Physical Properties of Elements for HSC Chemistry

You are given three grey solids with the labels removed. One is aluminium, one is silicon, and one is sulfur. You are not allowed to identify them by reacting them with anything. You can only look at them, bend them, test whether they conduct electricity, and compare other physical properties.

Could you still work out which is which?

Probably. But the useful skill is not memorising that “metals are shiny”. Plenty of things are shiny. The real skill is combining several physical properties, then using the element’s likely position in the periodic table to check whether your classification makes sense.

01What counts as a physical property?

A physical property is something you can observe or measure without changing the substance into a different chemical substance.

For an element, useful physical properties include:

• state at room temperature: solid, liquid, or gas
• colour and appearance
• lustre, meaning how shiny the surface is
• electrical conductivity
• thermal conductivity
• malleability, meaning whether it can be hammered or pressed into a new shape
• ductility, meaning whether it can be drawn into a wire
• brittleness
• density
• melting point
• boiling point

Suppose you bend a piece of copper wire. It is still copper, so you have investigated a physical property.

If you burn copper and form a new copper compound, you are investigating chemical behaviour instead.

That distinction matters because physical properties let us classify a substance without requiring a chemical reaction.

02Start with the broad pattern in the periodic table

Before thinking about individual properties, picture the periodic table.

The broad pattern is:

Hydrogen is an important exception. It sits on the left because of its electron configuration, but it is a non-metal.

You can think of the periodic table as a map of neighbourhoods. Knowing someone’s suburb does not tell you their exact house, but it gives you useful information about where to look. In the same way, knowing where an element sits gives you a strong first prediction about its physical properties.

The analogy stops there. Elements do not have properties because of a label on the periodic table. Their properties come from their atomic structure and bonding.

03What would you predict for a metal?

Imagine an unknown element that is shiny, can be flattened without shattering, and conducts electricity well.

Before reading on, what would you predict?

It is probably a metal.

That conclusion becomes much stronger because three different properties point in the same direction.

Typical physical properties of metals

Most metals are:

• solid at room temperature
• lustrous when freshly cut or polished
• good electrical conductors
• good thermal conductors
• malleable
• ductile

Many metals also have relatively high densities and melting points, although those properties vary considerably.

The reason metals conduct electricity is especially important.

In a metal, some outer electrons are delocalised. They are not fixed to one particular atom. When a potential difference is applied, these charged particles can move through the metallic structure and carry electrical charge.

That is why conductivity is usually a much stronger clue than shininess alone.

But the first model has exceptions

“Metals are solid” is useful, but it is not completely true.

Mercury, \(\ce{Hg}\), is a liquid at room temperature.

“Metals have high melting points” is also too simple. Some metals melt at relatively low temperatures.

And “metals are dense” is not reliable by itself. Lithium, sodium, and potassium are low-density metals.

The better rule is:

Classify an element using a pattern of properties, not one property in isolation.

Check your understanding

An unknown element is silver-grey, conducts electricity strongly, and can be hammered into a thin sheet without breaking. Is it most likely a metal, non-metal, or metalloid?

04What makes non-metals different?

Now consider another unknown element.

It is a brittle solid. It does not conduct electricity under ordinary conditions. When struck, it cracks rather than flattening.

What would you predict?

A non-metal is the most likely answer.

Typical physical properties of non-metals

Non-metals commonly:

• conduct electricity poorly
• conduct heat poorly
• are brittle when solid
• are not malleable
• are not ductile

Their physical states are much more varied than those of metals.

At room temperature, non-metals include:

• gases such as oxygen, \(\ce{O2}\), and nitrogen, \(\ce{N2}\)
• solids such as carbon, \(\ce{C}\), and sulfur, \(\ce{S}\)
• one familiar liquid element, bromine, \(\ce{Br2}\)

This variety is one reason “state at room temperature” is not enough to classify an element.

A tempting misconception: all non-metals are dull insulators

That is a useful beginner’s pattern, but it is not an exact rule.

Graphite is made only from carbon, which is a non-metal, yet graphite conducts electricity.

Why?

