Antimony (Sb)
Antimony (Sb) is a metalloid with atomic number 51 and a standard atomic weight of 121.76. At room temperature it is a solid that melts at 631 °C and boils at 1,587 °C.
Antimony key facts
Antimony at a glance
Identity
| Symbol | Sb |
|---|---|
| Atomic number (Z) | 51 |
| Atomic weight | 121.76 ± 0.01 (CIAAW 2024) |
| Category | Metalloid |
| Group | 15 — pnictogens (nitrogen group) |
| Period | 5 |
| Block | p-block |
| CAS number | 7440-36-0 |
Atomic properties
| Electron configuration | [Kr] 4d10 5s2 5p3 Full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p3 |
|---|---|
| Electrons per shell | 2, 8, 18, 18, 5 |
| Oxidation states | -3, +3, +5 Less common: -2, -1, 0, +1, +2, +4 |
| Electronegativity (Pauling) | 2.05 |
| First ionisation energy | 8.608 eV (830.6 kJ/mol) 2nd: 16.626 eV · 3rd: 25.323 eV |
| Electron affinity | 1.046 eV |
| Atomic radius (empirical) | 145 pm |
| Covalent radius | 140 pm |
| Van der Waals radius | 206 pm |
Physical properties
| State at 25 °C | Solid |
|---|---|
| Melting point | 630.6 °C · 1,167.1 °F · 903.78 K |
| Boiling point | 1,587 °C · 2,888.6 °F · 1,860.15 K |
| Density | 6.68 g/cm³ |
| Specific heat capacity | 0.207 J/(g·K) |
| Thermal conductivity | 24.43 W/(m·K) |
| Crystal structure | Rhombohedral |
Isotopes & abundance
| Natural occurrence | Primordial |
|---|---|
| Radioactive | No |
| Natural isotopes (abundance) | 121Sb 57.21%, 123Sb 42.79% |
| Stable isotopes | 4 |
| Known isotopes | 41 |
| Abundance in Earth's crust | 0.2 mg/kg |
| Abundance in seawater | 0.00024 mg/L |
Atomic weights: CIAAW Abridged Standard Atomic Weights 2024. Other properties: standard reference compilations, checked October 2026; predicted values are marked.
Antimony electron configuration
Antimony has 51 electrons. Written in short form, its ground-state configuration is [Kr] 4d10 5s2 5p3 — the electrons of the noble gas krypton, plus 4d10 5s2 5p3.Written out in full: 1s2 2s2 2p6 3s2 3p6 3d10 4s2 4p6 4d10 5s2 5p3.
As a member of group 15 it has 5 valence electrons in its outer shell (shell 5), which largely decide its chemistry.
Discovery and name
Antiquity — Known since antiquity (compounds); described as an element in the 17th century.
Possibly Greek anti + monos ('not alone'); the symbol Sb is from Latin stibium.
Known for
Antimony trioxide is a flame retardant; antimony hardens lead in batteries and bullets. Stibnite was used as kohl eye make-up in antiquity.
Isotopes of antimony
Naturally occurring antimony is a mix of 2 isotopes. Each has 51 protons; they differ in the number of neutrons:
| Isotope | Protons | Neutrons | Natural abundance | Stability |
|---|---|---|---|---|
| 121Sb | 51 | 70 | 57.21% | stable |
| 123Sb | 51 | 72 | 42.79% | stable |
41 isotopes of antimony are known in our data set (half-lives as published in nuclear data tables).
Antimony compared with group 15
The other elements in group 15 (pnictogens (nitrogen group)), from top to bottom of the table:
| Element | Z | Atomic weight | Melting point | Boiling point | Density | Electronegativity |
|---|---|---|---|---|---|---|
| Nitrogen | 7 | 14.007 | -210 °C | -196 °C | 1.2506 g/L | 3.04 |
| Phosphorus | 15 | 30.974 | 44 °C | 281 °C | 1.823 g/cm³ | 2.19 |
| Arsenic | 33 | 74.922 | — | 614 °C | 5.75 g/cm³ | 2.18 |
| Antimony | 51 | 121.76 | 631 °C | 1,587 °C | 6.68 g/cm³ | 2.05 |
| Bismuth | 83 | 208.98 | 271 °C | 1,564 °C | 9.79 g/cm³ | 2.02 |
| Moscovium | 115 | [290] | — | — | 13.5 g/cm³ (pred.) | — |