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:

Physical Properties of Metals, Semi-metals and Non-metals Explained

This video explains the similarities and differences in physical properties amongst the different elements in the Periodic Table.

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

Physical vs Chemical Properties

Physical properties are characteristics of matter that can be observed or measured without changing the substance’s composition or identity. For example, melting and boiling points of a substance are temperatures at which a substance changes between states of matter. When a substance melts, it transitions from its solid state to liquid state without altering its chemical composition. 

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

• Reactivity
• Electronegativity
• Ionisation Energy

This is discussed separately here.

Both physical and chemical properties depend on the chemical structure of an element or compound. The chemical structure of an element depends on its position on the periodic table.

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 and a continuous network called metallic lattice. The electrons of these atoms are delocalised instead of fixed in an atom's orbit.

• 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, germanium and arsenic. Atoms of semi-metal elements are held together by covalent bonds to form a continuous network, similar to covalent network structures of non-metal elements.

• 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 in detail separately here. The physical properties of non-metal elements vary greatly depending on the specific chemical structure it occupies.

 

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.

Metals

Metals are mostly solids at room temperature. Metals have variable melting points.

For example, Tungsten (W), has one of the highest melting points (3400 ºC) whereas Mercury (Hg), has one of the lowest melting points for metals at –38 ºC). Mercury is actually a liquid at room temperature.

Non-metals

Most non-metals are gases at room temperature and pressure. However, the melting and boiling points of non-metals vary widely.

Non-metals with covalent molecular structures are held together by weaker intermolecular forces and therefore have relatively low melting and boiling points. Hydrogen exists as diatomic hydrogen molecules which have very weak intermolecular forces. The melting point of hydrogen molecules is –260 ºC.

In contrast, non-metal elements with covalent network structures are held together by stronger covalent bonds, and therefore have relatively high melting and boiling points.

Carbon, in its diamond form, has an extremely high melting point due to the strong covalent bonds in its crystal lattice.

Semi-metals

The melting and boiling points of metalloids tend to be lower than most metals but higher than those of most non-metals in covalent molecular structures, reflecting their intermediate bonding characteristics. Semi-metals tend to have consistently high melting and boiling points. The melting point of Germanium is 938 ºC.

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.

Density

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

 

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

 

Density is heavily influenced by temperature, pressure as well as the allotropic form of the element. The former two affect the state of matter of an element. Solids have higher density than liquids which in turn have higher density than gases.

 

 

• Metals have variable densities but collectively tend to have higher densities than semi-metals and non-metals. For example, Tungsten (W) has a very high density of 19.3 g/cm³ whereas aluminium has a lower density of 2.6 g/cm³.

• Non-metals have variable densities. Non-metals in covalent molecular structures have lower densities, whereas those in network structures have higher densities.  For example, Hydrogen gas has a very lower density of 0.09 g/cm³ whereas diamond (allotrope of carbon) has much higher density of 3.5 g/cm³.

• Semi-metals tend to have consistently densities in between the high end of metals and majority of non-metals. For example, germanium's density is 5.2 g/cm³.

Electrical Conductivity

Electrical conductivity refers to an element's ability to conduct electricity (movement of charges). The main type of charges that can move freely in an electrical conductor are electrons. Therefore, electrical conductivity is a measure of how easily electrons can move through a substance. This is also a physical property as movement of electrons does not alter the chemical composition of the 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 there is a temperature difference between its two ends. 

Energy is transferred is the form of either vibration of atoms or movement of charges. 

Generally, elements that are good electrical conductors are also good conductors of heat because mobile electrons can facilitate transfer of energy (heat). However, the converse is not true, not all good thermal conductors are good electrical conductors. Diamond, a non-metal, is an excellent thermal conductor but is a poor conductor of electricity.

 

 

• 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 non-metals occurs through slower processes such as lattice vibrations. Again, graphite is a notable exception to this as its mobile electrons make it a relatively good thermal conductor.
• 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.

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

Metals generally possess high tensile strength, attributable to the strong network between positive metal atoms and surrounding negative electrons. When layers of metal atoms slide past each other due to an external tensile stress, the attraction between atoms and electrons is maintained, hence the metal is able to withstand the stress without deformation & fracture.

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.

While some metals with extremely high tensile strengths e.g. Tungsten, they are not used as commonly due to their relatively low strength to mass ratio. Titanium (Ti) has the highest strength to mass ratio, and is used in a wide range of applications in aerospace engineering, medicine, jewellery and sports equipment.

Semi-metals

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.

Non-metals

Non-metals, particularly in their solid forms, are very brittle and 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.

Hardness

Hardness is a measure of how resistant a material is to deformation, scratching, or indentation. It is determined by the strength of the bonds between atoms and the overall structure of the material. Hardness can be classified into different types:

• Mohs hardness: Measures scratch resistance on a scale from 1 (talc) to 10 (diamond). Note that this is an ordinal scale, meaning the assigned values do not depict the absolute difference in hardness between materials.

• Brinell and Vickers hardness: Used for metals, measuring indentation resistance under a specific load.

 

 

The hardness of elements varies significantly based on their chemical structures and bonding types. 

• Metals have variable hardness. For example, Tungsten has a hardness on Mohs scale of 7.5, whereas Lithium has a hardness of 0.6.

• Semi-metals have consistently high hardness. The Mohs hardness of silicon and germanium are 7 and 6 respectively.

• Non-metals have variable hardness. Notably, diamond has a very high hardness of 10 on Mohs scale. This is attributed to the strong covalent bonds in the lattice structure of diamond. In contrast, the Mohs hardness of sulfur solids, which occupies a covalent molecular structure, is 2.

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.

  

 

Metals

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 attraction between the metallic atoms and surrounding electrons.

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).

Semi-metals

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

Non-metals

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.

Solubility

Solubility of elements depends on their chemical structure as well as the nature of solvent tested i.e. whether it is polar (water) or non-polar (oil). Polar solutes dissolve in polar solvents; non-polar solutes dissolve in non-polar solvents.

In general:

• Metals and semi-metals are insoluble in both polar and non-polar solvents

• Covalent network substances are insoluble in both polar and non-polar solvents

• The solubility of covalent molecules varies depending on the polarity of the molecule. Non-metals which form covalent molecules tend to be non-polar which means they have limited solubility in polar solvents like water, but dissolve better in non-polar solvents like oil.

 

RETURN TO MODULE 1: PROPERTIES AND STRUCTURE OF MATTER