Arrangement of Elements in the Periodic Table
Senior 1 · Chemistry
Arrangement of Elements in the Periodic Table
Lesson 4 of 4
25 min
Electric wires in Rwandan homes are made of copper, and cooking pots of aluminium, while the charcoal in an imbabura stove and the yellow sulfur used to dust crops are very different materials. One quick way to sort elements is to test whether they conduct electricity.
Experiment. Build the circuit in Figure 4.1. Clip each sample in turn between the crocodile clips and watch the bulb. Test magnesium ribbon, aluminium foil, copper wire, an iron nail, a lump of sulfur and the graphite "lead" of a pencil (graphite is a form of carbon).
Figure 4.1: A simple conductivity tester.
Warning
Safety. Use only dry cells, never the mains socket. Do not hold a sample in your hand while it is clipped in: some wires get warm. Wash your hands after handling sulfur, and do not let magnesium ribbon near a flame.
Table 4.1: Typical results
| Sample | Does the bulb light? | Appearance |
|---|---|---|
| Magnesium ribbon | yes, brightly | shiny grey (after rubbing with sandpaper) |
| Aluminium foil | yes, brightly | shiny silver |
| Copper wire | yes, brightly | shiny orange-brown |
| Iron nail | yes | shiny grey |
| Sulfur lump | no | dull yellow, crumbles when pressed |
| Graphite (pencil lead) | yes, dimly | dull grey-black, snaps easily |
The four metals conduct well, sulfur does not conduct at all, and graphite is an exception: it is a form of carbon, a non-metal, but it conducts. This is why one property is never enough to classify an element.
Table 4.2: Typical properties of metals and non-metals
| Property | Metals | Non-metals |
|---|---|---|
| State at room temperature | solids (except mercury, a liquid) | many are gases; some solids; bromine is a liquid |
| Appearance | shiny (lustrous) when freshly cut | usually dull |
| Electrical and heat conduction | good conductors | poor conductors (graphite is an exception) |
| Effect of hammering | malleable (can be beaten into sheets) and ductile (can be drawn into wires) | solids are brittle and break |
| Sound when struck | sonorous: they ring | not sonorous |
| Melting point | usually high | usually low |
| Ions formed | lose electrons to form positive ions, e.g. | gain electrons to form negative ions, e.g. , or share electrons |
Definition
Metalloid: an element with some properties of metals and some of non-metals. Example: silicon is shiny like a metal but brittle like a non-metal, and it conducts electricity only weakly; this makes it useful in computer chips and solar panels. Of the first 20 elements, boron and silicon are metalloids.
Look back at Figure 3.1. The metals (blue) are on the left; the non-metals (orange) are on the right, together with hydrogen; the metalloids boron and silicon lie on the border between them. Of the first 20 elements, 7 are metals, 2 are metalloids and 11 are non-metals. In the whole periodic table the balance is the other way round: more than three-quarters of all elements are metals.
Metallic character is how readily an atom loses its outer electrons to form positive ions. Non-metallic character is how readily it gains electrons to form negative ions.
Figure 4.2: Trends in metallic character. The most metallic elements are at the bottom left; the most non-metallic are at the top right (leaving out the noble gases).
Table 4.3: Across period 3
| Element | Configuration | Outer electrons | Usual behaviour | Class |
|---|---|---|---|---|
| Sodium | 2,8,1 | 1 | loses 1: | metal |
| Magnesium | 2,8,2 | 2 | loses 2: | metal |
| Aluminium | 2,8,3 | 3 | loses 3: | metal |
| Silicon | 2,8,4 | 4 | shares electrons | metalloid |
| Phosphorus | 2,8,5 | 5 | gains 3 or shares | non-metal |
| Sulfur | 2,8,6 | 6 | gains 2: | non-metal |
| Chlorine | 2,8,7 | 7 | gains 1: | non-metal |
| Argon | 2,8,8 | 8 (full) | does not react | noble gas |
Worked example
Problem. Which element in each pair is more metallic? Explain. (a) Sodium or magnesium; (b) lithium or potassium.
Solution. (a) Sodium. Both are in period 3, but sodium is further left: it has to lose only 1 electron, and its nucleus has one proton fewer holding it, so it loses its outer electron more easily. (b) Potassium. Both are in group 1, but potassium (2,8,8,1) has two more shells than lithium (2,1). Its outer electron is further from the nucleus and more screened, so it is lost much more easily. This is why potassium reacts with water far more violently than lithium.
Worked example
Problem. Which is more non-metallic, fluorine (2,7) or chlorine (2,8,7)? Explain.
Solution. Fluorine. Both need one electron to complete their outer shell, but fluorine's outer shell is the second one, much closer to the nucleus. An extra electron is attracted more strongly, so fluorine gains an electron more readily than chlorine. Fluorine is the most reactive non-metal of all.
Worked example
Problem. Gasana says: "Aluminium is a non-metal, because it is in group 13 with boron, and boron is not a metal." Evaluate his argument.
Solution. His argument is not sound. Elements in a group are similar, but metallic character increases down a group, so a lower member can be more metallic than the top one. The evidence about aluminium itself matters most: it is shiny, conducts electricity and heat well (it is used for power cables and cooking pots), is malleable enough to make foil, and forms ions. By every test aluminium is a metal. Boron, at the top of the group, is a metalloid.
Warning
Common mistake: classifying an element from a single property. Graphite conducts, yet carbon is a non-metal; silicon is shiny, yet it is a metalloid. Use several properties, and the element's position, together.
Tip
Metallic character grows towards the bottom left; non-metallic character grows towards the top right. Leaving out the noble gases, the most non-metallic element is fluorine (top right) and, of the first 20, the most metallic is potassium (bottom left).
MooMooMath and Science: How to identify metals, nonmetals and metalloids on the periodic table (3 min). Watch for the staircase line that separates the metals from the non-metals.
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