Trends in Properties of Elements in the Periodic Table
Senior 2 · Chemistry
Trends in Properties of Elements in the Periodic Table
Lesson 2 of 4
22 min
In a school laboratory, sodium is kept in a bottle of paraffin oil, because it reacts with the water vapour and oxygen in air. Potassium, just below sodium in group 1, is kept even more carefully, because it is more reactive still. At the other side of the table, chlorine is added in small, controlled amounts to Rwanda's piped water to kill germs, while fluorine, just above it in group 17, is far too dangerous to handle that way.
These are not random facts. They follow trends: regular changes in properties as you move down a group or across a period. If you understand the trends, you can predict how an element will behave even if you have never seen it, for example the rare metal francium at the bottom of group 1.
By the end of this lesson you will be able to explain the trends in reactivity of metals and non-metals using electron arrangement.
Chemical reactions involve only the outer-shell electrons (also called valence electrons) of atoms. Look at the electron arrangements of three group 1 metals and three group 17 non-metals:
| Group 1 | Electron arrangement | Group 17 | Electron arrangement |
|---|---|---|---|
| Lithium (Li) | 2,1 | Fluorine (F) | 2,7 |
| Sodium (Na) | 2,8,1 | Chlorine (Cl) | 2,8,7 |
| Potassium (K) | 2,8,8,1 | Bromine (Br) | 2,8,18,7 |
Every group 1 atom has one outer electron and every group 17 atom has seven. For groups 1 and 2 the number of outer electrons equals the group number; for groups 13 to 18 it equals the group number minus 10. The number of occupied shells equals the period number. This is why elements in the same group react in similar ways.
When a metal reacts, its atoms lose their outer electrons to form positive ions. The easier an atom loses its outer electrons, the more reactive the metal.
The group 1 metals (the alkali metals) all react with cold water to form an alkaline hydroxide solution and hydrogen gas:
2Na(s) + 2H2O(l) → 2NaOH(aq) + H2(g)
Lithium fizzes steadily. Sodium melts into a silver ball that races across the surface. Potassium reacts so violently that the hydrogen catches fire with a lilac flame. Reactivity increases down group 1 because the single outer electron is further from the nucleus and more shielded, so it is lost more easily. The same trend is found in group 2: calcium is more reactive than magnesium.
Rubidium (Rb) lies below potassium in group 1. Predict how it reacts with water and write the equation.
Solution. Rubidium has one outer electron, like potassium, so it reacts in the same way to form rubidium hydroxide and hydrogen. It has one more shell than potassium, so its outer electron is further from the nucleus and is lost even more easily: the reaction is more violent than potassium's (it can explode).
2Rb(s) + 2H2O(l) → 2RbOH(aq) + H2(g)
When a non-metal reacts with a metal, its atoms gain electrons to form negative ions. The more strongly an atom attracts an extra electron, the more reactive the non-metal.
In group 17 (the halogens), reactivity decreases down the group: fluorine > chlorine > bromine > iodine. A small atom such as fluorine has its outer shell close to the nucleus, so it attracts an incoming electron strongly. In a large atom such as iodine the outer shell is far away and well shielded, so the attraction is weaker.
You can see this trend in displacement reactions: a more reactive halogen takes the place of a less reactive one in its salt.
Cl2(aq) + 2KBr(aq) → 2KCl(aq) + Br2(aq)
The colourless potassium bromide solution turns orange-brown as bromine forms. Chlorine also displaces iodine from potassium iodide, and bromine displaces iodine, but iodine cannot displace chlorine or bromine.
Moving across period 3 from sodium to argon:
Helium, neon, argon, krypton, xenon and radon (group 18) have full outer shells (helium has 2 electrons; the others have 8). They have no tendency to lose, gain or share electrons, so they are very unreactive. Argon fills ordinary light bulbs because it will not react with the hot filament; helium fills balloons because it is light and does not burn.
Which is more reactive, magnesium (atomic number 12) or calcium (atomic number 20)? Explain.
Solution. Magnesium is 2,8,2 and calcium is 2,8,8,2. Both are in group 2 with two outer electrons. Calcium has an extra shell, so its outer electrons are further from the nucleus and more shielded. They are lost more easily, so calcium is more reactive. (Calcium reacts steadily with cold water; magnesium reacts only very slowly.)
Tip
Opposite jobs, opposite trends. Metals lose electrons, and losing is easier when the electrons are far from the nucleus, so metals get more reactive down a group. Non-metals gain electrons, and gaining is easier when the shell is close to the nucleus, so non-metals get more reactive up a group.
Exam technique: a full-mark explanation mentions three things: the number of shells (atomic size), the distance of the outer electrons from the nucleus, and shielding by inner shells.
Try each item before you read the solution.
An element has the electron arrangement 2,8,6. State its group and period, and say whether it is a metal or a non-metal.
Solution: It has 6 outer electrons, so it is in group 16 (6 + 10). It has 3 occupied shells, so it is in period 3. It is sulfur, a non-metal.
Explain why potassium reacts more vigorously with water than sodium does.
Solution: Both have one outer electron. Potassium (2,8,8,1) has one more shell than sodium (2,8,1), so its outer electron is further from the nucleus and more shielded by inner electrons. It is lost more easily, so potassium reacts faster and more violently.
Bromine water is added to potassium iodide solution, and the colourless solution turns brown. Write a balanced equation and explain the observation.
Solution: Br2(aq) + 2KI(aq) → 2KBr(aq) + I2(aq). Bromine is more reactive than iodine (it is higher in group 17), so it displaces iodine from its salt. The iodine formed colours the solution brown.
Would you expect iodine to react with potassium chloride solution? Explain.
Solution: No. Iodine is less reactive than chlorine (it is lower in group 17, with larger atoms that attract electrons less strongly), so it cannot displace chlorine from potassium chloride.
Arrange Na, Mg and Al in order of decreasing reactivity and explain the order.
Solution: Na > Mg > Al. All three are in period 3. Moving across, the nuclear charge increases and the atoms get smaller, and each atom must lose more electrons (1, 2 and 3), so the electrons are lost less easily.
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