Difficulty: Beginner to Intermediate | Prerequisites: Parts 1 and 2 of these Chapter 17 notes (Metals and Nonmetals).
This is the final part of Chapter 17. It covers the "mixed groups" (Groups 13 through 16), where metals, nonmetals, and metalloids coexist within the same group. You will also learn about semiconductors and transuranium elements. The key idea is that the stair-step line on the periodic table is not a clean divide: elements near it share properties of both metals and nonmetals.
Metalloids sit along the stair-step line and have properties of both metals and nonmetals. They conduct electricity better than nonmetals but worse than metals, making some of them semiconductors (silicon, germanium). Groups 13 through 16 are "mixed" because each contains metals, nonmetals, and metalloids. Transuranium elements (atomic number above 92) are all synthetic and radioactive.
Metalloid
An element that shares some properties with metals and some with nonmetals. Think of it as: a hybrid. It looks a bit metallic but does not conduct heat or electricity as well as a true metal.
Semiconductor
A material that conducts an electric current under certain conditions but not others. In simple terms, a semiconductor is a conditional conductor. Change the temperature, add an impurity, or apply a voltage, and it switches from insulator to conductor. This is the basis of every computer chip.
Allotrope
Different molecular structures of the same element. Diamond and graphite are both pure carbon, arranged differently. Think of it as: same ingredient, different recipe.
Transuranium element
Any element with more than 92 protons (atomic number greater than uranium's). All transuranium elements are synthetic and radioactive. In simple terms, these elements do not exist in nature. They are made in laboratories and are unstable.
Metalloids are located along the stair-step (zigzag) line on the periodic table.
They can conduct electricity better than most nonmetals but not as well as metals.
Groups 13 through 17 are considered "mixed groups" because they contain metals, nonmetals, and metalloids.
Elements: boron, aluminium, gallium, indium, thallium.
Boron is a metalloid found in borax and boric acid. Used as a water softener in laundry products, a mild antiseptic, and in heat-resistant glass.
Aluminium is a metal and the most abundant metal in the Earth's crust. Used in soft-drink cans, foil, pans, bicycle frames, and aeroplanes.
Gallium is used in electronic components.
Indium and thallium are rare metals.
Elements: carbon, silicon, germanium, tin, lead.
Carbon is a nonmetal that occurs as an element in coal and as a compound in oil, natural gas, and foods. All organic compounds contain carbon, but not all carbon compounds are organic. Carbon is an allotrope: diamond and graphite are two different structural forms.
Silicon is a metalloid, second only to oxygen in abundance in the Earth's crust. Found in sand, rocks, and soil. It is the main component in semiconductors.
Germanium is also used to make semiconductors.
Tin is a metal used to coat other metals to prevent corrosion. Combined with other metals, it forms bronze and pewter.
Lead was once widely used in paint but is no longer used for that purpose because it is toxic. Still used in car batteries.
Elements: nitrogen, phosphorus, arsenic, antimony, bismuth.
Nitrogen is a nonmetal used to make ammonia and nitrates for fertilisers. It is the fourth most abundant element in the human body.
Phosphorus is a nonmetal with three allotropes. Its compounds are used in water softeners, fertilisers, match heads, and fine china.
Arsenic and antimony are metalloids. Arsenic was formerly used in paints and semiconductors but is toxic. Antimony is used to lower melting points in alloys.
Bismuth is a metal used in automatic fire-sprinkler heads.
Elements: oxygen, sulfur, selenium, tellurium, polonium.
Oxygen is a nonmetal that always exists as O₂ in its elemental form. Found in ozone, water, and hydrogen peroxide.
Sulfur is a nonmetal used for paint pigments.
Selenium is found in multivitamins.
Tellurium and polonium are metalloids.
These are elements with more than 92 protons (higher atomic number than uranium).
All are synthetic (made in laboratories) and radioactive.
They do not occur naturally.
Silicon's semiconductor properties are the foundation of the entire electronics industry. Every phone, laptop, and server runs on silicon chips.
Aluminium foil, drink cans, and aeroplane fuselages all exploit aluminium's combination of light weight, malleability, and resistance to corrosion.
Carbon's allotropes have wildly different uses: diamond in cutting tools and jewellery, graphite in pencils and lubricants.
Lead-acid batteries still power most car starters, despite lead's toxicity in other applications.
Nitrogen-based fertilisers feed a large fraction of the world's population. The Haber process, which converts nitrogen gas into ammonia, is one of the most important industrial chemical reactions.
Students sometimes think metalloids are a separate category unrelated to metals and nonmetals. They are not. Metalloids share properties of both, which is exactly why they sit on the boundary line.
"Semiconductor" does not mean "half a conductor." It means the material conducts only under specific conditions (temperature, voltage, doping with impurities).
Students often assume all carbon compounds are organic. They are not. Carbon dioxide and calcium carbonate, for example, are inorganic.
It is a common mistake to think that every element in a mixed group (13 to 16) is a metalloid. Each group contains metals, nonmetals, and metalloids, depending on where the element falls relative to the stair-step line.
⚠️ Know what a metalloid is and where metalloids are found on the periodic table (along the stair-step line).
⚠️ Be able to define "semiconductor" and name at least two examples (silicon, germanium).
⚠️ Know the number of outer electrons for each mixed group: Group 13 = 3, Group 14 = 4, Group 15 = 5, Group 16 = 6.
⚠️ Carbon as an allotrope (diamond vs. graphite) is a frequent exam topic.
⚠️ Know that transuranium elements are synthetic and radioactive, with atomic numbers above 92.
⚠️ Expect a question distinguishing "all organic compounds contain carbon" from "all carbon compounds are organic" (the second statement is false).
True or False: Metalloids are located along the stair-step line on the periodic table.
Fill in the blank: A material that conducts electricity only under certain conditions is called a ________.
True or False: All carbon compounds are organic.
Fill in the blank: Elements with an atomic number greater than 92 are called ________ elements.
True or False: Silicon is the most abundant metal in the Earth's crust.
Answers: 1. True. 2. Semiconductor. 3. False (all organic compounds contain carbon, but not all carbon compounds are organic). 4. Transuranium. 5. False (silicon is a metalloid; aluminium is the most abundant metal).
Q: What is a metalloid, and where are metalloids found on the periodic table?
A: A metalloid is an element that has some properties of metals and some of nonmetals. Metalloids are located along the stair-step line that separates metals from nonmetals.
Q: Explain why silicon is important in technology.
A: Silicon is a semiconductor, meaning it conducts electricity under certain conditions. This property makes it the foundation of computer chips and virtually all modern electronics.
Q: Carbon has two common allotropes. Name them and explain what "allotrope" means.
A: Diamond and graphite. An allotrope is a different molecular structure of the same element. Both are pure carbon, but the atoms are arranged differently, giving them very different physical properties.
Q: Why are Groups 13 through 17 called "mixed groups"?
A: Because each of these groups contains a combination of metals, nonmetals, and metalloids, unlike Groups 1 and 2 (all metals) or Group 18 (all nonmetals).
Q: What defines a transuranium element, and can you find one in nature?
A: A transuranium element has more than 92 protons (atomic number higher than uranium). You cannot find one in nature; they are all synthetic (made in laboratories) and radioactive.
Metalloids and semiconductors connect directly to physics and materials science: understanding how silicon conducts under certain conditions leads into the study of diodes, transistors, and integrated circuits. Carbon's allotropes connect to organic chemistry, where the structure of carbon compounds is the central topic. The mixed groups also reinforce the broader periodic trend that metallic character increases as you move down a group and to the left across a period, a pattern you will revisit in discussions of electronegativity and ionisation energy.
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