Revision sheet: metals, nonmetals and metalloids (202-SN1-RE)
This revision sheet covers metals, nonmetals and metalloids in CEGEP general chemistry: the staircase of the periodic table, the physical states of the elements, metallic bonding, the charge of cations and anions, the activity series and single displacement, the oxidizing power of the halogens, semiconductors and the extraction of metals. It is written for CEGEP students in General Chemistry 202-SN1-RE (formerly 202-NYA).
The thread: the position of an element in the periodic table tells you whether it gives or takes electrons, and therefore everything else, its physical state, its conductivity, the charge of its ion, its strength as a reducing or an oxidizing agent, and even the price of its metal. Once you have read the sheet, the corrected exercise set on the same chapter puts every reflex to the test.
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The thread of the chapter
The position of an element in the periodic table tells you whether it gives or takes electrons, and therefore everything else: its physical state, its conductivity, the charge of its ion and its strength as a reducing or an oxidizing agent.
•The staircase starts between boron and aluminum and runs down towards polonium. To its left are the METALS, to its right the NONMETALS, and on its steps the six METALLOIDS: boron, silicon, germanium, arsenic, antimony, tellurium.
•A metal LOSES its valence electrons to take on the configuration of the noble gas that PRECEDES it. A nonmetal GAINS electrons to take on that of the noble gas that FOLLOWS it.
•The charge of the ion is therefore read from the group number: +1, +2, +3 for groups 1, 2 and 13; −3, −2, −1 for groups 15, 16 and 17.
•Only two elements are LIQUID at 25 °C: mercury, a metal, and bromine, a nonmetal. Eleven are gases, and all of them are nonmetals: H, N, O, F, Cl and the six noble gases.
•No metal is a gas at 25 °C: a metal is a collective lattice, a nonmetal a collection of small molecules that simple London dispersion forces are enough to separate.
Metals to the left of the staircase, nonmetals to the right, the six metalloids on its steps; the charge of the ion is read above the column.
Metallic bonding and what it explains
•A metal is a lattice of CATIONS bathed in a sea of delocalized electrons, free to move throughout the crystal.
•CONDUCTION: what decides is not the presence of charges but their MOBILITY. Solid salt contains ions and does not conduct; molten or dissolved, it conducts very well.
•The conductivity of a metal DECREASES as the temperature rises: the electrons are already free and fixed in number, and the vibrating cations hinder them. A semiconductor does the opposite.
•MALLEABILITY: metallic bonding is not directional, so a plane that slides finds the same environment again. In an ionic crystal, the same slip brings charges of the same sign face to face and the crystal cleaves.
•MELTING: it rises with the charge of the cation and the number of electrons given up. Sodium 98 °C, magnesium 650 °C, aluminum 660 °C.
In a metal, a sliding plane finds the same neighbours in the sea of electrons; in an ionic crystal, the same slip brings charges of the same sign face to face.
The surest test for classifying an unknown sample is to measure its conductivity at two temperatures: it falls for a metal, it rises for a metalloid, it stays at zero for a nonmetal.
Reducing and oxidizing agents: the two series
•Activity series, from strongest to weakest reducing agent: K, Ca, Na, Mg, Al, (C), Zn, Fe, Pb, (H), Cu, Ag, Au.
•A metal displaces from its compounds any metal located BELOW it, never one above. Zinc displaces copper, copper does not displace zinc.
•A metal above hydrogen attacks nonoxidizing acids. A metal below it does not: a copper wire does nothing in hydrochloric acid.
•A metal below carbon can be smelted in a blast furnace. A metal above it needs electricity, hence the historical gap between iron and aluminum.
•Halogens, decreasing OXIDIZING power: F2>Cl2>Br2>I2, the electron that is gained sitting farther and farther from the nucleus and more and more shielded. The REDUCING power of the ions follows the opposite order.
Two landmarks are enough in the series: hydrogen tells you which metals attack acids, carbon tells you which can be smelted in a blast furnace.
The mistakes that cost marks
These are the errors I correct most often in session. Each one costs marks on a paper, even when the reasoning behind it is right.
1.Reversing the direction of single displacement
the whole question, and a reaction announced that does not take place
What not to write
“Copper is a metal, so it will certainly displace zinc from its sulfate.”
