Group Properties & Trends
The periodic table arranges elements into groups where properties follow predictable patterns — and understanding why these patterns exist is the key to predicting how any element will behave.
Chapter at a Glance
Group I (Alkali Metals)
Soft, reactive metals that form alkaline solutions with water. Reactivity increases down the group.
Group VII (Halogens)
Poisonous non-metals with 7 outer electrons. Reactivity decreases down the group (opposite to Group I!).
Group VII Displacement
More reactive halogens displace less reactive ones from solutions — a redox reaction you can observe as color changes.
Transition Elements
Form ions with variable charges, act as catalysts, and form colored compounds — properties unique to this block.
Noble Gases (Group VIII)
Completely unreactive because they have full outer electron shells — no need to gain, lose, or share electrons.
Group I: The Alkali Metals
What Are Alkali Metals?
Group I elements are called alkali metals because when they react with water, they form alkaline solutions — solutions with a high pH. The main ones you'll study are lithium (Li), sodium (Na), and potassium (K). Further down the group are rubidium, caesium, and francium (though francium is rare and radioactive).
All Group I metals share one crucial feature: they have exactly 1 electron in their outer shell. This outer electron is what makes them so reactive — they're desperate to lose it.
Key Property: Group I metals have 1 electron in their outer shell, which makes them highly reactive. They easily lose this electron to form positive ions with a +1 charge.
Physical Properties: Soft, Shiny, and Getting Softer
If you've ever seen sodium or potassium being cut, you know Group I metals are surprisingly soft — you can cut them with a knife. Here are the physical properties that define this group:
- Soft and easy to cut — they get softer as you move down the group (lithium is harder, potassium is softer)
- Shiny silvery surface when freshly cut — but they quickly tarnish because they react with oxygen in the air
- Conduct heat and electricity — they're metals, after all
- Low melting points for metals — lithium melts at 180°C, sodium at 98°C, potassium at 63°C. Potassium can literally melt on a hot day!
- Low densities — lighter than most other metals. So light they float on water (and then react with it)!
Notice the trend: as you go down the group, metals get softer and have lower melting points. This is because atomic radius increases, and the single outer electron is held less tightly by the nucleus.
Why stored under oil?
Alkali metals are stored under oil to prevent them from reacting with oxygen and water vapor in the air. Cut sodium exposed to air will quickly oxidize and become a dull, white crust. The oil acts as a barrier.
Chemical Properties: Reactivity with Water
This is where Group I gets dramatic. All alkali metals react vigorously with water to produce:
Group VII: The Halogens
What Are Halogens?
halogens
diatomic