Properties: Characteristics of Elements
The Fundamental Identity of an Element
At the heart of every element is the atom. What makes one element different from another is the number of protons in the nucleus of its atoms. This number is called the atomic number. For example, every single atom of hydrogen has 1 proton, so its atomic number is 1. Every atom of carbon has 6 protons, so its atomic number is 6. This number is the element's unique fingerprint; it never changes.
The total number of protons and neutrons in an atom is called the mass number. The mass of an electron is so small that it is generally ignored in this calculation. The average mass of all the naturally occurring atoms of an element is its atomic mass. You can find both the atomic number and atomic mass on the periodic table.
Physical Properties: Observing Without Changing
Physical properties are characteristics that scientists can measure or observe without changing the composition of the matter. In other words, you are not turning the element into a different substance when you measure these. Think about an ice cube melting; it changes from solid to liquid water, but it's still H_2O. That's a physical change.
| Property | Description | Example |
|---|---|---|
| State of Matter | Whether the element is a solid, liquid, or gas at room temperature (20°C). | Iron is a solid, mercury is a liquid, neon is a gas. |
| Density | The mass of a substance per unit volume ($ D = \frac{m}{V} $). | Lead is very dense (11.3 g/cm³), lithium is not (0.53 g/cm³). |
| Melting & Boiling Point | The temperature at which a solid becomes a liquid and a liquid becomes a gas. | Tungsten melts at 3422°C, helium boils at -269°C. |
| Luster | How shiny a material is when light reflects off its surface. | Gold and silver are very lustrous (metallic luster). Sulfur is dull. |
| Malleability & Ductility | Ability to be hammered into thin sheets (malleable) or drawn into wires (ductile). | Gold is the most malleable metal. Iodine crystals are brittle. |
| Electrical & Thermal Conductivity | Ability to conduct electricity and heat. | Copper is excellent for both. Sulfur is an insulator. |
Chemical Properties: The Behavior in Reactions
Chemical properties describe the ability of a substance to undergo changes that transform it into different substances. You can only observe a chemical property when a chemical change (a reaction) occurs. Burning, rusting, and tarnishing are all examples of chemical changes where the original substance is fundamentally altered.
| Property | Description | Example |
|---|---|---|
| Reactivity | The readiness with which an element combines with another substance. | Sodium reacts violently with water. Gold is very unreactive. |
| Flammability | The ability of a substance to burn in the presence of oxygen. | Carbon (as coal) is flammable. Helium is not flammable. |
| Toxicity | The degree to which a substance can damage an organism. | Mercury and lead are toxic. Nitrogen gas is non-toxic. |
| Stability | The tendency of an element not to undergo chemical change. | Noble gases like argon are very stable. Francium is radioactive and unstable. |
| Oxidation States | The possible charges an atom can have when it forms ions in compounds. | Iron can be +2 (Fe²⊕) or +3 (Fe³⊕). Sodium is almost always +1 (Na⊕). |
How Properties Relate to the Periodic Table
The Periodic Table of Elements is not just a random list; it is a powerful map that organizes elements based on their properties. Elements in the same vertical column, called a group or family, have similar chemical properties because they have the same number of electrons in their outer shell.
For instance, all elements in Group 1 (the alkali metals[1]) are soft, have low densities, and are extremely reactive with water. As you move down the group, the reactivity increases. In contrast, the elements in Group 18 (the noble gases[2]) are all colorless, odorless gases that are famously unreactive. This periodic, or repeating, pattern of properties is what gives the table its name.
Elements in Action: From Labs to Daily Life
We use the properties of elements every single day, often without realizing it. The aluminum foil in your kitchen is used because aluminum is malleable (can be rolled into thin sheets), has a high melting point (won't melt in the oven), and is relatively unreactive (won't tarnish or react with most foods). The helium in party balloons is used because it is less dense than air (so the balloon floats) and is non-flammable (unlike hydrogen, which is also light but dangerous). Copper is used in electrical wiring because of its excellent conductivity and ductility (it can be pulled into long, thin wires).
A more dramatic example is the use of titanium in artificial hip joints and aircraft. Titanium is chosen because it is strong, has a low density (it's light), and is highly resistant to corrosion (it doesn't rust easily). These physical and chemical properties make it ideal for these high-performance applications.
Common Mistakes and Important Questions
Q: Is the ability to rust a physical or chemical property?
Q: Can a property be both physical and chemical?
Q: Why is gold found as a pure metal in nature, but sodium is not?
Footnote
[1] Alkali Metals: The elements found in Group 1 of the periodic table (Lithium, Sodium, Potassium, etc.). They are soft, shiny metals that are highly reactive.
[2] Noble Gases: The elements found in Group 18 of the periodic table (Helium, Neon, Argon, etc.). They are inert, meaning they are very stable and rarely form chemical compounds.