Its carbon atoms are arranged so that some electrons can move through the structure.

So a conducting substance is not automatically a metal. You still need to combine its conductivity with other evidence and, where possible, its location in the periodic table.

Check your understanding

Element X is a yellow, brittle solid and conducts electricity very poorly. Where would you expect it to lie in the periodic table?

05Metalloids sit near the boundary

The division between metals and non-metals is not perfectly sharp.

Near the zig-zag boundary are elements commonly classified as metalloids. These have some properties associated with metals and some associated with non-metals.

Silicon is the most important example for HSC Chemistry.

Silicon is:

• solid at room temperature
• brittle rather than malleable
• capable of having a shiny surface
• a semiconductor

A semiconductor conducts electricity better than a typical non-metal but not as well as a typical metal under the same conditions. Its conductivity can also be deliberately controlled, which is one reason silicon is so important in electronics.

This is a good example of why appearance can mislead you. Silicon can look metallic, but its brittleness and electrical behaviour tell a more complete story.

06How position in the periodic table helps you predict properties

The periodic table does more than separate metals from non-metals.

There is a broad trend in metallic character, meaning how strongly an element behaves like a metal.

Across a period from left to right, metallic character generally decreases.

Down a group, metallic character generally increases.

So the overall direction is:

• towards the bottom-left: increasingly metallic behaviour
• towards the top-right: increasingly non-metallic behaviour

Why?

Across a period, the nucleus generally attracts the outer electrons more strongly. Those electrons become harder to remove, so the tendency to show metallic behaviour decreases.

Down a group, the outer electrons are farther from the nucleus and more shielded by inner electrons. They are generally easier to remove, which supports more metallic behaviour.

This is a useful explanation of the overall pattern. It does not mean every physical property changes smoothly across every row and column.

Melting point, density, hardness, and conductivity depend on the detailed structure and bonding of each element, so their trends can be much less simple.

07Do not invent a trend where there isn’t one

Students sometimes learn one periodic trend and then try to apply it to everything.

For example:

“Elements farther left have higher melting points because they are more metallic.”

That is not a valid general rule.

Consider Period 3. Its elements include sodium, magnesium, aluminium, silicon, phosphorus, sulfur, chlorine, and argon. Their structures are very different.

Some form giant metallic lattices. Silicon forms a giant covalent network. Several non-metals exist as small molecules or individual atoms.

Because melting depends on the forces that must be overcome, the melting points do not simply rise or fall according to metallic character.

So use position in the periodic table to predict broad classifications and broad behaviour, but use bonding and structure when explaining a particular physical property in detail.

Check your understanding

A student says, “Silicon is close to aluminium in the periodic table, so their physical properties should be almost the same.”

What is wrong with this reasoning?

09A practical decision rule for HSC questions

When a question gives you physical properties and asks you to classify an element, work in this order.

1. Check electrical conductivity. High conductivity strongly supports a metal. Intermediate semiconductor behaviour can suggest a metalloid.
2. Check malleability or brittleness. Malleability supports a metal. Brittleness supports a non-metal or metalloid.
3. Use appearance as supporting evidence. Lustre is useful, but not decisive.
4. Check state at room temperature. This can help, but remember mercury and bromine.
5. Use periodic table position to confirm your conclusion. Left and centre suggest metals, the zig-zag region suggests metalloids, and the upper right suggests non-metals.
6. Mention exceptions when they matter. Do not weaken a strong answer by pretending every element follows an oversimplified rule.

That gives you a much stronger answer than listing memorised properties without explaining how they connect.

10Physical properties become useful beyond classification

The same physical properties used to classify elements can also help you decide how substances can be separated.

Differences in boiling point, solubility, particle size, magnetism, and other physical properties allow mixtures to be separated without necessarily changing the substances chemically. If you are moving on to that topic, see How to Choose Separation Techniques in HSC Chemistry.

The next important step is to explain why elements have these physical properties. That takes you from recognising that aluminium conducts and silicon is brittle to explaining those observations using metallic bonding, covalent structures, electron movement, and the arrangement of atoms in solids.