What to write
“Copper is BELOW zinc in the series: it does not displace it. The reverse happens, zinc displaces copper.”
Zinc, higher in the series, gives its electrons to the copper ions and becomes coated with copper; the copper strip in zinc sulfate does nothing.
Why: The direction is recovered from the physical meaning: the higher a metal sits in the series, the less tightly it holds its electrons, so it is the one that gives them to the other. You do not memorize an arrow, you memorize that the stronger reducing agent gives.
2.Concluding that a set of isoelectronic ions has the same radius
2 marks, and a ranking turned completely upside down
What not to write
“N3−, O2−, F−, Na+ and Mg2+ all have ten electrons, so they all have the same radius.”
What to write
“They have the same number of ELECTRONS but not of PROTONS: 7 for nitrogen, 12 for magnesium. The radius decreases as Z increases, from 1.46 to 0.65 angstrom.”
Six ions, a single arrangement of ten electrons, and a ratio of nearly three between the largest and the smallest.
Why: It is the only case in which radii are compared without worrying about the period or the group, and that is precisely why it comes up on tests. The more highly charged the nucleus, the tighter it pulls the same cloud: an anion is always larger than its atom, a cation always smaller.
3.Believing that a semiconductor behaves like a metal when hot
2 marks, and the distinction between metal and metalloid is lost
What not to write
“Silicon conducts, so like any conductor it will conduct less well when it heats up.”
What to write
“It is the opposite: its electrons are engaged in bonds and must cross a band gap. Heating RELEASES new ones, so the conductivity RISES.”
Three families, three behaviours. Measuring at two temperatures is enough to classify an unknown sample.
Why: The two solids do not face the same problem. In a metal the carriers already exist in fixed number, and agitation only hinders them. In a semiconductor carriers must first be created, and agitation takes care of that. It is exactly this contrast that defines a metalloid.
4.Confusing n-type and p-type doping
1 to 2 marks, and the junction is described backwards
What not to write
“Silicon is doped with boron, which is in group 13: an electron is missing, so the carrier is negative.”
What to write
“A MISSING electron leaves a HOLE, which moves like a POSITIVE charge: that is p-type. n-type is obtained with phosphorus, from group 15, which supplies an extra electron.”
Phosphorus supplies a fifth electron, a negative carrier: n-type. Boron has only three, it leaves a hole that moves like a positive charge: p-type.
Why: The way never to go wrong is to count valence electrons against the four of silicon. Five, so one extra, negative carrier, n-type for negative. Three, so one missing, a positive hole, p-type for positive.
5.Believing that an abundant metal is an easy metal to obtain
the reasoning question, often worth 3 or 4 marks
What not to write
“Aluminum is the most abundant metal in the Earth's crust, so it has always been easy to produce.”
What to write
“It is at the very top of the activity series: no ordinary chemical reducing agent strips it from its oxide. It was first isolated only in 1825 and remained more expensive than gold until 1886.”
Why: Abundance and accessibility are two unrelated questions. The carbon of the blast furnace reduces iron because iron is BELOW it in the series; it can do nothing against aluminum, which is above. A reducing agent more powerful than any chemical reagent was needed, electric current.
6.Taking a physical state for the definition of a metal
1 mark on a course question, more if the conclusion rests on it
What not to write
“All metals are solid at room temperature, that is what defines them.”
What to write
“Mercury is liquid at 25 °C, and gallium and cesium melt only just above, at 29.8 and 28.5 °C. What defines a metal is metallic bonding, not its physical state.”
Mercury is liquid well below 25 °C, cesium and gallium melt only just above: the physical state does not define a metal.
Why: A definition based on an observable property is always fragile. Mercury conducts, it is malleable in the solid state, its ions are cations: it is a metal by its bonding, and it happens to melt at −39 °C. Conversely, bromine, the only liquid nonmetal, does not conduct.
7.Looking for oxidizing power on the side of the largest halogen
2 marks, and the ranking of the whole column is inverted
What not to write
“Iodine is the largest halogen, so it attracts an electron best and is the best oxidizing agent.”
What to write
“It is the opposite: the larger the atom, the farther the electron that is gained sits from the nucleus and the more shielded it is. Fluorine, the smallest, is the best oxidizing agent, to the point of oxidizing the oxygen of water.”
