The core idea
An element contains one kind of atom, defined by proton number. A compound contains different elements chemically combined in a fixed composition; its properties can differ greatly from those of its constituent elements.
1. An atom has structure
An atom is a basic chemical unit of an element. Its small central nucleus contains positively charged protons and, for most atoms, uncharged neutrons. Negatively charged electrons occupy the region around the nucleus. A neutral atom has equal numbers of protons and electrons. Proton number, called atomic number, identifies the element: every carbon atom has six protons. Changing electron number makes an ion rather than a different element. Atoms of the same element can have different neutron numbers; these are isotopes. The familiar solar-system sketch is only a model: electrons do not follow the neat planetary tracks shown in many school drawings.
Sources: NCERT via IIT Kanpur SATHEE: Atoms and Molecules ↗ · NCERT Exploration: Journey Inside the Atom ↗
2. Element does not mean single atom
An element is a pure substance whose atoms all have the same atomic number. A sample can contain an enormous number of those atoms. Some elements occur as separate atoms, while others occur as molecules: oxygen gas commonly contains O₂ molecules, each with two oxygen atoms. O₂ remains an element because only oxygen atoms are present. A molecule is a discrete group of bonded atoms; it can belong to an element or a compound. A metal such as copper forms an extended structure rather than separate Cu₂ molecules. Therefore “element”, “atom” and “molecule” answer different questions about a material.
3. Combining changes chemical identity
Water is a compound: each water molecule contains hydrogen and oxygen in a two-to-one atom ratio. Those atoms are chemically bonded, so a filter cannot separate the hydrogen from the oxygen. Sodium chloride is also a compound, but its solid structure is a repeating arrangement of sodium and chloride ions. NaCl gives their simplest one-to-one ratio, called a formula unit; it does not describe a separate little molecule in the crystal. A mixture differs because substances are together without becoming a single fixed-composition substance. Air, for example, contains several gases whose proportions can vary while each gas retains its chemical identity.
Sources: NCERT via IIT Kanpur SATHEE: Atoms and Molecules ↗ · NIOS: Matter in Our Surroundings ↗
4. Read one symbol at a time
Chemical symbols are case sensitive. C means carbon, O oxygen, H hydrogen and Na sodium. Co means cobalt, whereas CO is carbon monoxide containing carbon and oxygen. A subscript belongs to the symbol or bracket immediately before it: H₂O contains two H atoms and one O atom. No written subscript means one. In Ca(OH)₂ the outside 2 multiplies both atoms inside the brackets, giving one Ca, two O and two H. A coefficient before a formula multiplies the entire formula. Thus 3H₂O represents three water molecules in particle counting, not a new substance with a changed formula.
Read the chemical formula
| Formula | What one molecule contains | Classification |
|---|---|---|
| O₂ | 2 oxygen atoms | Element |
| H₂O | 2 hydrogen + 1 oxygen | Compound |
| CO₂ | 1 carbon + 2 oxygen | Compound |
5. Counting atoms is not weighing them
Atoms of different elements have different masses. The unified atomic mass unit, written u, is defined using one-twelfth of the mass of a carbon-12 atom. For introductory calculations we often round hydrogen to 1 u, carbon to 12 u and oxygen to 16 u. These are stated approximations, not gram masses of individual atoms. Molecular mass is obtained by adding the masses of the atoms in one molecule. Relative atomic mass is a ratio and has no unit; an atomic mass expressed in u does have a unit. Keep that distinction clear rather than attaching grams to every chemical number.
6. Worked example: count a collection
Illustrative counting problem: a drawing shows four CO₂ molecules and three H₂O molecules. Each CO₂ contributes one carbon and two oxygen atoms, so the first group has four carbon and eight oxygen atoms. Each H₂O contributes two hydrogen and one oxygen atom, so the second group has six hydrogen and three oxygen atoms. Totals are C = 4, H = 6, O = 11; there are 21 atoms in seven molecules. The whole collection is a mixture of two compounds. Adding their atom totals does not turn it into a new compound with a formula made by joining all those numbers together.
Sources: NCERT via IIT Kanpur SATHEE: Atoms and Molecules ↗ · NIOS: Matter in Our Surroundings ↗
7. Worked example: a mass ratio inside water
Using the stated rounded masses, one H₂O molecule has mass (2 × 1) + 16 = 18 u. Hydrogen contributes 2 u and oxygen 16 u, so their mass ratio is 2:16, simplified to 1:8. The atom ratio was 2:1; these ratios differ because an oxygen atom is much heavier than a hydrogen atom. In an illustrative 90 g sample of pure water, the corresponding rounded mass shares are 90 × 2/18 = 10 g hydrogen and 90 × 16/18 = 80 g oxygen. This describes elemental composition, not separate hydrogen and oxygen gases floating inside the water.
PUT IT INTO PRACTICE
Apply your understanding
- For an illustrative diagram containing five N₂ molecules and two CO₂ molecules, count molecules, atoms of each element, and all atoms. N is nitrogen.
- Classify each kind of molecule and then the whole collection. Explain why two atoms in N₂ do not make it a compound.
- Check: seven molecules; ten N, two C and four O atoms, sixteen atoms altogether. N₂ is elemental, CO₂ is a compound, and the collection is a mixture.
Check your understanding
Why does losing an electron not turn sodium into another element?
Element identity depends on proton number, which has not changed. Electron loss changes electrical charge, producing a positive ion with different behaviour.
Is every molecule a compound?
No. O₂ contains only one element. H₂O contains two elements chemically bonded, so it is a compound. Count kinds of elements, not merely atoms.
Why can H₂O not be changed to H₂O₂ just to improve a calculation?
Subscripts define composition. H₂O₂ is hydrogen peroxide, a different substance. Changing a coefficient alters amount; changing a subscript can alter identity.
Why is oxygen the larger mass share in water despite fewer oxygen atoms?
With the rounded values, one oxygen atom contributes 16 u while two hydrogen atoms contribute only 2 u together. Number share and mass share are different.
Can a filter extract the oxygen element from pure water?
No. The atoms are chemically combined within molecules. Filtration separates suitable particles in mixtures, not bonded elements within a compound.