Difficulty: Beginner | Prerequisites: Part 1 (Metals) of these notes, or a basic grasp of metallic bonding and the periodic table layout.
This is the second of three parts covering Chapter 17. Part 2 deals with nonmetals, the elements to the right of the stair-step line. You will learn what makes nonmetals different from metals, then focus on two important nonmetal families: the halogens (Group 17) and the noble gases (Group 18). Understanding how nonmetals gain or share electrons is the key idea here, and it sets up the concept of covalent bonding.
Nonmetals are usually gases or brittle solids, poor conductors, and found to the right of the stair-step line. They gain electrons (forming negative ions) when bonding with metals, and share electrons (forming covalent bonds) when bonding with other nonmetals. The halogens (Group 17) are highly reactive and need just one electron to complete their outer level. The noble gases (Group 18) are stable and unreactive because their outer energy levels are already full.
Nonmetal
An element that is usually a gas or a brittle solid at room temperature, is not malleable or ductile, is a poor conductor of heat and electricity, and is typically not shiny. In simple terms, nonmetals are the opposite of metals in almost every physical property.
Covalent bond
A bond formed when two nonmetal atoms share one or more pairs of electrons. Think of it as: neither atom is willing to give up electrons entirely, so they share them instead.
Noble gas
Any element in Group 18 of the periodic table. Noble gases have full outer energy levels and exist only as isolated atoms. In simple terms, they already have all the electrons they need, so they have no reason to bond with anything.
Halogen
Any element in Group 17 of the periodic table: fluorine, chlorine, bromine, iodine, and astatine. Each has seven outer electrons and needs one more to complete its level. Think of it as: they are one electron short of being as stable as a noble gas, which is why they are so reactive.
Diatomic molecule
A molecule made up of two atoms of the same element, joined by a covalent bond. In simple terms, some elements (like O₂, N₂, Cl₂) naturally travel in pairs.
Allotrope
Different molecular structures of the same element. Think of it as: diamond and graphite are both pure carbon, just arranged differently.
Nonmetals are usually gases or solids at room temperature. Bromine is the sole exception: it is a liquid.
Solid nonmetals are brittle or powdery, not malleable or ductile.
They are poor conductors of heat and electricity because their electrons are not free to move.
On the periodic table, nonmetals sit to the right of the stair-step line. Hydrogen is the exception: it is in Group 1 but is a nonmetal.
When a nonmetal bonds with a metal, the nonmetal gains electrons, becoming a negative ion. This produces an ionic bond.
When a nonmetal bonds with another nonmetal, they share electrons and form a covalent bond.
All noble gases are nonmetals.
They exist only as isolated atoms (they do not form molecules).
They are stable because their outermost energy levels are completely full.
When an electric current is passed through a noble gas, it glows. Neon lights are the classic example.
The halogens are fluorine, chlorine, bromine, iodine, and astatine.
They are very reactive in their elemental forms because each atom has seven outer electrons and needs just one more to complete its level.
When a halogen bonds with a metal, it gains that one electron and forms an ionic compound called a salt.
Fluorine: The most chemically active nonmetal element. Fluorides are added to toothpaste and city water to prevent tooth decay. Hydrofluoric acid (HF) is used to etch glass, frost light bulbs, and manufacture semiconductors.
Chlorine: The most abundant halogen. Obtained from seawater. Used to disinfect water and to whiten flour, clothing, and paper.
Bromine: The only nonmetal that is a liquid at room temperature. Extracted from seawater compounds. Formerly used in cosmetics and as flame retardants. Bromine compounds absorb fluorescent light and emit a reddish visible light.
Iodine: A solid at room temperature, obtained from seawater. It sublimes (goes directly from solid to gas) and turns purple. Essential in the human diet and used as a disinfectant.
Astatine: A rare, radioactive element and the last member of the halogen group.
Chlorine in your tap water is there to kill bacteria. The same principle applies to swimming pools.
Iodine is added to table salt ("iodised salt") because your thyroid gland needs it to function, and deficiency causes goitre.
Fluoride in toothpaste strengthens tooth enamel, directly applying the chemistry of fluorine compounds.
Noble gases are used in lighting (neon signs), welding shields (argon), and airship inflation (helium), all because of their reluctance to react.
Students often think hydrogen is a metal because it sits in Group 1. It is not. Hydrogen is a nonmetal, placed in Group 1 only because it has one electron.
"Noble gases never bond" is an oversimplification. Under extreme lab conditions, some noble gas compounds have been made, but for the purposes of this course, treat them as unreactive.
Bromine is sometimes mistaken for a metal because it is a liquid. Liquids can be nonmetals too; bromine is the only nonmetal liquid at room temperature.
Students confuse ionic and covalent bonds. If both atoms are nonmetals, the bond is covalent (sharing). If one is a metal and one is a nonmetal, it is ionic (transfer).
⚠️ Know the four key properties of nonmetals: usually gases or brittle solids, poor conductors, not shiny, not malleable/ductile.
⚠️ Be ready to explain the difference between ionic bonds (metal + nonmetal) and covalent bonds (nonmetal + nonmetal).
⚠️ Halogens have seven outer electrons. This number comes up frequently on exams.
⚠️ Noble gases are stable because their outer energy levels are full. Expect a question on why they do not react.
⚠️ Know the states of matter for the halogens at room temperature: fluorine and chlorine are gases, bromine is a liquid, iodine is a solid (and sublimes to a purple gas), astatine is a radioactive solid.
True or False: Nonmetals are good conductors of electricity.
Fill in the blank: When two nonmetals bond, they form a ________ bond.
True or False: Noble gases are highly reactive.
Fill in the blank: Each halogen has ________ electrons in its outer energy level.
True or False: Bromine is a solid at room temperature.
Answers: 1. False. 2. Covalent. 3. False (they are the least reactive). 4. Seven. 5. False (it is a liquid).
Q: Why are noble gases considered stable and unreactive?
A: Their outermost energy levels are completely full, so they have no need to gain, lose, or share electrons.
Q: What type of bond forms when chlorine reacts with sodium, and what is the product called?
A: An ionic bond forms. Sodium transfers an electron to chlorine, producing sodium chloride, which is a salt.
Q: Explain why nonmetals are poor conductors of electricity.
A: In nonmetals, electrons are not free to move between atoms the way they are in a metallic bond. Without mobile electrons, there is no pathway for electrical current.
Q: Which halogen is the most chemically active, and give one practical use of its compounds.
A: Fluorine is the most chemically active nonmetal. Fluoride compounds are added to toothpaste and city water systems to prevent tooth decay.
Q: Iodine is a solid at room temperature, yet it can turn into a purple gas without melting. What is this process called?
A: Sublimation. Iodine goes directly from a solid to a gas at room temperature.
Nonmetal behaviour is the mirror image of metal behaviour from Part 1. Together, they explain ionic bonding (metal gives electrons, nonmetal takes them). Covalent bonding between nonmetals is explored in much greater depth in later chapters on molecular compounds. The halogen family connects to acids (hydrofluoric acid, hydrochloric acid) and to everyday chemistry (water treatment, disinfection). Noble gas stability underpins the octet rule, which you will use throughout the rest of the course.
Nonmetals, halogens, noble gases, Group 17, Group 18, covalent bond, ionic bond, electron sharing, electron transfer, fluorine, chlorine, bromine, iodine, astatine, neon, argon, helium, diatomic molecule, sublimation, brittle, poor conductor, periodic table right side, chemistry chapter 17