0620 · 8.3Group VII properties

Halogen particles and physical properties

0:00Adding chlorine to potassium bromide solution produces an orange colour because bromine forms. Chlorine has displaced another element from its compound. This change reveals a useful pattern in the Periodic Table. By the end of this video, you will understand halogen properties, explain displacement reactions, and predict how other members of the group behave. Chlorine, bromine and iodine belong to Group VII, also called the halogens. They are non-metals with similar chemical properties because their atoms have seven electrons in the outer shell.

0:29Each needs to gain one electron to achieve a full outer shell. That connection between their atoms and electrons helps us understand their reactions. The position of an element within this group matters. Chlorine is above bromine, while iodine sits below both of them. Moving down the column gives us a sequence of related elements rather than unrelated substances. We can therefore use patterns among the familiar halogens to predict properties of less familiar members.

0:55Chlorine, bromine and iodine are diatomic non-metals. Diatomic means that each molecule contains two atoms. In a chlorine molecule, both atoms are chlorine, so the substance is still an element rather than a compound. Bromine and iodine also exist as molecules containing pairs of identical atoms. A molecule and an ion are different kinds of particle. The halogen molecules shown here have no overall electrical charge. When a halogen atom gains one electron, it becomes a halide ion with a 1− charge.

1:23Chlorine produces chloride ions, bromine forms bromide ions, and iodine produces iodide ions. Keep those names distinct because they tell us which particles are present. Chlorine is the element, whereas chloride is its negative ion. Potassium chloride contains potassium ions and chloride ions, not chlorine molecules trapped inside it. This distinction becomes essential when deciding whether a displacement reaction can happen. At room temperature and pressure, chlorine is a pale yellow-green gas.

1:53Under the same conditions, bromine exists as a red-brown liquid, while iodine forms a grey-black solid. These descriptions refer to the elements themselves, so always connect each colour with its correct physical state. Remember that the appearance of a pure element is not necessarily the appearance of its solution. Pure bromine is a red-brown liquid, but the bromine formed in our opening reaction makes the solution orange. There is no contradiction because the samples are different.

2:19In an answer, describe the substance under the conditions stated. These substances should not be handled casually because halogens can be harmful. Any practical work requires suitable laboratory precautions and teacher supervision. Never identify a sample by smelling it directly. We can understand the reactions from diagrams and recorded observations without attempting them outside a properly equipped laboratory.

Reactivity trends down Group VII

2:40Two important trends connect chlorine, bromine and iodine. Density increases down the group, while reactivity decreases. Density is the mass per unit volume, whereas reactivity is how readily a substance takes part in a chemical reaction. These trends describe different properties and must not be confused. Iodine has a greater density than chlorine, but it is less reactive. A denser substance is not automatically more reactive because chemical behaviour depends on its particles and electrons.

3:09For these three halogens, chlorine is the most reactive and iodine is the least reactive. A short electron explanation helps make that order understandable. Halogen atoms react by gaining an electron. Down the group, atoms have more occupied electron shells, placing the outer region farther from the nucleus. Inner electrons also shield an incoming electron from the attraction of the nucleus. That attraction becomes weaker down the group, so gaining an electron becomes less favourable.

3:35Chlorine therefore gains electrons more readily than bromine or iodine. This gives the order needed for displacement reactions. For this topic, the essential explanation is that a more reactive halogen displaces a less reactive one from its halide ions.

Halogen displacement reactions

3:49Consider chlorine added to potassium bromide solution. First, identify the incoming element, which is chlorine. Then identify the halide ions already present, which are bromide ions. Finally, compare chlorine with bromine in the group: chlorine is higher and more reactive, so displacement occurs. The word equation is chlorine plus potassium bromide gives potassium chloride plus bromine. Chlorine becomes chloride ions, while bromide ions become bromine molecules. Potassium ions remain in the solution throughout the reaction.

4:20The orange colour provides evidence that bromine has formed. The balanced symbol equation is on the board. One chlorine molecule reacts with two units of potassium bromide to produce two units of potassium chloride and one bromine molecule. The pairs matter because both elemental halogens are diatomic. Every substance here is aqueous, and balancing gives each type of atom the same total on both sides. The ionic equation on the board leaves out the potassium ions.

4:46At particle level, chlorine gains electrons from bromide ions. Each atom in a chlorine molecule gains one electron and becomes a chloride ion with a 1− charge. Meanwhile, each bromide ion loses an electron and becomes an uncharged bromine atom. Those bromine atoms form diatomic molecules, explaining why the element appears among the products. Displacement does not mean that one molecule physically pushes another out of the liquid. It is a chemical change involving electron transfer and the formation of different particles.

5:16The more reactive halogen can take electrons from the less reactive halogen’s ions. This is why the reactivity order determines whether the reaction happens. Now reverse the pairing by adding bromine to potassium chloride solution. The incoming element is bromine, and the solution contains chloride ions. Bromine is less reactive than chlorine, so it cannot displace chlorine from those ions. No displacement reaction occurs, and potassium bromide is not formed by this mixture.

5:43The same rule lets us predict every pairing among these three halogens. Chlorine displaces bromine from bromide ions and iodine from iodide ions. Bromine can displace iodine from iodide ions, but it cannot displace chlorine from chloride ions. Iodine cannot displace either chlorine or bromine because both are more reactive. A halogen also cannot displace itself from its own halide ions. Adding chlorine to chloride ions does not produce a different halogen.

6:09When reading this table, compare the element being added with the element named by the halide. Do not compare the halogen with potassium, which is unchanged in these reactions.

Predicting halogen reactions and properties

6:19An exam question might ask you to state whether bromine reacts with potassium iodide solution and explain your answer. Begin by changing the ion name iodide into the corresponding element name iodine. Compare that element with bromine in the reactivity order. Bromine is above iodine, so the incoming halogen is more reactive. A displacement reaction occurs. Bromine is more reactive than iodine. It displaces iodine from iodide ions. The word equation is bromine plus potassium iodide gives potassium bromide plus iodine.

6:53Pay attention to the command word used in a question. If asked to state an appearance, give the colour and physical state. To describe a trend, identify what increases or decreases down the group. When asked to explain displacement, connect the reaction outcome to the relative reactivities of the two halogens. Predictions extend the patterns beyond chlorine, bromine and iodine. Fluorine lies above chlorine, so we predict greater reactivity and lower density.

7:17For astatine, which lies below iodine, the trends point to lower reactivity and greater density. Given information about physical states, we would predict fluorine to be a gas and astatine to be a solid. Use a trend to predict a direction, not an unsupported exact value. Knowing that density increases does not tell us a precise density for astatine. Likewise, a question may supply extra information that helps you predict appearance.

7:43Base your answer on that evidence rather than inventing a colour or numerical measurement. The central pattern is now connected to the reactions. Chlorine, bromine and iodine are diatomic non-metals whose density increases and reactivity decreases down Group VII. Distinguish the uncharged elements from their negatively charged halide ions. A more reactive halogen displaces a less reactive one from its halide ions, allowing you to explain observations and predict unfamiliar reactions.