ChemistryAtoms Are the Building Blocks and Elements Are Their Types
Atlas stands inside a shimmering model of a copper wire, surrounded by billions of tiny glowing orange spheres — all identical — floating and vibrating in neat patterns, while holding one sphere up to the light with a magnifying glass and grinning with discovery.
- Explain what an atom is and why it is called the smallest unit of an element.
- Identify that each element is made of only one kind of atom with its own unique identity.
- Compare two different elements by describing how their atoms differ from each other.
- Predict what would happen to an element's identity if its atoms were changed into a different type.
Key terms
- Atom
- The smallest unit of an element that still has the properties of that element.
- Element
- A pure substance made of only one kind of atom.
- Proton
- A positively charged particle in the nucleus whose count defines the element.
- Atomic number
- The number of protons in an atom, the fingerprint that identifies its element.
- Mixture
- A material containing two or more different kinds of atoms combined together.
One Element, One Kind of Atom
A pure element is built from a single type of atom and nothing else. A bar of pure gold contains only gold atoms, and a balloon of pure helium contains only helium atoms. The instant you add even one atom of a different element, the sample stops being a pure element and becomes a mixture or a compound. This 'one kind of atom' test is the cleanest way to decide whether something is a pure element.
Protons Give Each Element Its Identity
What actually makes a gold atom different from a copper atom is the number of protons in its nucleus. Copper atoms always have 29 protons; gold atoms always have 79. This proton count, called the atomic number, never changes during ordinary chemical processes, which is why an element keeps its identity. Two metals can look similar yet still be completely different elements because their proton counts differ.
Worked examples
Is a steel nail a single element or a mixture? Explain using atoms.
- Recall that a pure element contains only one kind of atom.
- Steel is made of iron atoms with carbon atoms blended in.
- Because two different kinds of atoms are present, the 'one kind of atom' test fails.
Answer: Steel is a mixture, because it contains both iron atoms and carbon atoms rather than only one kind.
An atom is identified as having 29 protons. Which element is it?
- The number of protons equals the atomic number, which names the element.
- Look up atomic number 29 on the periodic table.
- Atomic number 29 corresponds to copper (symbol Cu).
Answer: It is copper, because 29 protons always means a copper atom.
Activity
Sort each sample card into the correct bin — 'Single Element' or 'Not a Single Element' — based on what its atoms look like.
Practice
Decide whether a jar of pure neon gas is a single element or a mixture, and justify it with atoms.
Explain why two atoms with different proton counts must be two different elements.
Common mistakes to avoid
- All metal atoms are basically the same.Each metal is a different element with its own proton count, so copper and gold atoms are not identical.
- Atoms slowly turn into other elements on their own.An element's proton count is fixed, so its atoms do not change type during ordinary chemical processes.
Check your understanding
A student says: 'I have a pure sample of sulfur.' Which statement best describes what is inside that sample?
Which of the following is the BEST definition of an atom?
Two students are arguing. Student A says copper and gold are made of the same kind of atom. Student B says they are made of different kinds of atoms. Who is correct, and why?
Recap
An atom is the smallest unit of an element, and a pure element contains only one kind of atom. The number of protons, called the atomic number, is each element's unchanging fingerprint and the reason elements keep their identity.
Reflect
How would you use the proton count to prove two samples are different elements?