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.
Example 1: water One water molecule consists of one oxygen atom and two hydrogen atoms.
One carbon dioxide molecule consists of one carbon atom and two oxygen atoms. Example 2: carbon dioxide
One ammonia molecule consists of one nitrogen atom and three hydrogen atoms. Example 3: ammonia
One argon molecule consists of one argon atom only. Example 4: argon
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.
… about elements Altogether, 109 different elements are known.
109 kinds of atoms are found. There are 109 elements Name of element = name of the atom
Most elements at room temperature are solids Hg Br liquids NOF Cl He Ne Ae Kr Xe Rn Gas
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…
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
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.
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
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
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)
… 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
If the atoms in a molecule are separated, the atoms formed will no longer have the properties of that substance.