The Periodic Table — groups & periods
1.5 · The Periodic Table We are here
The Periodic Table — groups & periods
By the end of this section:
- I can explain how the periodic table arranges the elements by their atomic structure, and use it to predict their physical and chemical properties
- I can say what ‘group’ and ‘period’ mean, and explain that elements in the same group have similar properties
- I can suggest the main physical and chemical properties of an element from its position in the periodic table
- I can describe the main differences between metals and non-metals, and say that metals are towards the left of the periodic table and non-metals towards the right
- I can explain that the noble gases (Group 18, called group 8 or group 0 in some books) are unreactive because their outer shell is full
1 · Read
The notes, the key idea and the words to knowThe Periodic Table lists every element in order of proton number (atomic number). That order is not just tidy — it groups elements so that ones with similar properties line up.
- Groups are the vertical columns (1–18). Elements in the same group have the same number of outer-shell electrons, so they have similar chemical properties.
- Periods are the horizontal rows (1–7). Across a period the proton number rises by one each step, and a new outer shell starts with each new period.
Metals sit to the left and centre; non-metals to the right. The noble gases (He, Ne, Ar, Kr, Xe, Rn) fill Group 18 — they have complete outer shells, so they barely react. (Some books call this group 8 or Group 0.)
Every ring on the diagram is a shell. The number of shells is the period.
Groups 1 and 2: the group number is the number of outer electrons.
Helium is the exception: its outer shell is full with just 2, so it sits in Group 18 with the noble gases.
Hydrogen is a non-metal even though it sits in Group 1. Boron and silicon are semi-metals: they act a little like both, and the booklet says not to worry about them yet.
| Typical metal (solid) | Typical non-metal (solid) |
|---|---|
| Good conductor of heat and electricity | Poor conductor of heat and electricity |
| Shiny surface | Dull surface |
| High density | Low density |
| Malleable and ductile | Brittle |
Why position predicts behaviourTap to open
The whole power of the table is this: an element’s place tells you how it behaves, because position reflects electron arrangement.
Same group → similar reactions. Chemical reactions are about outer-shell electrons. Every Group 1 atom has one outer electron, every Group 2 atom two, every Group 17 atom seven. Because reactions depend on those outer electrons, elements in a group react in the same way — which is why we can learn a group’s behaviour once and apply it to all its members (1.6).
Group number → outer electrons. For Groups 1 and 2 the group number is the number of outer electrons (Group 1 → 1; Group 2 → 2). For Groups 13 to 18 it is the last digit of the group number (Group 13 → 3; Group 17 → 7; Group 18 → 8, a full outer shell — helium is the exception, with 2). Period number → number of shells. So sodium (period 3, group 1) has three shells with one electron in the outermost: 2, 8, 1.
Why the noble gases are inert. A full outer shell is already stable, so a noble-gas atom has no tendency to lose or gain electrons — and reacting means losing or gaining electrons. A complete shell is the “goal” that drives every other element’s reactions.
Seven outer electrons (one short of a full shell) tells you chlorine is a reactive non-metal that tends to gain one electron.
-
Not thisElements in a group have the same number of shells.
ThisNo — that is a period. A group shares the same number of outer-shell electrons, but each element down a group has one more shell.
-
Not thisThe table is ordered by atomic mass.
ThisIt is ordered by proton number. Mass mostly rises too, but a few pairs (e.g. Ar and K) only sit correctly when ordered by protons.
-
Not thisNoble gases are unreactive because they have no electrons.
ThisThey are unreactive because their outer shell is full — the most stable arrangement, with no drive to gain or lose electrons.
-
Not thisThere is a sharp line between metals and non-metals.
ThisThe change is gradual; a “staircase” of metalloids (e.g. silicon) shows in-between behaviour.
- group
- a vertical column; same number of outer-shell electrons → similar properties
- period
- a horizontal row; the period number equals the number of occupied shells
- metal / non-metal
- metals (left and centre) conduct and are malleable; non-metals (right) do not and are brittle
- noble gas
- a Group 18 element with a complete outer shell, so very unreactive
- proton number (atomic number)
- the number of protons; sets each element’s place in the table
2 · Watch
Videos picked for this sectionScroll sideways for more videos
Videos in other languagesEspañol · Deutsch · Italiano · Português · العربية · 中文 · Русский
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. 19–22
- Knowledge Book pp. 19–22
- In your class workbook: Chapter 1.4 The Periodic Table of the elements (pp. 27–31) · 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.