Reactions with Oxygen: The Alkaline Earth Metals
Meet the Group 2 Family
Group 2 of the periodic table is a family of metallic elements that share similar properties. They are all shiny, silvery-white, and are good conductors of heat and electricity. However, you won't find them in their pure form in nature because they are too reactive; they are always bonded to other elements in compounds. The group includes beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra). For safety reasons, we will focus on the first five, as radium is radioactive.
The General Reaction: Metal + Oxygen → Metal Oxide
When a Group 2 metal reacts with oxygen, a new substance called a metal oxide is formed. This type of reaction is a combination reaction, where two or more substances combine to form a single product. We can write a general word equation and a general chemical equation for this process:
Chemical Equation: $2M + O_2 \rightarrow 2MO$
In the chemical equation, 'M' stands for any Group 2 metal. The '2' in front of the 'M' and 'MO' is needed to balance the equation because an oxygen molecule ($O_2$) has two oxygen atoms.
A Closer Look at Individual Reactions
While all Group 2 elements follow the general pattern, the details of their reactions with oxygen vary significantly. The reactivity increases as you move down the group from beryllium to barium. This trend is due to the increasing atomic size and the ease with which the outer electrons can be lost. Let's examine each reaction.
| Element | Reaction with Oxygen | Observations | Chemical Equation |
|---|---|---|---|
| Beryllium (Be) | Very slow, even at high temperatures. It forms a protective oxide layer. | The shiny metal surface becomes dull. | $2Be + O_2 \rightarrow 2BeO$ |
| Magnesium (Mg) | Burns with a brilliant white flame when heated in air. | Intense white light and a white powder (ash) are produced. | $2Mg + O_2 \rightarrow 2MgO$ |
| Calcium (Ca) | Burns readily with a brick-red flame. | A white powder with some red coloration is formed. | $2Ca + O_2 \rightarrow 2CaO$ |
| Strontium (Sr) | Burns vigorously with a crimson-red flame. | A white powder is produced. | $2Sr + O_2 \rightarrow 2SrO$ |
| Barium (Ba) | Burns very vigorously with a green flame. | A white powder is produced. | $2Ba + O_2 \rightarrow 2BaO$ |
Fireworks and Flames: A Practical Application
The vibrant colors produced when Group 2 metals burn are not just for laboratory demonstrations; they have a spectacular real-world application: fireworks! The different colored flames are used to create the stunning visual effects we see in firework displays. Strontium salts produce deep reds, barium salts create greens, and calcium compounds can give an orange-red color. Magnesium and aluminum are often added to create brilliant white sparks and flashes. So, the next time you watch a fireworks show, remember you are witnessing the controlled combustion of reactive metals like those in Group 2.
The Science Behind the Reaction and the Trend
Why does reactivity increase down Group 2? It all comes down to the structure of the atom. All Group 2 elements have two electrons in their outer shell. To form an oxide, each metal atom must lose these two electrons to an oxygen atom. As you go down the group, the number of electron shells increases. This means the outer electrons are further from the positive pull of the nucleus and are more shielded by the inner electron shells. Therefore, it becomes easier to remove the outer electrons, making the element more reactive. This is why barium reacts much more vigorously with oxygen than magnesium does.
Important Questions
Why is the reaction between magnesium and oxygen considered a combustion reaction?
Combustion is a chemical process where a substance reacts rapidly with oxygen and gives off heat and light. When magnesium ribbon burns in air, it reacts with oxygen to form magnesium oxide, and it releases a brilliant white flame, which is both heat and light. This fits the definition of combustion perfectly, similar to the burning of wood or fuel.
What is the white ash left after magnesium burns?
The white ash is the product of the reaction: magnesium oxide ($MgO$). It is a white, powdery solid that is completely different from the original shiny, silvery magnesium metal. This is a clear example of a chemical change, where new substances with new properties are formed.
Do these oxides have any uses?
Yes, they do! Magnesium oxide is used in medicine as an antacid to relieve heartburn. Calcium oxide, also known as quicklime, is a very important compound used in the manufacturing of cement and steel, and to treat soil in agriculture. Beryllium oxide is used in specialized ceramics and electronics because it is a good electrical insulator but also an excellent conductor of heat.
Footnote
[1] Alkaline Earth Metals: The elements found in Group 2 of the periodic table: Beryllium (Be), Magnesium (Mg), Calcium (Ca), Strontium (Sr), Barium (Ba), and Radium (Ra). They are named "alkaline" because their oxides form alkaline solutions in water, and "earth" was a historical term for oxides that were insoluble in water.
[2] Combustion: A high-temperature exothermic chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces heat and light, often in the form of a flame.
[3] Oxides: Chemical compounds that contain at least one oxygen atom and one other element in its chemical formula. For Group 2, the general formula is MO, where M is the metal.
[4] Periodic Table: A tabular arrangement of the chemical elements, organized by their atomic number, electron configuration, and recurring chemical properties. Elements are presented in order of increasing atomic number, which reveals periodic trends.