorine is a gas.
At normal atmospheric pressure (1 atm). The temperature scale is logarithmic. The particle box is schematic: real samples hold vastly more atoms, and spacing and speeds are not to scale.
Pull the electrons off
ClNeutral atom · no charge
A neutral chlorine atom. Removing its first electron takes 1,251 kJ/mol.
Chlorine’s outer shell holds 7 electrons. Electron 8 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 3× as much energy as the one before. Each dashed line on the chart marks another jump, into a shell even closer to the nucleus.
What does this tell us?
- The jumps let you count electrons. Between the dashed lines there are 7, then 8, then 2 bars. Read backwards, that’s 2, 8, 7: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The 7 electrons before the first big jump are chlorine’s valence electrons, the ones it uses in chemical reactions. That’s why chlorine sits in group 17: every element in the group has the same number of outer electrons, so they react in similar ways.
- Removing electrons from chlorine is expensive from the start (1,251 kJ/mol for the first). So in reactions chlorine tends to gain or share electrons rather than lose them.
- Within a shell the steps rise gently. Each electron removed leaves fewer electrons pushing the others away, while the nucleus pulls just as hard, so the next one is held a little more tightly.
Show the values as a table
| Electron | Shell | Energy (kJ/mol) | × previous | Value |
|---|---|---|---|---|
| 1 | 3 | 1,251 | – | Measured |
| 2 | 3 | 2,298 | 1.8 | Measured |
| 3 | 3 | 3,840 | 1.7 | Estimated |
| 4 | 3 | 5,137 | 1.3 | Estimated |
| 5 | 3 | 6,530 | 1.3 | Estimated |
| 6 | 3 | 9,353 | 1.4 | Estimated |
| 7 | 3 | 11,019 | 1.2 | Measured |
| 8 | 2 | 33,606 | 3 | Measured |
| 9 | 2 | 38,676 | 1.2 | Measured |
| 10 | 2 | 44,065 | 1.1 | Estimated |
| 11 | 2 | 51,137 | 1.2 | Estimated |
| 12 | 2 | 57,079 | 1.1 | Estimated |
| 13 | 2 | 63,323 | 1.1 | Estimated |
| 14 | 2 | 72,386 | 1.1 | Estimated |
| 15 | 2 | 78,076 | 1.1 | Calculated |
| 16 | 1 | 352,977 | 4.5 | Calculated |
| 17 | 1 | 380,759 | 1.1 | Calculated |
Successive ionization energies of gas-phase atoms and ions from the NIST Atomic Spectra Database. Solid bars are measured; hatched bars were estimated from nearby measurements or calculated from quantum theory, as is usual for highly charged ions. Electrons are taken from the outermost shell first, a simplification: electrons in different subshells of the same shell also differ in energy. On the log scale each gridline is 10 times the one below.
The halogens, down the group
Fluorine, chlorine, bromine and iodine sit in the same column, so their atoms all have seven outer electrons and all grab an eighth. But going down the column the atoms get bigger: the colour deepens, they melt and boil at higher temperatures, and they react less fiercely.
- FluorineGasMelts −220 °C · Boils −188 °CGrabs electrons: 3.98
- ChlorineGasMelts −102 °C · Boils −34 °CGrabs electrons: 3.16
- BromineLiquidMelts −7 °C · Boils 59 °CGrabs electrons: 2.96
- IodineSolidMelts 114 °C · Boils 184 °CGrabs electrons: 2.66
At room temperature you can see the whole trend at once: two gases, a liquid and a solid, getting darker down the group. Bigger atoms have more electrons, so the molecules cling to each other more strongly. The bar shows electronegativity: how strongly each atom grabs an extra electron. It falls down the group, and so does reactivity.
Who pushes who out?
The classic school experiment: add a halogen dissolved in water to a solution of a salt of another halogen. A more reactive halogen takes the electron and pushes the less reactive one out, and the colour changes. (Fluorine is left out: it reacts with the water itself.)
Ready to mix
Potassium iodide solution is colourless. The chlorine water is very pale yellow-green.
- Chlorine in water
- Bromine in water
- Iodine in water
Melting and boiling points from the element data. Colours are the familiar look of each substance; in the jars, the vapour above bromine and iodine is drawn stronger as they near their boiling points. Electronegativity on the Pauling scale.
How does it compare?
Density
0.0032 g/cm³8th lowest of 99
Melting point
172 K9th lowest of 96
Boiling point
239 K11th lowest of 97
Electronegativity
3.163rd highest of 96
First ionization energy
1,251 kJ/mol9th highest of 99
Elements 1–99 from the reference data; heavier elements are left out because only a few atoms have ever been made, so their bulk properties are predictions. First ionization energies come from NIST. Density uses a logarithmic scale, because gases are around a thousand times less dense than solids; gas densities are at 0 °C and 1 atm. Melting and boiling points are at normal pressure. Everyday temperatures and light sources in the pictures are approximate. Hover over a dot to see the element, or click it to compare.
Keep exploring
Explore the stories, imagery, and reference data in more depth. Properties with no value in the reference are marked unavailable.
- ChlorineRoyal Society of Chemistry
- ChlorineWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database