Chemistry · Some Basic Concepts Of Chemistry · NEET
| Type of atoms | Only one type of atom | Two or more different elements |
| Composition | Cannot be broken into simpler substances | Can be broken into its elements chemically |
| Ratio | No ratio needed (single element) | Elements in a fixed, definite ratio |
| Properties | Its own basic properties | New properties, different from its elements |
| Examples | Na, Cu, O2, N2, S8 | H2O, CO2, NaCl, CH4 |
Oxygen gas is an ELEMENT, not a compound. Even though O2 is made of two atoms joined together, both atoms are the SAME type (oxygen). An element can exist as single atoms or as molecules of the same atom. A compound needs atoms of DIFFERENT elements. So O2 = element, but H2O (hydrogen + oxygen) = compound.
Water is a COMPOUND. It is made of two different elements: hydrogen and oxygen, joined in a fixed ratio of 2:1 (H2O). You can split water into hydrogen gas and oxygen gas by passing electricity (electrolysis), which proves it is not a single element. Any substance made of two or more different elements in a fixed ratio is a compound.
No. An element CANNOT be broken into anything simpler by ordinary chemical methods, because it already has only one type of atom. A compound CAN be broken down into its elements by chemical means. This is one of the cleanest ways to tell them apart: if a chemical reaction can split it, it is a compound; if not, it is an element.
A compound has its elements joined in a FIXED ratio by chemical bonds, and its properties are totally different from the elements inside it (example: sodium metal + poisonous chlorine gas make edible salt, NaCl). A mixture has substances mixed in ANY ratio with no chemical bonding, and each keeps its own properties. A compound needs a chemical reaction to separate; a mixture can be separated by physical methods like filtering.
No, and this is a favourite NEET point. A compound shows NEW properties that are different from its elements. For example, water (a liquid that puts out fire) is made from hydrogen (which burns) and oxygen (which supports burning). Because bonds form, the compound behaves like a brand-new substance, not like a simple mix of its parts.
Because of the Law of Definite Proportions (Proust's law): a given compound always contains the same elements combined in the same fixed ratio by mass, no matter the source. Water is always 2 hydrogen : 1 oxygen. If the ratio can change freely, it is a mixture, not a compound. This fixed-ratio rule is what defines a true compound.
Try the real previous-year questions from this chapter — each with the answer and a full solution.
An element is a pure substance made of only one type of atom that cannot be broken into anything simpler (example: gold, oxygen). A compound is a pure substance made of two or more different elements joined in a fixed ratio (example: water H2O, salt NaCl).
Yes. Both elements and compounds are classified as PURE SUBSTANCES because each has a fixed, definite composition. Pure substances split into these two groups: elements and compounds. Mixtures are the other category, which are NOT pure substances.
Elements: sodium (Na), copper (Cu), oxygen (O2). Compounds: water (H2O), carbon dioxide (CO2), sodium chloride (NaCl). Remember the compound examples always contain symbols of two or more different elements.
There are about 118 known elements, but they combine to form millions of compounds. This is why NCERT says chemistry is the science of the infinite variety of molecules built from about one hundred elements.
NEET tests it inside 'classification of matter' and in tricky traps like 'is O2 a compound?'. Getting the element vs compound and fixed-ratio idea right also builds the base for the Law of Definite Proportions and molecular formula questions later in the chapter.
