Unit 2: Particles, Bonding, and Structures

Understanding how atoms combine to form the diverse materials around us.

2.1 Elements, Compounds & Mixtures

In chemistry, understanding the fundamental ways in which matter is classified is crucial. All matter can be broadly categorized into pure substances and mixtures. Pure substances are further divided into elements and compounds, while mixtures can be homogeneous or heterogeneous.

An element is a pure substance made up of only one type of atom. Elements cannot be broken down into simpler substances by chemical means. Examples include carbon (C), oxygen (O), and iron. Different forms of the same element, like diamond and graphite for carbon, are called allotropes.

Elements are the building blocks of all matter. Each element is defined by its atomic number, which is the number of protons in its atoms. Examples include hydrogen (H), oxygen (O), and gold (Au).

Examples:

  • Metals: Iron (Fe), Copper (Cu), Aluminum (Al)
  • Non-metals: Oxygen (Oâ‚‚), Nitrogen (Nâ‚‚), Carbon (C)
  • Metalloids: Silicon (Si), Germanium (Ge)
A compound is a pure substance composed of two or more different elements chemically bonded together in a fixed proportion by mass. Compounds have distinct properties from their constituent elements and can only be separated by chemical reactions.

Chemical bonding (ionic, covalent, metallic) holds the atoms in a compound together. The formation of a compound involves a chemical reaction, and energy changes (release or absorption) typically accompany this process.

Examples:

  • Water (Hâ‚‚O): Formed from hydrogen and oxygen atoms chemically bonded. Its properties (liquid at room temp) are very different from gaseous hydrogen and oxygen.
  • Sodium Chloride (NaCl): Table salt, formed from sodium and chlorine.
  • Carbon Dioxide (COâ‚‚): A gas formed from carbon and oxygen.
A mixture consists of two or more elements or compounds that are not chemically bonded. Mixtures have no fixed composition and their components retain their individual properties. They can be separated by physical means such as filtration, evaporation, or distillation.

Mixtures can be classified based on their uniformity:

Homogeneous Mixtures (Solutions)

These mixtures have a uniform composition and appearance throughout. The components are evenly distributed at a microscopic level.

Examples:

  • Saltwater: Salt (solute) dissolves uniformly in water (solvent).
  • Air: A mixture of nitrogen, oxygen, argon, etc.
  • Brass: An alloy (solid solution) of copper and zinc.

Heterogeneous Mixtures

These mixtures have a non-uniform composition, and their components can often be seen as separate phases.

Examples:

  • Sand and water: Sand particles settle at the bottom.
  • Oil and water: Form distinct layers.
  • Granite: Contains visible grains of different minerals.
Solved Examples:
  1. Classify air and explain your reasoning.
    Solution: Air is a mixture. It consists of various gases like nitrogen, oxygen, argon, and carbon dioxide, which are not chemically bonded and are not in a fixed ratio.
  2. Is glucose ($C_6H_{12}O_6$) an element, compound, or mixture?
    Solution: Glucose is a compound. It is made of three different elements (carbon, hydrogen, and oxygen) that are chemically bonded together in a fixed ratio (6:12:6).
  3. How can you separate the components of salt water? What does this imply about its classification?
    Solution: You can separate salt and water by distillation or evaporation. Since it can be separated by a physical process, salt water is a mixture (specifically, a solution).
  4. Explain why diamond and graphite are both considered forms of the element carbon.
    Solution: Both diamond and graphite are made of only one type of atom: carbon atoms. They are different structural forms of the same element, known as allotropes. Their different properties arise from the different ways the carbon atoms are bonded to each other.
  5. Classify bronze, which is made from copper and tin.
    Solution: Bronze is an alloy, which is a type of mixture. It is a solid mixture of two metals (copper and tin) that are not chemically bonded in a fixed ratio, so its composition can vary.

2.2 Laws of Chemical Combination

The formation of compounds is governed by fundamental principles known as the Laws of Chemical Combination. These laws describe the quantitative relationships of elements in compounds.

The Law of Constant Composition (or Definite Proportions)
This law states that "a given chemical compound always contains its component elements in a fixed ratio (by mass), regardless of its source or method of preparation".

For example, any pure sample of water ($H_2O$) will always contain approximately 88.8% oxygen and 11.2% hydrogen by mass.

The Law of Multiple Proportions
This law states that if two elements can combine to form more than one compound, then "the ratios of the masses of the second element which combine with a fixed mass of the first element will be ratios of small whole numbers".

For instance, carbon combines with oxygen to form carbon monoxide (CO) and carbon dioxide ($CO_2$). For a fixed mass of carbon (e.g., 12g), the mass of oxygen in $CO_2$ (32g) is exactly double the mass of oxygen in CO (16g), a simple 2:1 ratio.