From fluorine to iodine the atom grows: the electron gained sits farther from the nucleus, and the oxidizing power falls. The smallest, F2, is the strongest.
Why: A single cause explains three trends down column 17: the increasing radius lowers electron affinity, electronegativity and oxidizing power, all together. And the order of the ions follows by symmetry: the strongest oxidizing agent has the weakest conjugate reducing agent.
8.Forgetting to simplify after crossing the charges
1 mark per formula, and every molar mass that follows from it
What not to write
“Magnesium and nitrogen: Mg2+ and N3−, I cross over, so Mg3N2... or Mg2N3?”
What to write
“You cross the VALUES of the charges and swap them: the charge 3 of nitrogen becomes the subscript of magnesium, and the charge 2 of magnesium the subscript of nitrogen. Hence Mg3N2, and 3(+2)+2(−3)=0.”
Why: Crossing over is a memory aid, not a rule: the real rule is neutrality. The check takes three seconds and never fails: multiply each subscript by the corresponding charge and the sum must be zero.
Which method to choose
Does this displacement reaction take place?
The relative positions of the two species in the activity series
If the free metal is ABOVE the metal ion → the reaction takes place, and the free metal dissolves, giving up its electrons
Example: Zn+CuSO4→ZnSO4+Cu
If the free metal is BELOW the metal ion → nothing happens, and you write that there is no reaction
Example: Cu+ZnSO4 gives nothing
If the free metal is above hydrogen and the species is an acid → hydrogen gas is released
Example: Mg+2HCl→MgCl2+H2
If a free halogen faces the ion of a halogen located below it → the displacement takes place, and you recognize it by the colour
Example: Cl2+2KBr→2KCl+Br2
The two series work in exactly the same way, one for reducing agents, the other for oxidizing agents. In both cases you read the position, you do not guess.
Classifying an unknown sample
Its electrical conductivity, measured at two temperatures
Salt contains ions but conducts only once molten or dissolved: it is the MOBILITY of the charges that decides, and this is the branch that gets forgotten.
If it conducts well, and LESS well when heated → it is a metal: the carriers are already free, agitation hinders them
Example: copper, iron, aluminum
If it conducts poorly when cold and BETTER when heated → it is a semiconductor, hence a metalloid of the staircase
Example: silicon, germanium
If it does not conduct, whatever the temperature → it is a nonmetal: its band gap is too wide
Example: sulfur, diamond
If it does not conduct as a solid but conducts once molten → it is an IONIC solid, not an element: its ions were locked in place
Example: sodium chloride
The last branch is the one that gets forgotten, and it separates a question about elements from a question about compounds. A salt is neither a metal nor a nonmetal, it is an assembly of the two.
How the answer is expected to be written
A marker ticks steps. Here they are in order, with the concluding sentence expected word for word.
Predicting the formula of an ionic compound
When to use it: The question gives two elements and asks for the formula of the compound they form.
1Place each element: to the left of the staircase it loses, to the right it gains.
2Read the charge of each ion from the group number, and write it down.
3Swap the values of the charges to get the subscripts.
4Simplify if the two subscripts have a common divisor.
5Check neutrality by multiplying each subscript by its charge: the sum must be zero.
Concluding sentence
“Magnesium, in group 2, gives Mg2+; nitrogen, in group 15, gives N3−. Swapping the charges gives Mg3N2, and 3(+2)+2(−3)=0: the compound is neutral.”
The trap: Forgetting to simplify. Calcium and oxygen give Ca2O2 by crossing over, but the formula of an ionic lattice is always EMPIRICAL: you write CaO.
Marking: 1 mark for each charge correctly identified, 1 mark for the subscripts, 1 mark for the simplification when it is needed, 1 mark for the neutrality check, which is almost always counted separately.
Justifying why a metal cannot be extracted with carbon
When to use it: The question contrasts two metals, typically iron and aluminum, and asks why one has been known since antiquity and the other only since 1886.
1Place carbon in the activity series, between aluminum and zinc.
2Place the two metals relative to it.
3Write the reduction that works, with its oxidation numbers, and check the electron balance.
