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Elements & Compounds


Elements


AN ATOM

Elements

  • Atom - smallest unit of all matter, that is composed of 3 sub-atomic particles called protons, electrons and neutrons
  • Proton - the 'heavy' positively-charged particle in the nucleus of an atom
  • Electron - the very 'light' negatively-charged particle that orbits the nucleus of an atom
  • Neutron - the 'heavy' uncharged particle in the nucleus of an atom
  • Uncharged or unreacted atoms have the same number of positive protons and negative electrons.
  • Approximate size of atoms - Millions of atoms could fit on the sharp point of a needle. Also, if you imagine that an atom is the size of an oval, a proton and a neutron would be the size of footballs in the middle of the oval, and the electron would be the size of a rice grain racing around the running lane.

ATOMIC NUMBER AND ATOMIC MASS

  • An Example from the Periodic Table

Elements

  • Atomic Number - the number of protons in an unreacted atom
  • Mass Number - the number of protons and neutrons together

ATOMIC DIAGRAMS

  • Protons and neutrons are in the nucleus.
  • Protons are p+
  • Neutrons are n
  • Electrons orbit the nucleus in electron levels or 'rings'.
  • Electrons are e-
  • A limited number of electrons are situated in each electron 'ring'.
First ring - maximum of 2 electrons
Second ring - maximum of 8 electrons
Third ring - maximum of 8 electrons
Fourth ring - maximum of 18 electrons
  • Electron rings close to the nucleus are filled first.

ELEMENTS AND COMPOUNDS

  • Element - An element is a substance composed of the same type of atoms (e.g. gold Au, oxygen O2).
  • Compound - A compound is a substance made of more than one type of atom (e.g. water H2O, carbon dioxide CO2).
  • Molecule - A molecule is the smallest particle of either an element or a compound.

INERT OR NOBLE GASES

  • Inert or Noble Gases are unreactive gases. They do not corrode nor react.
  • Examples of Noble Gases are:
He - Helium
Ne - Neon
Ar - Argon
Kr - Krypton
Xe - Xenon
Rn - Radon
  • The electron rings of these unreactive gases are full, therefore they become stable.

IONS (CHARGED ATOMS)

  • When atoms react, they may either gain or lose electrons. Electrons have a negative charge. An atom gaining or losing electrons will get an overall charge.
  • Positive Ions are atoms that have lost electrons (e.g. sodium Na1+)
  • Negative Ions are atoms that have gained electrons (e.g. chlorine Cl1-)
  • In chemical reactions, atoms tend to gain or lose electrons to resemble the electron numbers of the stable Noble Gases.

COVALENT AND IONIC COMPOUNDS

  • Covalent Compound - a compound where electrons are shared between the atoms (e.g. carbon dioxide CO2)
  • Ionic Compound - a compound formed from the attraction between positive and negative ions. For example in the ionic compound sodium chloride NaCl, the chlorine ion (Cl1-) gains one electron that was given by the sodium ion (Na1+).

COMMON ELEMENTS AND SYMBOLS TO LEARN

Element Symbol

Element Name

Element Symbol

Element Name

H Hydrogen Mn Manganese
He Helium Fe Iron
Li Lithium Co Cobalt
C Carbon Ni Nickel
N Nitrogen Cu Copper
O Oxygen Zn Zinc
F Fluorine Br Bromine
Ne Neon Ag Silver
Na Sodium Sn Tin
Mg Magnesium I Iodine
Al Aluminium Ba Barium
Si Silicon W Tungsten
P Phosphorus Pt Platinum
S Sulphur / Sulfur Au Gold
Cl Chlorine Hg Mercury
Ar Argon Pb Lead
K Potassium Cr Chromium
Ca Calcium Ti Titanium
Pu Plutonium U Uranium

NAMING COMPOUNDS

PREFIX OR
SUFFIX

MEANING

EXAMPLE

Mono- There is 1 atom of that type in that molecule Carbon monoxide (CO)
Di- There are 2 atoms of that type in the molecule Carbon dioxide (CO2)
Bi- Hydrogen is present in the molecule Sodium bicarbonate

(NaHCO3)

-ide There are only 2 types of atoms present in the molecule Lead oxide

(PbO)

-ate There are 3 or more types of atoms in the molecule, and 1 type is oxygen Calcium carbonate

(CaCO3)


VALENCY TABLE

  • Valency - the charge of an ion or radical which has either lost or gained electrons
  • Note that metals lose electrons easily to become positive ions. This is why most metals are good conductors of electricity.

