KS3 · Chemistry
Molecular elements
Oxygen is an element — yet none of its particles is a single oxygen atom. Each is a joined pair. Helium's are single atoms. So how can you tell?
Chemistry · Molecular elements
One atom, or a joined pair?
Pick an element, then choose its group. Use the formula AND the picture — they should tell you the same thing.
Still to sort
Monatomic — single atoms (0)
'Mon' means one. Each particle is one atom on its own, and the symbol has no number after it.
Where the line is: A lone atom, not joined to anything. If the picture shows two atoms stuck together, it doesn't belong here.
Diatomic — joined pairs (0)
'Di' means two. Each particle is two atoms of the same element joined together, and the formula ends in a small 2.
Where the line is: Exactly two atoms, joined. The small 2 and the joined pair in the picture are the same fact, shown two ways.
Oxygen is an element — but its particles are not single oxygen atoms. Sort these seven non-metal elements by what their particles are actually like.
Predict, then check
Scientists draw molecules in different styles. One thing never changes.
Picture one oxygen molecule drawn three ways: as a 3D model, as two flat shaded circles, and as a quick outline sketch. They look quite different — but one feature appears in all three, and it's the feature that makes the drawing show a molecule. Which is it?
Formula and picture
Explore
Tap a part of the diagram, or a label below.
Left: one nitrogen molecule and its formula. Right: one neon atom and its formula.
WHAT YOU'VE LEARNED
A quick recap of today's lesson.
Some elements are single atoms. Others come as joined pairs — and the little 2 in O₂ tells you which.
What you need to know
- A molecule is two or more atoms joined together. This usually happens with non-metals.
- Monatomic elements exist as single atoms ('mon' means one). The noble gases, in the right-hand column of the periodic table — helium, neon, argon, krypton, xenon and radon — are monatomic.
- Diatomic elements exist as molecules of two atoms of the same element joined together ('di' means two), for example O₂, N₂, H₂ and Cl₂.
- The subscript 2 in a formula means two atoms of that element joined together. A monatomic element such as Ne or Ar has no number after its symbol.
- In a drawing of a molecule the atoms overlap or join, and the element's symbol can be written in each circle.
The big picture
Some non-metal elements exist as single atoms — they are monatomic, like the noble gases. Others exist as diatomic molecules: two atoms of the same element joined together, like O₂, N₂, H₂ and Cl₂. The subscript 2 in the formula and the joined pair in a drawing show the same thing.
Key points
Worked example
Problem
A student writes the formula for chlorine as CL2 and draws one chlorine particle as two circles with a gap between them. Find the three mistakes and fix them.
⚠ Watch out
Mixing up gaps and joins. In a drawing of a gas there are gaps between the particles, but inside a diatomic molecule the two atoms overlap. Draw a gap inside O₂ and you've drawn two separate atoms instead of one molecule.
Memory hook
Mon = one, di = two. Monatomic is a loner; diatomic is a duo — and the little 2 in O₂ is the duo written down.
Check yourself
Without looking back: what does the 2 in H₂ tell you about hydrogen, and how would you draw one hydrogen particle?
Flashcards
(14)What is a molecule?
What does monatomic mean?
What does diatomic mean?
Name the six noble gases. Are they monatomic or diatomic?
Give four diatomic elements with their formulae.
What does the subscript 2 in N₂ tell you?
Why is there no number after Ar?
How is an element symbol written?
How must the 2 in O₂ be written?
Three ways to represent a molecule?
In a drawing of a molecule, what shows the atoms are joined?
How do drawings of helium gas and oxygen gas differ?
Which two elements are liquids at room temperature?
Which elements exist as single atoms or as molecules?
Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.
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Keep me postedMore KS3 Chemistry topics
- Acids, bases and alkalis
- Boiling and condensing
- Changes of state: energy and evaporation
- Characteristics of chemical reactions
- Chemical formulae and symbols
- Chromatography
- Combustion
- Composition of the atmosphere
- Compounds and their formation
- Conservation of mass and balanced equations
- Displacement of metals
- Dissolving
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