Sodium
Sodium /ˈsoʊdiəm/ is a chemical element with symbol Na (from Ancient Greek Νάτριο) and atomic number 11. It is a soft, silver-white, highly reactive metal. In the Periodic table it is in column 1 (alkali metals), and shares with the other six elements in that column that it has a single electron in its outer shell, which it readily donates, creating a positively charged atom - a cation.
At a glance
- Atomic number
- 11
- Atomic mass
- 22.989769 u
- Phase
- Solid
- Density
- 0.968 g/cm³
- Melting point
- 370.944 K
- Boiling point
- 1,156.09 K
- First ionization energy
- 495.8 kJ/mol
- Electronegativity
- 0.93
- Electron affinity
- 52.867 kJ/mol
- Molar heat capacity
- 28.23 J/(mol·K)
- Period
- 3
- Appearance
- silvery white metallic
Conditions and isotopes affect measured properties. Some properties of short-lived elements are predicted. Electronegativity uses the Pauling scale.
Sodium’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…
11 protons12 neutrons
Sodium-23: 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 °FSolid
At this temperature, sodium is a solid.
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
NaNeutral atom · no charge
A neutral sodium atom. Removing its first electron takes 496 kJ/mol.
Sodium’s outer shell holds 1 electron. Electron 2 is the first from an inner shell: it sits closer to the nucleus with less shielding, so removing it takes 9.2× as much energy as the one before. That’s why sodium forms Na⁺ ions. 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 1, then 8, then 2 bars. Read backwards, that’s 2, 8, 1: the same electrons per shell as the shell model above. Jumps like these are part of the evidence that shells exist.
- The single electron before the first big jump is sodium’s valence electron, the one it uses in chemical reactions. That’s why sodium sits in group 1: every element in the group has the same number of outer electrons, so they react in similar ways.
- The numbers explain the charge. Removing electron 2 as well, to make Na²⁺, would cost 4,562 kJ/mol. Forming a compound gives back far less energy than that, so Na²⁺ doesn’t form in ordinary chemistry.
- 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 | 496 | – | Measured |
| 2 | 2 | 4,562 | 9.2 | Measured |
| 3 | 2 | 6,910 | 1.5 | Estimated |
| 4 | 2 | 9,546 | 1.4 | Estimated |
| 5 | 2 | 13,354 | 1.4 | Measured |
| 6 | 2 | 16,618 | 1.2 | Estimated |
| 7 | 2 | 20,118 | 1.2 | Estimated |
| 8 | 2 | 25,491 | 1.3 | Estimated |
| 9 | 2 | 28,932 | 1.1 | Calculated |
| 10 | 1 | 141,361 | 4.9 | Calculated |
| 11 | 1 | 159,076 | 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.
A fingerprint made of light
Almost all of sodium’s visible light comes from two yellow lines very close together, at 589.0 and 589.6 nm. That’s why old sodium streetlights glow one deep yellow, and why a pinch of salt turns a flame yellow.
Heated or electrified, sodium gives off light only at the exact wavelengths below. Each one comes from an electron falling between two energy levels, so the pattern is unique to the element, like a fingerprint.
Click a line to see where it comes from.
589 nmyellow
An electron falls from the 3p level down to the 3s level. It loses 2.11 eV of energyAn electronvolt (eV) is a tiny unit of energy used for atoms: the energy one electron gains crossing 1 volt. A particle of visible light carries between about 1.7 eV (red) and 3.3 eV (violet). and gives it off as one particle of yellow light. Bigger drops give bluer light.
The strongest visible lines of neutral sodium from the NIST Atomic Spectra Database. Line brightness follows NIST’s relative intensities, which only compare lines of the same element. Colours are approximate, and lamp colours are as they look to the eye.
Drop it in water
Ready to drop a small piece of sodium.
Floats, melts into a shiny ball and whizzes around the surface, fizzing hard. Held still, for example on wet paper, the hydrogen catches fire with a yellow-orange flame.
- In water it
- Floats0.968 g/cm³, water is 1
- From the heat it
- Melts into a ballmelts at 98 °C
- How violent
2Na + 2H₂O → 2NaOH + H₂
Hydrogen gas fizzes off, and the metal hydroxide dissolves, making the water alkaline: a few drops of phenolphthalein indicator turn it pink.
Why it gets more violent down the group
Every alkali metal has one electron in its outer shell, and reacting means giving it away. Further down the group that electron sits in a shell further from the nucleus, shielded by more electrons inside, so it takes less energy to remove and the reaction runs faster.
- Lithium520 kJ/mol
- Sodium496 kJ/mol
- Potassium419 kJ/mol
- Rubidium403 kJ/mol
- Caesium376 kJ/mol
Energy to remove the outer electron (first ionization energy, NIST). Speed isn’t the same as energy: per atom, lithium’s reaction releases slightly more energy than sodium’s, it just releases it slowly. Francium’s value (393 kJ/mol) is a little higher than caesium’s, because of relativistic effects, so it may not be the most violent after all.
Don’t try this at home. Demonstrations use rice-grain-sized pieces behind safety screens; the heavier metals are only shown on video.
How does it compare?
Density
0.968 g/cm³14th lowest of 99
Melting point
371 K19th lowest of 96
Boiling point
1,156 K24th lowest of 97
Electronegativity
0.937th lowest of 96
First ionization energy
496 kJ/mol5th lowest 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.
- SodiumRoyal Society of Chemistry
- SodiumWikipedia
- Quantum numbers and electron configurationsPurdue
- Hydrogenic wavefunctionsNIST
- Ionization energiesNIST Atomic Spectra Database
- Spectral linesNIST Atomic Spectra Database