C. Chemistry: The Nature of Matter · Atoms, Elements and CompoundsChemical Families: Patterns in the Periodic Table
C2.5 C2.4 A1.2 A2.2About 70 minutes
Learning goals- I can describe the properties of the alkali metals, alkaline earth metals, halogens and noble gases.
- I can describe the trend in reactivity down a group and explain it using atomic structure.
- I can use the periodic table to predict how an element will behave.
- I can describe some uses of elements in each family.
Words to know
chemical family
A group of elements in the same column of the periodic table, with similar properties because they have the same number of valence electrons.
alkali metals
Group 1 elements (lithium to francium): soft, very reactive metals with 1 valence electron.
alkaline earth metals
Group 2 elements (beryllium to radium): reactive metals with 2 valence electrons, less reactive than the alkali metals.
halogens
Group 17 elements (fluorine to astatine and beyond): very reactive nonmetals with 7 valence electrons.
noble gases
Group 18 elements (helium to radon and beyond): very unreactive gases with full valence shells.
reactivity
How easily a substance takes part in a chemical reaction.
diatomic
Made of two atoms joined together, like Cl₂.
Family resemblance
Members of a family often share features: the same laugh, the same curly hair. Elements in the same group of the periodic table are like that. They are called a chemical family because they have similar properties. Last lesson you learned the reason: they have the same number of valence electrons.
Today you'll meet four famous families, and you'll see that family members are similar, but not identical: there is a trend as you go down each group.
Group 1: the alkali metals
Lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs) and francium (Fr).
- Soft metals that can be cut with a knife, shiny when freshly cut but they tarnish within seconds in air.
- Very reactive, so they are never found as pure elements in nature and are stored under oil to keep air and water away.
- They react with water to produce hydrogen gas and a basic solution (a metal hydroxide). For example: sodium + water → sodium hydroxide + hydrogen.
- Reactivity increases going down the group. Lithium fizzes steadily on water; sodium whizzes around and melts into a ball; potassium reacts so violently that the hydrogen catches fire with a lilac flame.
- 1 valence electron, which they lose easily.
Group 2: the alkaline earth metals
Beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba) and radium (Ra).
- Harder and denser than the alkali metals, and less reactive. Magnesium reacts only slowly with hot water; calcium reacts steadily with cold water.
- Reactivity increases going down the group.
- 2 valence electrons.
- Their compounds are everywhere: calcium carbonate is limestone and chalk; calcium compounds make bones and teeth strong; magnesium burns with a dazzling white light and is used in some fireworks and flares.
Group 17: the halogens
Fluorine (F), chlorine (Cl), bromine (Br), iodine (I) and astatine (At).
- Very reactive nonmetals. They exist as diatomic molecules: F₂, Cl₂, Br₂, I₂.
- At room temperature they show all three states: fluorine is a pale yellow gas, chlorine a yellow-green gas, bromine a red-brown liquid and iodine a grey-black solid.
- They react strongly with metals to form compounds called salts: sodium + chlorine → sodium chloride (table salt). "Halogen" means "salt-former".
- Reactivity decreases going down the group: fluorine is the most reactive nonmetal of all.
- 7 valence electrons, so they tend to gain 1 electron.
- Uses: chlorine compounds disinfect drinking water and swimming pools; fluoride compounds in toothpaste help prevent tooth decay; iodine is used as an antiseptic, and iodine compounds are added to table salt.
Group 18: the noble gases
Helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe) and radon (Rn).
- Colourless, odourless gases that are extremely unreactive, because their valence shells are full.
- Uses: helium fills balloons and cools the magnets in MRI machines; neon glows red-orange in signs; argon protects hot metal from air during welding and fills some light bulbs.