Solved Examples:
  1. In one sample, 10g of methane contains 7.5g of carbon. Another 40g sample of methane contains 30g of carbon. Show this is consistent with the Law of Constant Composition.
    Solution: - In the first sample, the percentage of carbon is $(7.5g / 10g) \times 100\% = 75\%$. - In the second sample, the percentage of carbon is $(30g / 40g) \times 100\% = 75\%$. Since the percentage by mass of carbon is the same in both samples, the data supports the Law of Constant Composition.
  2. Sulfur and oxygen form two different compounds. In Compound A, 50g of sulfur reacts with 50g of oxygen. In Compound B, 50g of sulfur reacts with 75g of oxygen. Show this demonstrates the Law of Multiple Proportions.
    Solution: The mass of the first element (sulfur) is fixed at 50g in both cases. We compare the masses of the second element (oxygen): - Mass of Oxygen in A = 50g - Mass of Oxygen in B = 75g The ratio of the masses of oxygen is $75 : 50$, which simplifies to $3 : 2$. This is a ratio of small whole numbers, confirming the Law of Multiple Proportions.
  3. A 30g sample of magnesium oxide (MgO) contains 12.1g of oxygen. How much magnesium is in a 100g sample of MgO?
    Solution: First, find the mass of magnesium in the 30g sample: $30g - 12.1g = 17.9g$ Mg. The percentage of magnesium is $(17.9g / 30g) \times 100\% = 59.67\%$. By the Law of Constant Composition, a 100g sample will also be 59.67% magnesium. So, the mass of magnesium is $0.5967 \times 100g = 59.67g$.
  4. Iron forms two common chlorides. In one, 10g of iron combines with 12.7g of chlorine. In the other, 10g of iron combines with 19.05g of chlorine. What is the simple whole-number ratio of chlorine masses?
    Solution: The mass of iron is fixed at 10g. The ratio of the chlorine masses is $19.05 : 12.7$. To simplify, divide both by the smaller number: $19.05 / 12.7 = 1.5$ and $12.7 / 12.7 = 1$. The ratio is $1.5 : 1$. To get whole numbers, multiply by 2: $3 : 2$. This supports the Law of Multiple Proportions.
  5. A scientist claims to have created a new type of water where 100g of it contains 80g of oxygen. Why is this claim inconsistent with the Law of Constant Composition?
    Solution: Pure water is a compound with a fixed composition of about 88.8% oxygen by mass. The scientist's substance, at 80% oxygen by mass, has a different composition. Therefore, it cannot be the compound water, as this would violate the Law of Constant Composition.

2.3 Separation Techniques

Mixtures, unlike compounds, can be separated into their constituent substances by physical means. The choice of separation technique depends on the properties of the components in the mixture.

Separation of Heterogeneous Mixtures

Heterogeneous mixtures have visibly distinct components, making their separation often straightforward.

1. Decantation
Decantation is a process for separating mixtures, typically solid-liquid or liquid-liquid, by pouring off the top layer (liquid) while leaving the bottom layer (solid or denser liquid) undisturbed. This method relies on differences in density.

Example: Separating sand from water. Allow the sand to settle at the bottom of the beaker, then carefully pour the water out, leaving the sand behind.

2. Filtration
Filtration is a technique used to separate insoluble solid particles from a liquid or gas by passing the mixture through a filter medium that retains the solid particles and allows the fluid to pass through.

Example: Separating chalk powder from water. Pour the mixture through filter paper in a funnel. The water (filtrate) passes through, while the chalk powder (residue) is retained on the filter paper.

3. Magnetism
Magnetism is used to separate magnetic materials from non-magnetic materials in a mixture.

Example: Separating iron filings from sulfur powder. A magnet is passed over the mixture; the iron filings will stick to the magnet, leaving the sulfur behind.

Separation of Homogeneous Mixtures (Solutions)

Homogeneous mixtures, where components are uniformly mixed, require more sophisticated techniques that exploit differences in physical properties like boiling points or solubility.

1. Evaporation
Evaporation is a process used to separate a soluble solid from a liquid in a solution. The liquid solvent is heated until it turns into a gas, leaving the solid residue behind.

Example: Obtaining salt from saltwater. Heat the saltwater in an evaporating dish. The water evaporates, leaving the solid salt crystals behind.

2. Simple Distillation
Simple Distillation is a method used to separate a liquid from a solution containing a dissolved solid, or to separate two liquids with significantly different boiling points. The liquid is vaporized and then condensed back into a liquid (distillate).