1+

2+

3+

1-

2-

3-

H 1+ Mg 2+ Al 3+ F 1- O 2-

oxide

PO4 3-

phosphate

Na 1+ Ca 2+ Fe 3+

ferric

Cl 1- S 2-

sulphide

 
Li 1+ Cu 2+   Br 1- CO3 2-

carbonate

 
K 1+ Zn 2+   OH 1-

hydroxide

SO4 2-

sulphate

 
Ag 1+ Pb 2+   NO3 1-

nitrate

   
NH4 1+

ammonium

Fe 2+

ferrous

  HCO3 1-

bicarbonate

   

WORKING OUT FORMULAE OF IONIC COMPOUNDS
(THE CROSS-OVER METHOD)

  • Step 1 - In the ionic compounds to be learnt in junior science, there are two parts to the ionic compound - the first is a positive ion (usually a metal e.g. Na1+) and the second is a negative ion (e.g. Cl1-).
  • Step 2 - Using the valency table, write the two ions and their valencies.
  • Step 3 - Now ignore the positive and negative signs. Cross-over the top valency number to the bottom of the other ion symbol. Do this for both.
  • Step 4 - Write the completed formulae with those same numbers at the bottom.
  • Step 5 - If the numbers on each part are the same (e.g. Na1 Cl1 or Mg2 O2), ignore them and rewrite the formulae without them (e.g. Na Cl or Mg O).
  • Step 6 - Brackets may be used around radicals (groups of atoms that are charged e.g CO3).

Elements


EXAMPLES OF CHEMICAL NAMES OF COMPOUNDS

CHEMICAL FORMULA

CHEMICAL NAME

CO2 carbon dioxide
CO carbon monoxide
Na Cl sodium chloride
Cu O copper oxide
Ag Br silver bromide
K I potassium iodide
H Cl hydrogen chloride (hydrochloric acid)
NH4 Cl ammonium chloride
K OH potassium hydroxide
Na OH sodium hydroxide
Ca (OH)2 calcium hydroxide
Ca S calcium sulphide
Na NO3 sodium nitrate
H NO3 hydrogen nitrate (nitric acid)
Na HCO3 sodium bicarbonate
Zn SO4 zinc sulphate
Mg CO3 magnesium carbonate
Ca SO4 calcium sulphate
Cu CO3 copper carbonate
Al PO4 aluminium phosphate
Fe SO4 iron sulphate
Fe CO3 iron carbonate
NH4 NO3 ammonium nitrate
NH4 HCO3 ammonium bicarbonate
H2 SO4 hydrogen sulphate (sulphuric acid)
Na2 SO4 sodium sulphate
(NH4)2 CO3 ammonium carbonate

EXAMPLES OF NUMBERS AND TYPES OF ATOMS
IN VARIOUS ELEMENTS AND COMPOUNDS

NAME OF
SUBSTANCE

CHEMICAL
FORMULA

ELEMENT OR COMPOUND

NUMBER AND TYPE
OF ATOMS IN MOLECULE

Hydrogen H2 element 2 hydrogen atoms
Carbon dioxide CO2 compound 1 carbon atom
2 oxygen atoms
Water H2O compound 2 hydrogen atoms
1 oxygen atom
Methane CH4 compound 1 carbon atom
4 hydrogen atoms
Sodium hydroxide NaOH compound 1 sodium atom
1oxygen atom
1 hydrogen atom
Calcium hydroxide Ca(OH)2 compound 1 calcium atom
2 oxygen atoms
2 hydrogen atoms