4Explain that the symmetrical reaction does not take place, and name the reducing agent that had to replace it.
Concluding sentence
“Iron is below carbon, so carbon monoxide reduces it: Fe2O3+3CO→2Fe+3CO2, with 2×3=6 electrons gained and 3×2=6 given up. Aluminum is above carbon: the reaction would go the other way, and electrolysis, in 1886, was needed.”
The trap: Invoking abundance or the melting point. Neither explains anything here, and the only accepted answer is the position in the activity series.
Marking: 1 mark for the position of carbon, 2 for the balanced blast furnace equation, 1 for the electron balance, 2 for the explanation of the aluminum case.
Check before you hand in
Five minutes of checking recover more marks than one more problem started in a hurry.
The neutrality of the compound formed
Multiply each subscript by the charge of its ion and add: the sum must be zero. The check takes three seconds and catches any crossing-over error.
Mg3N2: 3(+2)+2(−3)=0, the formula holds.
The direction of the series, rechecked on a known case
Before concluding about an unknown pair, test your reasoning on an obvious pair. If the rule you applied says that copper displaces zinc, it has been reversed.
Zinc displaces copper, and an iron nail becomes coated with silver in silver nitrate.
The consistency of the ion's electron count
The ion formed must have the number of electrons of the nearest noble gas. If the count does not come out, the charge is wrong.
Mg2+ has 12−2=10 electrons, those of neon.
The maximum oxidation number of a main-group element
It never exceeds the number of valence electrons. A result beyond that signals an error in the charge or in the sum.
Chlorine in group 17 rises at most to +7, in perchlorate.
The order of magnitude of an extraction yield
Compare the mass of ore with that of the metal obtained. A ratio below one is impossible, and a ratio well above ten must be justified by the grade of the ore.
Nearly four tonnes of bauxite at 50% alumina for one tonne of aluminum.
The typical problem, taken apart
Four elements of the third period, identified from their properties alone
A is a soft solid that can be cut with a knife, reacts violently with water releasing a flammable gas, and has an oxide that gives a basic solution.
B conducts current only once it is heated and is used to make chips. C is a bleaching yellow-green gas. D is a yellow solid that burns with a blue flame.
Electronegativities available: 0.93; 1.90; 2.58; 3.16. Atomic masses: Na 22.99; S 32.07; Cl 35.45.
Step 1
A is SODIUM: soft, reacting with water by releasing hydrogen gas, basic oxide; these are the four marks of an alkali metal.
Why
Only one alkali metal belongs to the third period. The four clues cross-check each other, and none is decisive on its own.
Step 2
B is SILICON: conduction that appears only when hot is the signature of a semiconductor, hence a metalloid of the staircase.
Why
It is the most reliable classification criterion, and it requires no chemical knowledge, only a measurement at two temperatures.
Step 3
C is CHLORINE, the yellow-green gas of group 17, and D is SULFUR, a yellow solid whose combustion gives sulfur dioxide with its pungent odour.
Why
Colour and physical state are enough here: chlorine is one of the eleven elements that are gases at 25 °C, and sulfur is a molecular solid.
Step 4
Electronegativity increases from left to right, so Na0.93, Si1.90, S2.58, Cl3.16.
Why
You do not trust the order of the statement, which lists the elements out of order: you put them back in the order of the period and assign the values.
Step 5
Compound of A and C: Na+ and Cl−, hence NaCl. Compound of A and D: Na+ and S2−, hence Na2S.
Why
The charges are read from columns 1, 16 and 17. No simplification is needed here.
Step 6
Electronegativity differences: 3.16−0.93=2.23 and 2.58−0.93=1.65, both above or close to the threshold of 1.7.
Why
The check confirms the ionic character of both compounds, the sulfide already being at the border of polar covalent.
Step 7
M(NaCl)=22.99+35.45=58.44 g/mol, so 58.4422.99×100=39.34% sodium.
Why
The mass percent is the most direct way to check a proposed formula against an elemental analysis.
Step 8
M(Na2S)=45.98+32.07=78.05 g/mol, so 78.0545.98×100=58.91%.
Why
The sulfide is richer in sodium: it contains two per formula unit, and sulfur is lighter than chlorine.
