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Revision from Grade 8
Definitions: Element: Consists of one kind of atom only Compound: Consists of 2 or more different kinds of elements (as listed on the Periodic Table) chemically bonded together Question 1: Identify the following as an element or a compound: CH 4 NH 3 O 2 H 2 SO 4 KMnO 4 N 2 HCl NaOH Ne Si Question 2: Identify the following as a diatomic molecule or a compound: CH 4 NH 3 O 2 H 2 SO 4 KMnO 4 N 2 HCl NaOH By looking at Question 1 and Question 2’s answers, can you see that that O 2 and N 2 (and every other diatomic molecule) can also be called a diatomic element because it consists of two (di) atoms of the same kind (the definition of an element).
The names of ELEMENTS are abbreviated by using
SYMBOLS.
Examples: Neon’s symbol is:
Ne
This means there is one neon ATOM as there is no number as a subscript on the right Drawing:
Si
This means there is one silicon ATOM as there is no number as a subscript on the right Drawing:
O
This means there is one oxygen ATOM as there is no number as a subscript on the right Drawing:
The names of COMPOUNDS are abbreviated by using a
FORMULA. The plural of a formula is many formulae
Examples:
NO 2 :
This means that there is one nitrogen atom chemically bonded together with two oxygen atoms (because of the subscript two on the right of oxygen) It also means that in total there are three atoms and for every one nitrogen atom that I count there are two oxygen atoms. (Nitrogen and oxygen are in a ratio 1:2) It also means that there is one molecule of the compound nitrogen dioxide (because there is no number in front of the whole formula) Drawing:
2H 2 SO 4 :
This means that there are two hydrogen atoms chemically bonded together with one sulphur atom and four oxygen atoms (because of the subscripts on the right) in one molecule of sulphuric acid. It also means that there are twelve atoms in total: (2x2)+(2x1)+(2x4). For every four hydrogen atoms I count, there are two sulfur atoms and eight oxygen atoms, so they are in a ratio of 4:2:8. Simplified this ratio is 2.1.4. Therefore, in each molecule of sulphuric acid the ratio of hydrogen:sulphur:oxygen is 2.1.4. It also means that there are two molecules of the compound sulphuric acid (because of the number two in front). Diagram:
When drawings of an atom or diagrams of certain formulae are asked for in “Revision of Grade 8 ONE and TWO” please use the following key: KEY: Ne Si O N Cl H S C Mg K Mn Cu Ca F Example: Ne This means there is one neon ATOM as there is no number as a subscript on the right Drawing:
