Elements, Compounds, Symbols and Formulae
ICSE Class 6 · Chemistry · Chapter 4
What this chapter covers
- Elements: made of one kind of atom
- Symbols of elements and where they come from
- Names and symbols of the first 20 elements
- Molecules of elements
- Compounds: fixed proportion and new properties
- Molecules and formulae of compounds
Key definitions
An atom is the smallest particle of an element.
An element is a pure substance that is made up of only one kind of atom.
A symbol is a short way of writing the name of an element, using one or two letters.
A molecule is the smallest particle of a substance that can exist on its own and still has the properties of that substance.
A compound is a pure substance formed when two or more elements join together chemically in a fixed proportion.
A formula is a short way of writing a molecule of a substance using the symbols of its elements and small numbers.
Key points
- An atom is the smallest particle of an element.
- An element is a pure substance made of only one kind of atom, such as iron, oxygen or carbon.
- A symbol is a one- or two-letter short form for the name of an element; it also stands for one atom.
- Symbols come from the first letter, the first letter with another letter of the name, or the Latin name (Na, K, Fe).
- The first letter of a symbol is always a capital and the second is always small.
- The first 20 elements, in order, are H, He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, K and Ca.
- A molecule is the smallest particle of a substance that can exist on its own.
- Molecules of elements contain atoms of one element only, such as O2, N2 and H2.
- A compound forms when two or more elements join chemically in a fixed proportion.
- A compound has new properties, different from those of its elements.
- A formula shows a molecule using symbols and small numbers, such as H2O, CO2, NO2, CaO and ZnCl2.
Questions answered
What is an element? Give two examples.
An element is a pure substance made up of only one kind of atom. Iron and oxygen are elements.
Write the symbols of sodium, chlorine, magnesium and potassium.
Na, Cl, Mg and K.
Why is the symbol of sodium Na and not S?
S is already the symbol of sulphur. Sodium's symbol comes from its Latin name, natrium.
A student wrote the symbol of calcium as CA. What mistake did the student make?
The second letter of a symbol must be a small letter. The correct symbol is Ca.
What does N2 stand for? Is nitrogen an element or a compound?
N2 stands for one molecule of nitrogen, made of two nitrogen atoms. Nitrogen is an element, because its molecule has atoms of only one kind.
What is a compound? Name one compound and the elements it is made of.
A compound is a pure substance formed when two or more elements join chemically in a fixed proportion. Water is a compound of hydrogen and oxygen.
Hydrogen burns and oxygen helps burning, yet water puts out fire. What does this tell you about compounds?
A compound has new properties. The properties of the elements it is made from are lost.
Name the elements in CaO and count the atoms of each in one molecule. How many oxygen atoms are there in 4 molecules of CO2?
CaO has 1 calcium atom and 1 oxygen atom. Each CO2 molecule has 2 oxygen atoms, so 4 molecules have 4 × 2 = 8 oxygen atoms.