- Radon is radioactive. It forms naturally from uranium in rocks and soil and can build up in basements. Health Canada recommends that every home be tested for radon.
| Family | Group | Valence e⁻ | Metal or nonmetal | Reactivity | Trend down the group |
|---|
| Alkali metals | 1 | 1 | Metal | Very high | More reactive |
| Alkaline earth metals | 2 | 2 | Metal | High | More reactive |
| Halogens | 17 | 7 | Nonmetal | Very high | Less reactive |
| Noble gases | 18 | 8 (He: 2) | Nonmetal | Almost none | Remain unreactive |
Explaining the trends with atomic structure
Metals react by losing valence electrons. Going down group 1, each atom has one more energy level, so the single valence electron is farther from the nucleus and is shielded by more inner electrons. The nucleus holds it less tightly, so it is easier to lose. That's why potassium is more reactive than sodium, and sodium more than lithium.
Halogens react by gaining an electron. Going down group 17, the atoms get bigger, so the nucleus attracts an incoming electron less strongly. Fluorine, the smallest, grabs electrons most strongly, so it is the most reactive. The trend goes the opposite way from the alkali metals.
Worked exampleExample 1: Predict a reaction
Rubidium (Rb) is below potassium in group 1. Predict how rubidium reacts with water, and name the products.
- Rubidium is an alkali metal (group 1), so it reacts with water like Li, Na and K.
- It is lower in the group than potassium, so its valence electron is farther from the nucleus and lost even more easily: it should react more violently than potassium.
- Products follow the same pattern: metal + water → metal hydroxide + hydrogen.
Rubidium reacts even more violently than potassium, producing rubidium hydroxide and hydrogen gas.
Worked exampleExample 2: Which is more reactive?
Which is more reactive, chlorine or bromine? Explain using atomic structure.
- Both are halogens with 7 valence electrons; both react by gaining one electron.
- Chlorine (2, 8, 7) has 3 energy levels; bromine has 4. Bromine's outer level is farther from its nucleus.
- Chlorine's nucleus pulls an incoming electron in more strongly.
Chlorine is more reactive than bromine: halogen reactivity decreases down the group.
Safety note
The violent reactions of alkali metals with water are done only by trained people, behind safety shields, with tiny pieces. Many videos online show them: watching is the safe way to see the trend. Never try these reactions yourself.
Hands-on activityFamily data detective
You need: paper, a pencil, and the data below (or a periodic table that lists melting points).
Melting points of the alkali metals: lithium 181 °C, sodium 98 °C, potassium 63 °C, rubidium 39 °C, caesium 28 °C.
- Make a bar graph of melting point (y-axis) for each element in order down the group (x-axis).
- Describe the trend in one sentence.
- Room temperature is about 20 °C and a very hot summer day in Ontario might reach 35 °C. Which alkali metal would melt on a hot day if you held it in a sealed container? (Hint: also think about your hand at 37 °C.)
- Now watch a video of lithium, sodium and potassium reacting with water (search for one made by a university or a science centre). Record what happens with each. Do your observations show the reactivity trend described in this lesson?
- Challenge: predict whether francium would melt below or above 28 °C.
What to look for: melting points decrease going down the group. Caesium (28 °C) would melt on a 35 °C day and at body temperature; rubidium (39 °C) needs a little more than body temperature. The videos should show reactions becoming more violent from lithium to potassium. Francium should melt below 28 °C if the trend continues (francium is so rare and radioactive that it has never been measured directly; scientists predict it from the trend).
Just for funNeon signs are famous, but neon gas glows only red-orange. Most other colours come from other gases, such as argon, or from coatings on the inside of the glass. So plenty of "neon" signs have no neon in them at all.
Your turn
1Name the four families in this lesson and their group numbers.
Alkali metals (1), alkaline earth metals (2), halogens (17), noble gases (18).
2Why are alkali metals stored under oil?
They are so reactive that they would react with oxygen and water vapour in the air. The oil keeps the air away.
3What are the products when lithium reacts with water?
Lithium hydroxide and hydrogen gas.
4Why are the noble gases so unreactive?
Their valence shells are full, so they don't tend to gain, lose or share electrons.