Step 9
2Na+2H2O→2NaOH+H2: sodium goes from 0 to +1, hydrogen from +1 to 0, so 2 electrons are exchanged.
Why
Oxygen stays at −2 on both sides: it takes no part. The sodium hydroxide formed explains the basicity, the hydrogen gas the ignition.
The conclusion, written out
“The four elements are sodium, silicon, sulfur and chlorine, in this order of increasing electronegativity, 0.93, 1.90, 2.58 and 3.16. Sodium forms NaCl at 39.34% sodium and Na2S at 58.91%, and its reaction with water is a redox reaction involving two electrons.”
The classic mistake on this problem: Identifying B as a metal because it conducts. A conductor whose conductivity INCREASES with temperature is not a metal: that is exactly the opposite behaviour, and it is the definition of a semiconductor.
Learn by heart
•The staircase separates metals from nonmetals; its six steps are the metalloids.
•The charge of the ion is read from the group: +1, +2, +3 on the left, −3, −2, −1 on the right.
•Two liquids at 25 °C: mercury and bromine. Eleven gases, all nonmetals.
•What decides conduction is the MOBILITY of the charges, not their presence.
•Metal: conductivity falls when hot. Semiconductor: it rises. Insulator: it stays at zero.
•Isoelectronic series: for equal electrons, the radius decreases as Z increases.
•A metal displaces those BELOW it, never those above.
•Above hydrogen: it attacks acids. Below carbon: it can be smelted in a blast furnace.
•F2>Cl2>Br2>I2 in oxidizing power; the ions follow the opposite order.
•n-type: phosphorus, one extra electron. p-type: boron, one hole.
•Recycling costs only the melting: the metal is already at oxidation number 0.
Frequently asked questions
How can you tell whether an element is a metal, a nonmetal or a metalloid?
Find the staircase that starts between boron and aluminum and runs down towards polonium. Metals are to its left, nonmetals to its right, and on its steps are the six metalloids: boron, silicon, germanium, arsenic, antimony and tellurium. The most reliable experimental criterion remains conductivity measured at two temperatures: it falls for a metal, it rises for a metalloid, and it stays at zero for a nonmetal.
Why do isoelectronic ions not have the same radius?
Because the number of protons changes while the number of electrons stays the same. The nitride ion has seven protons for ten electrons, so the nucleus holds a cloud that is too large for it poorly, and the cloud swells. The aluminum ion has thirteen protons for the same ten electrons, so it pulls much harder and tightens the cloud. The radius therefore decreases steadily as the atomic number increases, by a ratio of nearly three between the two extremes.
How can you tell whether a metal reacts with an acid?
Look at its position relative to hydrogen in the activity series. If it is above, it is a stronger reducing agent than hydrogen and it attacks nonoxidizing acids, releasing hydrogen gas: this is the case for magnesium, zinc and iron. If it is below, like copper, silver and gold, nothing happens, which is why a copper wire stays intact in hydrochloric acid whereas an iron nail fizzes in it.
Why was aluminum more expensive than gold in the nineteenth century?
Because it sits at the very top of the activity series, above carbon. The carbon of the blast furnace reduces iron because iron lies below it, but it can do nothing against aluminum oxide. A reducing agent more powerful than any chemical reagent was needed, electric current, with the electrolysis process developed in 1886. It is this date that made the price of the metal collapse, not the discovery of new deposits.
What is the difference between n-type and p-type doping?
It comes down to the number of valence electrons of the added impurity, compared with the four of silicon. Phosphorus, in group 15, supplies five: the fifth stays free and becomes a negative charge carrier, hence the name n-type. Boron, in group 13, supplies only three: one electron is missing, and this absence, called a hole, moves through the crystal as a positive charge would, hence p-type. The junction between the two regions is the basic component of all electronics.
Why does recycling a metal use so little energy?
Because the used metal is already at oxidation number zero. All the expense of primary production goes into giving electrons back to a metal that is present as an oxide in its ore, and only electricity can do that in large quantity for the strongest reducing metals. The metal to be recycled has no electrons left to receive: it only has to be melted, which requires only an ordinary latent heat. For aluminum, the ratio is about one to twenty.
Practise it
Corrected exercises: Metals, nonmetals and metalloids, 202-SN1-RE
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