Trends in Properties of Elements in the Periodic Table
Senior 2 · Chemistry
Trends in Properties of Elements in the Periodic Table
Lesson 1 of 4
20 min
Rwanda earns a large share of its export income from three metals dug out of its hills: tin, tantalum and tungsten. The copper wire in your house, the aluminium saucepan in the kitchen, the charcoal in the stove and the silicon chip inside a mobile phone are all made from elements, and each was chosen because of the way it behaves.
Chemists sort the 118 known elements into three classes: metals, non-metals and metalloids. Once you know which class an element belongs to, you can predict a great deal about it before you ever see it: whether it will conduct electricity, whether it will bend or shatter, and what kind of compounds it will form.
By the end of this lesson you will be able to locate each class on the periodic table, compare their physical properties, and classify an unknown element from observations.
The periodic table arranges the elements in order of increasing atomic number. A vertical column is a group (numbered 1 to 18) and a horizontal row is a period (numbered 1 to 7). Elements in the same group have similar chemical properties.
A zig-zag line, often called the staircase, runs from boron (B) down to astatine (At).
A physical property is one you can observe or measure without changing the substance into a new substance, such as colour, melting point or electrical conductivity.
| Property | Typical metal | Typical non-metal |
|---|---|---|
| State at room temperature | Solid (except mercury, a liquid) | Many are gases (O2, N2, Cl2); bromine is a liquid; some are solids (carbon, sulfur, phosphorus, iodine) |
| Appearance | Shiny (lustrous) when freshly cut or polished | Dull (iodine and graphite have a slight shine) |
| Electrical conductivity | Good conductor | Poor conductor (insulator), except graphite |
| Thermal conductivity | Good conductor of heat | Poor conductor of heat |
| Behaviour when hammered or pulled | Malleable (can be beaten into sheets) and ductile (can be drawn into wires) | Solids are brittle: they shatter or crumble |
| Sound when struck | Sonorous (rings like a bell) | Not sonorous |
| Melting and boiling points | Usually high | Usually low (carbon and silicon are exceptions) |
| Density | Usually high | Usually low |
Silicon looks like a metal: it is grey and shiny. Hit it with a hammer, though, and it shatters like a non-metal. It conducts electricity, but only weakly, and its conductivity increases when it is heated or when light falls on it. A substance like this is called a semiconductor. That is why silicon is used in the solar panels on Rwandan health centres and in every computer chip.
Because of these exceptions, never classify an element from one property alone. Use several properties together.
A laboratory technician in Huye tests three unlabelled samples.
| Sample | Observations |
|---|---|
| X | Grey and shiny; shatters when hit; conducts electricity weakly, and better when warm |
| Y | Yellow solid; crumbles when hit; melts at 115 °C; does not conduct |
| Z | Silvery; bends and flattens without breaking; conducts well; melts at 660 °C |
Solution.
Tip
The staircase rule. Put your finger on boron and walk down the stairs to astatine. Left of the stairs: metals. Right of the stairs (plus hydrogen): non-metals. Standing on the stairs: metalloids. Aluminium touches the staircase but is a metal.
Exam technique: when asked to classify an element, quote at least two properties as evidence, for example "It is a metal because it conducts electricity and is malleable."
Try each item before you read the solution.
Classify each element as a metal, non-metal or metalloid: magnesium, chlorine, germanium, iron, phosphorus.
Solution: Magnesium (group 2) and iron (a transition metal) are metals. Chlorine (group 17) and phosphorus (group 15, right of the staircase) are non-metals. Germanium lies on the staircase and is a metalloid.
Explain why copper, not sulfur, is used to make electrical wires. Give two properties.
Solution: Copper is a good conductor of electricity and is ductile, so it can be drawn into thin wires. Sulfur is a non-metal: it does not conduct electricity and is brittle, so it would crumble instead of forming a wire.
A student says, "Graphite conducts electricity, so carbon must be a metal." Is the student right? Explain.
Solution: No. Graphite is an exception. Carbon is a non-metal: it is right of the staircase, it is brittle, it is not malleable or ductile, and in its diamond form it does not conduct at all. One property is not enough to classify an element.
Element Q is a shiny grey solid. Its electrical conductivity is low at 20 °C but rises when it is warmed. When hit with a hammer it breaks into pieces. What class of element is Q, and name one use of such an element.
Solution: Q is a metalloid: it has a metallic shine but is brittle, and it is a semiconductor (conductivity rises with temperature). Metalloids such as silicon are used to make solar cells and computer chips.
Summary
Sources and credits
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