5Which is the most reactive halogen, and why?
Fluorine. It is the smallest halogen, so its nucleus attracts an incoming electron most strongly.
6Give one use each for chlorine, helium and argon.
Chlorine: disinfecting drinking water or pools. Helium: balloons or cooling MRI magnets. Argon: shielding gas in welding or filling light bulbs.
7Which is more reactive, magnesium or calcium? Why?
Calcium. It is lower in group 2, so its valence electrons are farther from the nucleus and lost more easily.
More practice: warm-up
1How many valence electrons do the halogens have?
2Which family is in group 2?
The alkaline earth metals.
3Name a halogen that is a liquid at room temperature.
4Which noble gas is radioactive and can build up in basements?
5Does reactivity increase or decrease down group 1?
More practice: on your own
1Predict the products of potassium reacting with water.
Potassium hydroxide and hydrogen gas (the hydrogen often catches fire).
2Explain why the trend in reactivity goes in opposite directions for group 1 and group 17.
Group 1 metals react by losing an electron, which gets easier as atoms get bigger (so reactivity increases down the group). Group 17 nonmetals react by gaining an electron, which gets harder as atoms get bigger (so reactivity decreases down the group).
3Why is helium used in party balloons instead of hydrogen, even though hydrogen is lighter?
Hydrogen is flammable and can explode. Helium is a noble gas, so it does not burn or react. It is also less dense than air, so balloons still float.
4Astatine is below iodine in group 17. Predict whether it is more or less reactive than iodine, and whether it is likely a gas, liquid or solid.
Less reactive (reactivity decreases down the group). Since F and Cl are gases, Br is a liquid and I is a solid, astatine is likely a solid.
5Why is the reaction of sodium with chlorine a good example of "halogen = salt-former"?
Sodium (an alkali metal) and chlorine (a halogen) react to form sodium chloride, which is table salt.
Challenge
1Strontium compounds give fireworks a red colour. Strontium is in group 2, period 5. Predict two properties of strontium metal and explain your reasoning.
It is an alkaline earth metal, so it should be shiny when freshly cut, a good conductor, and react with water (giving hydrogen and a basic solution). Being lower than calcium, it should react more readily than calcium. (The firework colour comes from its electrons jumping between energy levels, from the Bohr lesson.)
2Using the melting point data in the activity, estimate the melting point of potassium if you had only the values for sodium (98 °C) and rubidium (39 °C). How close is your estimate?
Halfway between 98 and 39 is (98 + 39) ÷ 2 = 68.5 °C. The real value is 63 °C, so the estimate is about 5 °C too high, but close. Trends let scientists make useful estimates.
3A student says, "Noble gases never react with anything." Is that completely true? How should she phrase it more scientifically?
Not completely: chemists have made a few compounds of the heavier noble gases, especially xenon, under special conditions. A better statement: "Noble gases are extremely unreactive because they have full valence shells; only a few compounds of them are known."
Quiz
1. Which family has 1 valence electron and reacts vigorously with water?
2. Which statement about group 17 is true?
3. Why is potassium more reactive than sodium?
4. Which element is a noble gas?
5. What gas is produced when an alkali metal reacts with water?
6. Halogens exist as diatomic molecules. Which formula shows this?
Big idea: C2.5 asks students to identify patterns in the periodic table. Chemical families are the clearest example: elements in a group share properties, and reactivity changes in a predictable way down the group. C2.4 is reinforced by explaining why using atomic structure.
Common difficulties:
- Thinking all trends go the same way. Metals get more reactive down a group; halogens get less reactive.
- Putting hydrogen in the alkali metal family.
- Explaining trends with "more electrons = more reactive". The real reason is the distance of the valence electrons from the nucleus.
Safety: these reactions must not be tried at home. Videos from universities or science centres are the safe way to see them. Consider testing your home for radon (test kits are sold in hardware stores); it connects this lesson to real life.