Physical Properties and Types of Elements

 

This is part of Year 11 HSC Chemistry course under the topic of Properties of Matter.

HSC Chemistry Syllabus

• Classify the elements based on their properties and position in the periodic table through their:

This topic assumes knowledge of the Bohr model of the atom.

Understanding the Periodic Table

 

The periodic table is structured in rows called periods and columns known as groups or families. Elements are arranged in order of increasing atomic number (the number of protons in the nucleus) from left to right and top to bottom. This arrangement is not arbitrary; it reflects patterns in the elements' properties, which recur periodically.

• Groups/Families: Elements in the same group share similar chemical properties due to having the same number of valence electrons. This is discussed in greater detail in chemical properties of elements.

• Periods: Moving across a period, properties transition from metallic to nonmetallic. The number of valence electrons increases from left to right, influencing the type of ions the elements tend to form.

Metals, Non-metals, and Metalloids

 

• Metals are located on the left (exception of hydrogen) and in the centre of the periodic table. The structure of pure metals consists of the same atoms arranged in layers. The electrons of these atoms are delocalised instead of fixed in an atom's orbit.

• Non-metals are found on the right side of the periodic table. The structure of non-metals varies between covalent molecular and covalent lattice network. This is discussed separately here.

• Metalloids (semi-metals) are situated along the zig-zag line separating metals and nonmetals, metalloids exhibit properties intermediate between those of metals and nonmetals and include elements like silicon and arsenic.

Physical vs Chemical Properties

Physical properties are characteristics of matter that can be observed or measured without changing the substance’s composition.

Chemical properties describe how an element behaves in a chemical reaction, including:

• Reactivity
• Electronegativity
• Ionisation Energy

This is discussed separately here.

Appearance (Lustre and Dullness)

The appearance of an element, particularly its lustre or dullness, is a noticeable physical property that contributes to its identification and applications.

 

 

• Metals are characterised by their shiny appearance, known as metallic lustre. This lustre is due to the ability of the free electrons within the metal to absorb and re-emit light, resulting in a surface that reflects light, producing the characteristic shine. While most metals are lustrous, they can become dull when oxidised or corroded, losing their shiny appearance due to surface reactions with oxygen or other chemicals in the environment.

• Non-metals generally lack metallic lustre and instead have a dull appearance. Their surfaces do not reflect light in the same way metals do, making them appear matte or even transparent in the case of gases. 

• Metalloids can have a varied appearance, with some exhibiting a more metallic lustre and others being more dull. Silicon (Si), for example, has a metallic lustre, though less pronounced than true metals. Germanium is another example of metalloid that can be mistaken for a metal due to its lustrous appearance.

State of Matter

The state of matter of an element refers to whether it exists as a solid, liquid, or gas under particular conditions such as temperature and pressure. This property is a fundamental aspect of an element's physical identity.

• Most metals are solid at room temperature, known for their structural rigidity and high melting points, exceptions being mercury (Hg), which is liquid.

• Non-metals exhibit more diversity; they can be gases (e.g., nitrogen, N₂, and oxygen, O₂), liquids (e.g., bromine, Br₂), or solids (e.g., carbon, C, in the form of graphite or diamond).

• Metalloids, like silicon (Si) and arsenic (As), are solid at room temperature but have a more brittle structure compared to metals.

Melting and Boiling Points

Melting point is the temperature at which a solid becomes a liquid, and the boiling point is the temperature at which a liquid turns into a gas.

• Generally, metals have high melting and boiling points due to the strength of the metallic bonds holding their atoms together. Tungsten (W), for instance, has one of the highest melting points.

• The melting and boiling points of non-metals vary widely. Noble gases have very low boiling points, while carbon, in its diamond form, has an extremely high melting point due to the strong covalent bonds in its crystal lattice.

• The melting and boiling points of metalloids tend to be lower than those of metals but higher than those of most nonmetals, reflecting their intermediate bonding characteristics.

Density

Density is defined as mass per unit volume and is a key identifier of how compact the matter is within a substance.

 

$$\text{Density} = \frac{m}{V}$$

 

• Metals typically have high densities due to the close packing of their atoms and the presence of metallic bonding.

• Non-metals, particularly the gases, have much lower densities. Solid non-metals like sulfur (S) and phosphorus (P) have relatively low densities compared to metals.

