4. the smallest particle of a substance that can exist alone and still have the chemical properties of that substance. Example: water the smallest part of an element, which possesses the properties of that element.

5. Example 1: water One water molecule consists of one oxygen atom and two hydrogen atoms.

6. One carbon dioxide molecule consists of one carbon atom and two oxygen atoms. Example 2: carbon dioxide

7. One ammonia molecule consists of one nitrogen atom and three hydrogen atoms. Example 3: ammonia

8. One argon molecule consists of one argon atom only. Example 4: argon

9. Most metals are composed of atoms as shown.

11.  2.1What are elements? An element is a substance which are made up of only one kind of atoms. Example oxygen and hydrogen A model of hydrogen molecules: every hydrogen molecule is made up of two hydrogen atoms. A model of oxygen molecules: every oxygen molecule is made up of two oxygen atoms.

12.  … about elements Altogether, 109 different elements are known.

13. 109 kinds of atoms are found. There are 109 elements Name of element = name of the atom

14. Most elements at room temperature are solids Hg Br liquids NOF Cl He Ne Ae Kr Xe Rn Gas

15. NOF Cl He Ne Ae Kr Xe Rn Gas

16. NOF Cl He Ne Ae Kr Xe Rn Gas noble gases

17. NOF Cl He Ne Ae Kr Xe Rn Gas fluorine and chlorine

18. NOF Cl He Ne Ae Kr Xe Rn Gas oxygen

19. NOF Cl He Ne Ae Kr Xe Rn Gas nitrogen

20. Symbols of Elements: Common metals

21. Symbols of Elements: Common non-metals

22. Symbols of Elements: Common semi- metals

24. A compound is a substance whose molecules contain more than one kind of atoms. Examples: water and carbon dioxide A model of water molecule A model of carbon dioxide molecule

26. A group of atomic symbols which represent a molecule of an element or a compound Example: O2O2 H2OH2O H 2 SO 4 The molecule contains two oxygen atoms which are chemically combined together. The molecule contains two hydrogen atoms and one oxygen atom… The molecule contains two hydrogen atoms, one sulphur atom and four oxygen atoms…

27. One molecule of phosphorus consists of four phosphorus atoms,….. P4P4 One molecule of helium contains one helium atom only. He Carbon dioxide molecule contains one carbon atom and two oxygen atoms CO 2

28. The number of atoms in a molecule is called its atomicity. Substances of monoatomic molecules: He, Ne, Ar, Kr, Xe, RnAtomicity of them is ONE. Substances of diatomic molecules: O 2, H 2, HI, HCl, etc. Atomicity of them is TWO. Substances of triatomic molecules: H 2 O, H 2 S, CO 2, etc.Atomicity of them is THREE. Molecule containing many atoms is called polyatomic molecule. e.g. S 8 Atomicity of is EIGHT.

30. Examples (molecule containing two atoms only): 1. CO 2 2. NO 4. PCl 3 3. PCl 5 Carbon dioxide Nitrogen monoxide Phosphorus pentachloride Phosphorus trichloride Name of the first element: no change

31. Examples (molecule containing two atoms only): 1. CO 2 2. NO 4. PCl 3 3. PCl 5 Carbon dioxide Nitrogen monoxide Phosphorus pentachloride Phosphorus trichloride Name of the first element: no change Name of the second element: end with -ide

32. HCl : hydrogen chloride N 2 O: dinitrogen oxide More examples HF: hydrogen fluoride SO 2 : sulphur dioxide OCl 2 : oxygen dichloride Sometimes, the prefix mono- is omitted (simple hydrogen compounds) (no. of atoms of the first element is more than one while the second has one atom only)

34. Carbon disulphide Hydrogen fluoride Sulphur hexachloride Dinitrogen pentoxide Sulphur trioxide Phosphorus tribromide Hydrogen sulphide Nitrogen monoxide Carbon tetrachloride Nitrogen dioxide CS 2 HF N2O5N2O5 SO 3 PBr 3 SCl 6 NO 2 CCl 4 NO H2SH2S

36.  … about elements 94 of the elements are obtained from the Earth’s crust, the sea and the atmosphere. Others are artificially made. Aluminium and carbon are common to us. They are elements. Aluminium foil

37. If the atoms in a molecule are separated, the atoms formed will no longer have the properties of that substance.