Example: Obtaining pure water from saltwater. Heat the saltwater; water vaporizes, condenses in a condenser, and is collected as pure liquid water, leaving the salt in the original flask.

3. Fractional Distillation
Fractional Distillation is an enhanced distillation technique used to separate liquids with boiling points that are relatively close to each other. It uses a fractionating column to provide a large surface area for repeated vaporization and condensation cycles.

Example: Separating ethanol from water. Since ethanol and water have different boiling points (78.37°C and 100°C respectively), fractional distillation can be used to separate them based on their vapor pressures.

4. Chromatography
Chromatography is a powerful analytical technique used to separate components of a mixture based on their differential partitioning between a stationary phase and a mobile phase. Different components travel at different speeds, leading to their separation.

Example: Separating the different colored pigments in ink using paper chromatography. A drop of ink is placed on filter paper, and the paper is dipped into a solvent. As the solvent moves up the paper, it carries the different pigments at different rates, separating them into distinct bands.

5. Crystallization
Crystallization is a separation technique that purifies a solid by dissolving it in a hot solvent and then allowing the solution to cool. As the solution cools, the solubility of the solid decreases, causing it to crystallize out of the solution in a purer form.

Example: Purifying sugar. Dissolve impure sugar in hot water, then allow the solution to cool slowly. Pure sugar crystals will form as the solution cools, leaving impurities dissolved in the remaining liquid.

Knowledge Check (20 Questions)

Answer: An atom is the smallest unit of an element. An element is a pure substance made of only one type of atom.

Answer: Compounds have strong chemical bonds that require significant energy to break, whereas mixtures are held by weaker physical forces.

Answer: A compound (iron sulfide). A chemical reaction occurs, forming a new substance with distinct properties (e.g., non-magnetic, fixed composition), unlike the original components..

Answer: True. Compounds are composed of molecules (or formula units). However, molecules can also be elements (e.g., Oâ‚‚), so not all molecules are compounds.

Answer: A compound has entirely new physical and chemical properties. Example: Sodium (reactive metal) + Chlorine (toxic gas) combine to form Sodium Chloride (stable table salt).

Answer:
  • Melting ice: Physical (Hâ‚‚O remains Hâ‚‚O).
  • Burning wood: Chemical (new substances like COâ‚‚ formed).
  • Dissolving sugar: Physical (sugar remains sugar, forms a mixture).

Answer:
  • Petroleum refining: Separating crude oil (mixture) into useful compounds/mixtures by distillation.
  • Metal extraction: Chemically reducing metal oxides (compounds) to pure metals (elements).

Answer: The Law of Constant Composition.

Answer: Homogeneity and sharp melting point indicate a pure substance. Decomposition by electrolysis (chemical means) into new substances confirms it's a compound

Answer: Allotropy is when an element exists in different forms in the same physical state (e.g., carbon as diamond and graphite), having different structures and physical properties.

Answer: Compounds have a fixed, unchangeable ratio of elements by mass because they are chemically bonded. Mixtures have variable ratios as their components are only physically combined.

Answer: Test its boiling point. A pure compound will have a sharp, fixed boiling point, while a homogeneous mixture will boil over a range of temperatures.

Answer: Decantation relies on components settling into distinct layers due to density differences (e.g., sand and water). In homogeneous mixtures, components are uniformly dispersed and don't form separate layers to decant.

Answer: An element is made of only one type of atom, while a compound is made of two or more different types of atoms chemically bonded together.

Answer: A compound's formula represents the fixed, definite ratio of its chemically bonded elements. Air, being a mixture, has variable proportions of its components, making a single overall "formula" meaningless.

Answer: Because its components (copper and tin) are not bonded in a fixed ratio by mass.

Answer:Reaction mixtures often contain unreacted starting materials, byproducts, and catalysts, all physically mixed with the desired compound. Separating these often requires complex, energy-intensive physical methods like fractional distillation, chromatography, or crystallization to achieve high purity.

Answer:
  • Magnetism: Use a magnet to separate iron filings (element).
  • Dissolution & Filtration: Add water to dissolve the salt (compound). The sand (mixture of compounds) will not dissolve. Filter the mixture; sand remains on filter paper.
  • Evaporation: Evaporate the water from the filtrate to obtain the dissolved salt

Answer: Fractional distillation, which separates miscible liquids based on their different boiling points. The substance with the lower boiling point(ethanol) vaporizes first, is condensed, and collected.

Answer: Use a strong magnet. Pass the magnet over the mixture.
Collection: The iron filings will stick to the magnet (principle: magnetic properties - iron is magnetic, chalk is not). Carefully remove the magnet and brush off the iron filings into a separate container.
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