• Metalloids have densities that are higher than nonmetals but generally lower than metals, reflecting their intermediate atomic structure.

Electrical Conductivity

Electrical conductivity refers to an element's ability to conduct electricity (movement of charges). It's a measure of how easily electrons can move through a substance.

 

 

• Metals are known for their excellent electrical conductivity. The metallic structure contains a sea of delocalised electrons surrounding metal atoms. These electrons are mobile and account for metals' relatively high electrical conductivity.

• Most non-metals are poor conductors because their electrons are more tightly bound and not free to move. Graphite, a form of carbon, is a notable exception due to its unique structure that allows electrons to move freely within layers.

• Metalloids have variable conductivity, which can often be enhanced by adding impurities in a process known as doping. Silicon (Si), for example, is a semiconductor widely used in electronic devices.

Thermal Conductivity

Thermal conductivity is a physical property that measures a material's ability to conduct heat. Heat refers to the transfer of energy from a region of high temperature to a region of low temperature. Thermal conductivity indicates how quickly heat can pass through a material when one side is heated.

 

• Metals typically exhibit high thermal conductivity, meaning they can transfer heat efficiently. This is due to the presence of free electrons within their structure, which can move freely and transfer heat rapidly across the material.

• Most non-metals have low thermal conductivity. They lack the free electrons that in metals facilitate the transfer of heat. Instead, heat transfer in nonmetals occurs through slower processes such as lattice vibrations.

• Metalloids generally have thermal conductivity values that are between those of metals and nonmetals. Their semi-metallic bonding allows some degree of heat transfer through free electrons or lattice vibrations, but not as efficiently as in metals. Common metalloids used in electronics, such as silicon (Si) and germanium (Ge), have moderate thermal conductivity. This property is essential in semiconductors, where managing heat is crucial to maintain performance and reliability.

 

Generally, materials that are good thermal conductors are also good electrical conductors. However, there are exceptions to this rule. Diamond, a non-metal, is an excellent thermal conductor but is a poor conductor of electricity.

Malleability and Ductility

Malleability is the ability of a material to deform without breaking while under compression. For instance, a malleable metal can be worked with a hammer and beaten into thin sheets.

Ductility is the property of a material that indicates how easily it will plastically deform (but not break) under tensile (pulling or stretching) stress. A ductile material can be easily drawn into shapes such as wires without fracturing.

  

 

• Most metals are both ductile and malleable. This is because the structure of most metals (repeating atoms layered on top of each other) allows for relatively easy sliding movement between them without breaking the metallic bonds in between. Most metals deform well, whether being stretched (exhibiting ductility) or being compressed (which is malleability). Generally if a metal is ductile, it is also malleable (at the same temperature). However, lead is an example of a metal that is malleable (deforms under compressive stress), but not ductile (easily fractures under tensile stress).

• Non-metals are generally brittle in their solid forms and lack the malleability and ductility of metals. When force is applied, covalent bonds between atoms are more likely to break, leading to fracture.

• Metalloids are less malleable and ductile than metals. They are more likely to break or shatter when subjected to stress, similar to nonmetals.

Tensile Strength

Tensile strength refers to the maximum amount of tensile (stretching) stress that a material can withstand before failure, such as breaking or significant deformation.

• Metals generally possess high tensile strength, attributable to the strong bonds between metal atoms and the ability of these atoms to slide past each other without losing cohesion. This property makes metals ideal for construction materials, cables, and tools. Steel, an alloy of iron (Fe), is renowned for its exceptional tensile strength and is widely used in construction and manufacturing.

• Non-metals, particularly in their solid forms, tend to have lower tensile strength compared to metals. They are more likely to shatter or break when subjected to stress because of the directional nature of covalent bonds, which are strong but can lead to brittle structures if the atoms are not arranged in a network solid, like diamond, which is an exception with high tensile strength due to its strong covalent bonding in a three-dimensional lattice.

• Metalloids have variable tensile strengths that are generally lower than those of metals but can be higher than some nonmetals. Their structure and bonding characteristics give them a balance of properties, but they are not typically used in applications where high tensile strength is a critical requirement.

 

RETURN TO MODULE 1: PROPERTIES AND STRUCTURE OF MATTER