The Elusive Element: Francium (Fr)
Introduction to Francium
Francium (Fr), atomic number 87, stands as the most reactive of the alkali metals and is one of the rarest naturally occurring elements on Earth. It was discovered in 1939 by Marguerite Perey at the Curie Institute in Paris, France, and was named in honor of the country. Francium is highly radioactive and possesses no stable isotopes, meaning all its forms undergo radioactive decay. Its extreme scarcity and radioactivity limit its practical applications and make it a subject primarily of academic study. Francium is typically produced through the alpha decay of actinium-227 or artificially in nuclear reactions.
Atomic Number and Mass Number
The atomic number (Z) of Francium is 87. This number signifies that every atom of Francium contains exactly 87 protons within its nucleus. The atomic number fundamentally defines an element.
Francium, like other elements, exists in various isotopic forms. An isotope is an atom of an element that has the same number of protons but a different number of neutrons. All known isotopes of Francium are radioactive. The most stable and naturally occurring isotope is Francium-223 ($\text{Fr}^{223}$).
For Francium-223:
- Atomic Number (Z): 87
- Mass Number (A): 223 (This is the total number of protons and neutrons in the nucleus)
Composition of a Neutral Francium-223 Atom
For a neutral atom, the number of electrons is equal to the number of protons. For Francium-223:
- Number of Protons: 87 (determined by the atomic number)
- Number of Electrons: 87 (for a neutral atom)
- Number of Neutrons: Calculated by subtracting the atomic number from the mass number: A - Z = 223 - 87 = 136.
Therefore, a neutral atom of Francium-223 contains 87 protons, 87 electrons, and 136 neutrons.
Electron Configuration
The electron configuration describes the arrangement of electrons in an atom’s orbitals around the nucleus. For Francium, with 87 electrons, its configuration can be written using the noble gas shorthand notation. The noble gas preceding Francium in the periodic table is Radon (Rn), which has an atomic number of 86.
The full electron configuration for Francium is: $1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 3d^{10} 4p^6 5s^2 4d^{10} 5p^6 6s^2 4f^{14} 5d^{10} 6p^6 7s^1$
Using noble gas shorthand, this configuration is simplified to: $[Rn] 7s^1$
This notation indicates that the inner 86 electrons of Francium are arranged identically to those in a neutral Radon atom, followed by one electron in the 7s orbital. The ‘7’ indicates the principal energy level (shell), ‘s’ indicates the subshell type, and ‘1’ indicates the number of electrons in that subshell.
Valence Electrons
Valence electrons are the electrons located in the outermost shell or highest principal energy level of an atom. These electrons are primarily involved in chemical bonding and determine an element’s chemical properties and reactivity.
For Francium, based on its electron configuration $[Rn] 7s^1$, the highest principal energy level is 7. The only electron in this outermost shell is the one in the 7s orbital.
Therefore, Francium possesses 1 valence electron.
This single valence electron makes Francium highly reactive, as it readily loses this electron to form a positive ion with a +1 charge ($\text{Fr}^+$), achieving a stable electron configuration similar to that of Radon. This characteristic places Francium in Group 1 of the periodic table, known as the alkali metals.