Francium: atomic number 87, instability, uses and myths
This post helps you find francium on the interactive periodic table using the Category and Period controls.
You will also get a small data table, one worked half-life example and three myths to avoid in exams.
Find francium on the interactive table
The interactive table has controls above the tiles: Category, Block, Period, State and Natural / synthetic. There is also a search box for a name, symbol or atomic number.
Try these steps in order:
- Use the Category control and pick alkali metals. You will see the Group 1 metals. Francium is the last one in the column. Hydrogen also sits in Group 1, but it is not an alkali metal.
- Use the Period control and choose period 7. Francium is the first element of that row.
- Tap the francium tile. The quick-view card shows its atomic mass, electron configuration and physical state. Choose Read all data to open the full profile.
- Now use the Natural / synthetic control. Check which group francium falls in, and compare it with the elements near it.
Why do this? Francium is hard to study directly. The best way to understand it is to compare it with the metals above it: sodium, potassium, rubidium and caesium. The table keeps all of them in one column, so the trend is easy to see.
Notice also that the tile for francium shows [223], not a decimal like 22.99 for sodium. That bracket carries a message, and the next section explains it.
Francium data at a glance
Here are the key facts. Read the atomic mass row carefully, because it behaves differently from most elements.
| Property | Francium |
|---|---|
| Symbol | Fr |
| Atomic number | 87 |
| Atomic mass | [223] |
| Category | Alkali metal |
| Group and period | Group 1, Period 7 |
| Electron configuration | [Rn] 7s1 |
| Most stable isotope | Francium-223 |
| Half-life of that isotope | 22 minutes |
The square brackets around 223 matter. For most elements, atomic mass is a weighted average over stable isotopes. Isotopes are atoms of the same element with different numbers of neutrons. Francium has no stable isotope, so no standard atomic weight can be given. The bracketed number is the mass number of its most stable isotope.
The configuration [Rn] 7s1 means francium has the electron arrangement of radon, plus one electron in the 7s orbital. A single outer electron is the mark of Group 1. That is why francium is classed as an alkali metal.
The same bracket style appears on other tiles, such as technetium [98], astatine [210] and radium [226]. Whenever you see it, think: no stable isotope, so the number is a mass number.
Why francium is so unstable
Francium is radioactive. Its nuclei break down on their own and give out radiation. The time taken for half of the atoms in a sample to decay is the half-life.
For francium-223 the half-life is only 22 minutes. All isotopes of francium decay into astatine, radium or radon. Francium-223 also has a shorter half-life than the longest-lived known isotope of every element up to dubnium, element 105.
This is the main reason francium is so poorly known. Any sample disappears quickly, so we cannot study it the way we study sodium or potassium.
Worked example: how much francium-223 is left?
Question: A sample has 3200 atoms of francium-223. How many remain after 110 minutes? The half-life is 22 minutes.
Step 1. Find the number of half-lives: 110 ÷ 22 = 5.
Step 2. Halve the atoms five times: 3200 → 1600 → 800 → 400 → 200 → 100.
Step 3. Check with the formula: remaining = 3200 × (1/2)5 = 3200 ÷ 32 = 100.
Answer: 100 atoms remain. Always count the half-lives first. It keeps the arithmetic simple, and the halving method is easy to verify.
History and trace occurrence
Before francium was found, chemists called the missing element below caesium eka-caesium. They expected it to exist because of its gap in Group 1. This shows the periodic law making a prediction.
Marguerite Perey discovered francium in France on 7 January 1939, and the element takes its name from that country. It was the last element to be first discovered in nature rather than made by synthesis. Its isotope francium-223 was once called actinium K, after the natural decay chain in which it appears.
Francium does occur naturally, but only in trace amounts. It is found in uranium ores, where francium-223 from the uranium-235 family keeps forming and keeps decaying. It vanishes as fast as it forms, so the amount present at any moment is tiny. One estimate says as little as 28 g exists in the Earth's crust at a given time.
Astatine is even rarer in nature, because its natural isotopes have even shorter half-lives. Among elements whose most stable isotope is known with certainty, francium is the most unstable.
The largest amount ever produced in a laboratory was a cluster of more than 300,000 atoms. That is far too small to see.
Uses of francium
Francium has no everyday or industrial use. It is too rare and decays too fast. Its value lies in research, where small numbers of atoms are made in a laboratory and studied.
For a student, its real use is as a teaching example. It shows how trends continue down a group, and how instability limits what we can measure. Many of its physical properties are only estimates. For example, its melting point is uncertain because the element is so rare and radioactive, and different methods of estimating it give different answers. The same is true for its density.
The tool follows the same honest approach. Its notes say that for the heaviest synthetic elements, predicted values are marked and unmeasured properties are stated plainly. Use that habit in your own answers.
Three common misconceptions
Myth 1: Francium is a metal you can hold or store
No one has ever seen bulk francium. The heat released by its own decay would vaporise any visible amount at once. When books describe it as a reactive metal, they are presuming this from the other elements in its column.
Myth 2: Francium is the most reactive element and must be the most violent in water
This is a prediction, not an observation. Francium is the second most electropositive element, behind caesium. Electropositive means it loses electrons very easily. Its chemical properties mostly resemble those of caesium. Nobody has watched francium react with water in a visible quantity.
Myth 3: Francium is man-made only
Francium-223 and francium-221 occur naturally in trace amounts. Its other isotopes are entirely synthetic. So the correct statement is that natural francium exists, but only in extremely small amounts.
A good exam habit: write "predicted" or "expected" unless the property has actually been measured. Examiners value this care.
Key takeaways
- Francium is Fr, atomic number 87, an alkali metal in Group 1, Period 7, with configuration [Rn] 7s1.
- Its atomic mass is shown as [223] because it has no stable isotope.
- Francium-223 has a half-life of 22 minutes, so bulk francium has never been seen.
- It occurs in trace amounts in uranium ores and has no practical use outside research.
- Its reactivity is expected from caesium and the Group 1 trend, not directly observed.
Frequently asked questions
Why is francium's atomic mass written in square brackets?
Francium has no stable isotope, so no standard atomic weight exists. The bracketed [223] is the mass number of its most stable isotope, francium-223.
Which group and period is francium in?
Francium is in Group 1 and Period 7. Its configuration, [Rn] 7s1, has one valence electron, so it is classed as an alkali metal.
Is francium found in nature?
Yes, but only in trace amounts, mainly in uranium ores. Francium-223 forms continually in the uranium-235 decay family and then decays again within minutes.
Who discovered francium and when?
Marguerite Perey discovered it in France on 7 January 1939. Earlier, the undiscovered element was called eka-caesium.
Is francium more reactive than caesium?
This is not proven. Francium is the second most electropositive element, behind caesium. It has never been collected in visible amounts, so its reactivity is only expected from the Group 1 trend.
What to do next. Try it yourself on the page below.
Interactive Periodic Table — All 118 Elements →
