Chemistry Revision
Atomic Structure
Periodic Table
An atom has a central nucleus. This is surrounded by electrons arranged in shells.
The nuclei of all atoms contain subatomic particles called protons. The nuclei of most atoms also contain neutrons.
Subatomic particle, Relative mass, Relative charge
Proton 1 +1
Neutron 1 0
Electron Very small -1
The number of protons in an atom of an element is its atomic number
How to calculate the number of subatomic particles
number of protons = atomic number
number of electrons = atomic number
number of neutrons = mass number - atomic number
Atoms of the same element must have the same number of protons, but they can have different numbers of neutrons. Atoms of the same element with different numbers of neutrons are called isotopes.
Mendeleev
He published his first periodic table of the elements in 1869.
He arranged the elements in order of increasing atomic weights.
He also took into account the properties of the elements and their compounds.
Mendeleev left gaps in his table for elements not known at the time. By looking at the properties of the elements next to a gap, he could also predict the properties of these undiscovered elements.
Modern periodic table
Elements are arranged in rows, called periods, in order of increasing atomic number. Elements with similar properties are placed in vertical columns, called groups
Mendeleev did not know about isotopes, but their existence explains pair reversals.
Electrons
Electrons in atoms occupy energy levels, also called electron shells, outside the nucleus.
Different shells can hold different maximum numbers of electrons. The electrons in an atom occupy the lowest available energy level first.
Metals and nonmetals
Most elements are metals. In their chemical reactions, metal atoms lose electrons to form positive ions.
Elements that do not form positive ions in their chemical reaction are non-metals.
Propeties
Metals
Non-Metals
Good conductor of electricity
Good conductor of heat
Shiny
High density
Malleable
Ductile
Poor conductor of electricity
Poor conductor of heat
Dull
Low density
Brittle
Bonding
pure covalent bonds
polar covalent bonds
ionic bonds
A covalent bond is a shared pair of electrons, electrostatically attracted to the positive nuclei of two atoms.
Atoms can share electrons in order to achieve a stable outer electron arrangement (a noble gas arrangement).
Pure covalent bonds exist between two atoms with the same electronegativities. A pure covalent bond has no ionic character at all.
Usually, one of the atoms involved in the covalent bond will be more electronegative and will have a greater attraction for the bonding pair of electrons. This gives rise to polar covalent bonding.
A polar covalent bond is a bond formed when a shared pair of electrons are not shared equally.
The shared pair of electrons between an atom of hydrogen and an atom of bromine are not shared equally. Since bromine has a greater electronegativity than hydrogen, it will pull the bonding electrons towards itself.
Ionic bonds are usually (but not always) formed between a metal and non-metal with a large difference in electronegativity, eg Sodium Chloride.
Chlorine has a far larger negativity and so pulls the bonding electrons towards itself completely, thus gaining an electron and forming a negative ion.
The ionic bond is the electrostatic force of attraction between a metal ion and a non-metal ion. Ions are arranged into a three-dimensional ionic lattice of positive and negative ions.
All ionic compounds have a high melting point and boiling point. They conduct when molten or in solution as the ions are free to move.
Chemical Analysis
Gas Test
Chlorine
Chlorine has a characteristic sharp, choking smell. It also makes damp blue litmus paper turn red, and then bleaches it white.
Hydrogen
Carbon dioxide
Oxygen
A lighted wooden splint makes a popping sound in a test tube of hydrogen.
A glowing wooden splint relights in a test tube of oxygen.
Carbon dioxide turns limewater milky. A lighted wooden splint goes out in a test tube of carbon dioxide but this happens with other gases, too. So the limewater test is a better choice
Flame test
Strontium
Crimson
Potassium
Sodium
Copper
Calcium
Yellow
Lilac
Cyan
Brick red
Precipitate test
Calcium
Copper
Magnesium
Iron(II)
Aluminium
Iron(III)
Brown-Rusty
Green-Grey/green
White-White
Palue Blue- Pale Blue
White-dissolves
White-White
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Chromotography
1.draw a pencil line across the chromatography paper, 1 - 2 cm from the bottom
- use a pipette or capillary tube to add small spots of each ink to the line on the paper
- place the paper into a container with a suitable solvent in the bottom
- allow the solvent to move through the paper, but remove the chromatogram before it reaches the top
- allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent
Earths atmosphere
The Earth formed approximately 4.6 billion years ago. Scientists cannot be certain about what gases made up the Earth's early atmosphere. Ideas about how the atmosphere was produced and how it has changed have developed over time as new evidence has been discovered. There is still not enough evidence for scientists to be certain.
One theory suggests that the early atmosphere came from intense volcanic activity, which released gases that made the early atmosphere very similar to the atmospheres of Mars and Venus today:
a large amount of carbon dioxide
little or no oxygen
small amounts of other gases, such as ammonia and methane
Volcanic activity also released water vapour, which condensed as the Earth cooled to form the oceans
Carbon dioxide is a very soluble gas. It dissolves readily in water. As the oceans formed, carbon dioxide dissolved to form soluble carbonate compounds so the amount in the atmosphere decreased. Carbonate compounds were then precipitated as sedimentary rocks, eg limestone.
Carbon dioxide was also absorbed from the oceans when the first plants and algae carried out photosynthesis. Many of these organisms, and those in the food chains that they supported were turned into fossil fuels, eg crude oil, coal and natural gas, which all contain carbon.