Reactivity trends — groups 1, 2 & 17
1.6 · The Periodic Table
Reactivity trends — groups 1, 2 & 17
By the end of this section:
- I can describe the group 1 metals as the most reactive metals, and say how they are alike and how they differ in appearance and in their reaction with water
- I can describe how group 1 metals react more strongly with water further down the group, and say that groups 1 and 2 follow the same trend
- I can explain the reactivity trend of group 1 and 2 metals using their electron arrangements
1 · Read
The notes, the key idea and the words to knowBecause elements in a group share the same number of outer electrons (1.5), each group reacts in a characteristic way, and reactivity changes smoothly down the group.
- Group 1 — alkali metals. One outer electron. React with water, fizzing and giving off hydrogen, forming soluble hydroxides. Reactivity increases down the group (Li < Na < K).
- Group 2 — alkaline earth metals. Two outer electrons. React with water and acid to give hydrogen; form oxides and hydroxides. Reactivity also increases down the group.
- Group 17 — halogens. Seven outer electrons; coloured and diatomic (Cl2, Br2, I2). React by gaining one electron. Reactivity decreases down the group (F > Cl > Br).
What the Group 1 metals look like and do
- Soft — they can be cut with a knife.
- Shiny when freshly cut, but they quickly tarnish (turn dull) as they react with the air.
- Low density — lithium, sodium and potassium float on water.
- React with water, fizzing and giving off hydrogen (not oxygen), releasing heat, and leaving an alkaline solution — universal indicator turns from green to purple. Potassium reacts more vigorously than sodium, and sodium more than lithium.
sodium + water → sodium hydroxide + hydrogen
The Group 1 metals are the most reactive metals. The most reactive element of all is fluorine, a non-metal at the top of Group 17.
Reactive metals also react with acid to give hydrogen. Test five metals in hydrochloric acid and see what the fizzing tells you.
- Test each metal. Tap a tube to drop the metal into the acid. Watch how fast it fizzes.
- Put them in order. The more vigorous the reaction with acid, the more reactive the metal. Copper did not react at all. It is the least reactive of the five.
- Name the products. A reactive metal and an acid make a salt and hydrogen. Hydrochloric acid makes chlorides.
- Test the gas. Hold a lit splint at the mouth of the tube. Hydrogen burns with a squeaky pop.
Why reactivity changes down a group Try it: Down the group
Every trend here comes from one idea: how easily an atom loses or gains its outer electron, which depends on how far the outer shell is from the nucleus.
- Lithium (2 shells), sodium (3), potassium (4): the outer electron is further out each time, so it is held less tightly and lost more easily. Reactivity increases down Group 1.
- Fluorine (2 shells), chlorine (3), bromine (4), iodine (5): an incoming electron would sit further out each time, where the pull is weaker, so it is gained less easily. Reactivity decreases down Group 17.
Metals (Groups 1 and 2) react by losing electrons. The further the outer electron is from the nucleus — i.e. the lower down the group — the weaker the pull holding it, so it is lost more easily and the metal is more reactive. That is why potassium beats sodium beats lithium.
Halogens (Group 17) react by gaining an electron. The same distance argument runs the opposite way: lower down the group, the incoming electron would sit further from the nucleus, where the pull is weaker, so it is captured less easily. Reactivity therefore decreases down the group, with fluorine the most reactive at the top.
Why is potassium more reactive than lithium?
- Both are Group 1, so both have just one outer electron. Look at where that electron sits.
- Check the diagram: potassium has more shells than lithium, so its outer electron is further out, not closer.
- Both are Group 1, so both react by losing their one outer electron.
- In potassium that electron is in a shell further from the nucleus than in lithium.
- So it is held less tightly and lost more easily.
- Easier electron loss = more reactive.
Fluorine or iodine: which is more reactive?
- Halogens react by gaining an electron, not losing one. Lower down, the incoming electron would be further out, where the pull is weaker.
- Same group means a similar way of reacting, not the same reactivity. Reactivity decreases down Group 17.
- Both are Group 17 and react by gaining one electron.
- Fluorine’s outer shell is closest to the nucleus, so it pulls in an electron most strongly.
- Fluorine is therefore more reactive than iodine.
-
Not thisAll groups get more reactive down the group.
ThisMetals (1 and 2) get more reactive down; halogens (17) get less reactive down. The direction depends on whether the atom loses or gains electrons.
-
Not thisGroup 1 metals gain electrons.
ThisThey lose their single outer electron. It is the non-metals (like halogens) that gain electrons.
-
Not thisReactivity is about how heavy the atom is.
ThisIt is about how easily the outer electron is lost or gained — i.e. its distance from the nucleus, not the atom’s mass on its own.
-
Not thisFluorine is least reactive because it is at the top.
ThisThe opposite — for halogens the top is most reactive, because the outer shell is closest to the nucleus.
- reactivity
- how readily a substance takes part in a chemical reaction
- alkali metal (Group 1)
- a soft, very reactive metal with one outer electron; reacts with water to give hydrogen
- alkaline earth metal (Group 2)
- a reactive metal with two outer electrons
- halogen (Group 17)
- a reactive non-metal with seven outer electrons; reacts by gaining one electron
- diatomic
- existing as molecules of two atoms, e.g. Cl2
2 · Watch
Videos picked for this sectionScroll sideways for more videos
Videos in other languagesDeutsch
We try to help you in other languages. Learn the idea here in your own language, then learn the science words from the English or French video. Not all of these videos have been independently checked. If something does not make sense, or does not match what we did in class, ask your teacher.
Scroll sideways for more videos
- Booklet pp. 23–28
- Knowledge Book pp. 23–26
- In your class workbook: Chapter 1.8 Reactions of group 1 ‘alkali’ metals (pp. 32–39) · Revise this section
What this page stores about you
The notes, quizzes and simulations on this page work without an account. Two things need one: the class workbook (it belongs to the school, so you sign in to open it) and quiz scores (a quiz only counts for your class if you are signed in when you take it — as a guest, nothing is recorded).
If you are signed in and tick “I understand this” on a section, press “Mark as read”, or tick “I've watched this” on a video, we store three things each time: your account, which section or video it was, and the date. Nothing else — not how long you looked, not where you were.
Your teacher can see which sections you have ticked or marked as read, and which videos you have ticked as watched, so they know who to check in with. No other teacher and no other student can see it. It is deleted after 180 days. The exception is “Mark as read” and “I've watched this”: those are kept, so a section you opened stays open, and you can erase them yourself at any time. If you are not signed in, your ticks are kept only in this browser and nothing is sent to us.
You never have to tick anything, and nothing else on this page changes if you don't.
What you write in ‘Explain it’ stays in this browser; it is not sent to us or to your teacher.
Playing a video loads it from YouTube, and opening a simulation loads it from PhET (University of Colorado); neither is loaded until you click. Two kinds of picture are fetched when the page opens, or when you scroll down to them: the small preview picture on each video comes from YouTube's image server, and the picture of each simulation comes from PhET's.