Lanthanum (La)
Lanthanum is element 57, a lanthanide in the lanthanide series (period 6). A neutral atom has 57 protons, 82 neutrons and 57 electrons, 2 of them valence electrons.
Lanthanide- Atomic number
- 57
- Atomic mass
- 138.91 u
- Valence electrons
- 2*
- Protons · neutrons
- 57 · 82
- Electrons per shell
- 2, 8, 18, 18, 9, 2
- At room temperature
- solid
Bohr model of lanthanum
The Bohr model draws lanthanum as a nucleus of 57 protons and 82 neutrons, with its 57 electrons on 6 circular shells: 2 in shell 1, 8 in shell 2, 18 in shell 3, 18 in shell 4, 9 in shell 5, 2 in shell 6. Press Build the atom to watch the shells fill from the inside out. The outer ring (the P 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 lanthanum
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹
Short form (noble gas notation): [Xe] 6s² 5d¹. The [Xe] stands for the electrons of xenon, the noble gas before it.
Lanthanum breaks the usual filling order. The Aufbau rule predicts 1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 4f¹, but experiments show the configuration above. The energy levels of the subshells are so close here that a different arrangement wins.
How many valence electrons does lanthanum have?
Lanthanum has 2 valence electrons.
Lanthanum is a lanthanide, so its valence electrons are less clear-cut than for main-group elements. It has 2 electrons in its outermost shell (shell 6), and electrons in its inner f and d subshell can also take part in bonding.
Protons, neutrons and electrons
| Protons | 57 | equal to the atomic number |
|---|---|---|
| Electrons | 57 | equal to the protons in a neutral atom |
| Neutrons | 82 | mass number 139 − 57 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.
Lanthanum properties
| Symbol | La |
|---|---|
| Category | Lanthanide |
| Group · period · block | — · 6 · f |
| Atomic mass | 138.91 u |
| State at 25 °C | solid |
| Melting point | 1,191 K (918 °C, 1,680 °F) |
| Boiling point | 3,737 K (3,460 °C, 6,270 °F) |
| Density | 6.15 g/cm³ |
| Electronegativity (Pauling) | 1.1 |
| Atomic radius | 240 pm |
| First ionization energy | 5.577 eV |
| Electron affinity | 0.5 eV |
| Oxidation states | +3 |
| Common ion charge | +3 |
| Discovered | 1839 by Carl Gustaf Mosander |
Where lanthanum sits on the periodic table
Lanthanides: Cerium, Praseodymium, Neodymium, Promethium, Samarium, Europium, Gadolinium, Terbium.
Questions about lanthanum
How many valence electrons does lanthanum have?
Lanthanum has 2 valence electrons in its outer shell; inner d or f electrons can also bond.
How many protons, neutrons and electrons does lanthanum have?
A neutral lanthanum atom has 57 protons and 57 electrons. Its most common form has about 82 neutrons (mass number 139 − 57 protons).
What is the electron configuration of lanthanum?
1s² 2s² 2p⁶ 3s² 3p⁶ 4s² 3d¹⁰ 4p⁶ 5s² 4d¹⁰ 5p⁶ 6s² 5d¹, written in short form as [Xe] 6s² 5d¹.
What does the Bohr model of lanthanum look like?
A nucleus of 57 protons and 82 neutrons, circled by 6 electron shells holding 2, 8, 18, 18, 9, 2 electrons.
What is the atomic number of lanthanum?
57. The atomic number is the number of protons in the nucleus, and it is what makes an atom lanthanum.