The Periodic Table:
Everything on the earth is made of elements. More than 100 elements are known to extist. Each element has a name and a shorthand symbol. Usually the symbol is made up of the first one or two letters of its full name, They symbol for carbon is C, which is easy to understand. Some symbols however, are harder to work out; this is because ancient scientist preferred to use latin words. e.g Lead, the Latin name for lead is Plumbum, which gives it the symbol Pb.
It would not be very helpful for elemenst to be listed alphabetically, like the names in a phone book. The modern Periodic Table is a way of ordering elements. The elements are arranged in order of ascending atomic number to avoid elements being out of order when atomic mass is used.
There are 8 groups in the Periodic Table, a goup is a vertical column. All the elements have similar properties, They are a 'chemical family'. The grouos are numbered 1-7, 0 and the transition metals. Group 1: The Alkali Metals. Group 2: The Alkali Earth Metals. Group 7: The Halogens
Periods are the rows across the Periodic Table.Hydrogen and Helium make up the first period.
The periodic Table:
At the beginning of the 1800s only about 30 elements were found but by the turn of the century, almost all stable elements I earth were found.
Chemists found so many elements through patterns such as the properties of elements and the mass of their atoms.
As scientists couldn't measure the masses of atoms back then, they compared the masses to the lightest element - hydrogen. This is called relative atomic mass.
In the early 1800s, Johann Döberenier, a German scientist noticed there were several groups of elements with similar properties such as calcium, strontium and barium. For each of these groups, the relative atomic mass of the middle element was the mean of the other two elements.
Almost half a century later, an English chemist, John Newlands, arranged the known elements in order of relative atomic masses. Then he spotted the every eighth element had similarities. It only worked for the first 16 elements and so scientists didn't account the data.
Dimitri Mendeleev, a Russia scientist produced patterns of real meaning she ordering then in terms of relative atomic mass. He left gaps for missing elements to produce a sensible pattern and predicted the properties of missing elements.
With his ordering, Mendeleev spotted that at intervals along the line there were elements with similar properties.
Group 1: The Alkali Metals
The alkali metals are soft ans can be cut witha knife. They have low densities therefore float on water. They react with water producing a metal hydroxide and hydrogen. The alkali metals are corrosive ans higly flammable. For this reason, forceps should be used when handling these metals and goggles must be worn.
Reactions with chlorine - the alkali metals:
When sodium burns in chlorine, a bright yellow gas is produced.
All of the Group 2 metals produce a salt and water.
Lithium, sodium and potassium have a low density. They melting and boiling points decrease down the group (lithium's is higher than potassium)
Group 7: The Halogens
The Group 7 elements are very reactive non-metals. They include fluorine, chlorine, bromine and iodine. They are also known as the halogens. 'Halo-hen' means 'salt-former'.
They are so reactive that they are not found freely in nature, but in compounds with other materials.
When working with the halogens, eye protection must be worn and only be used in a fume cupboard as they are toxic gases. Liquid bromine is very corrosive and so chemical resistant gloves should be worn when dealing with them.
The halogens consist of diatomic molecules - they travel in pairs.
They have low melting a boiling points because the forces between the molecules are weak so it is easy to break the bonds and change the state. The melting and boiling points increase down the group.
The discovery of helium:
Robert Bunsen discovered that different chemicals produced characteristic flame colours. Later her and Kirchhoff discover that each element has it's own unique spectra when its light passes through a prism.
A Sun Element.
When there was a total eclipse of the sun in 1868, astronomers found an unexpected yellow line in the spectra. By 1895 it was announced that this new element in the sun was called helium after the Greek word Helios for Sun.
In 1803, John Dalton was studying mixtures of gases and their properties. He had a theory that everything is made of atoms and they cannot be broken down. The word 'atom' means 'indivisible'.
Later it became clear that atoms are not solid, indivisible spheres. From mid-nineteenth century, scientists were exploring the inside of atoms discovering sub-atomic particles.
In the model we use, the mass of the atom is concentrated into a tiny central nucleus. The nucleus consists of protons and neutrons. Protons have a positive charge and the neutrons are uncharged.
Around the nucleus are the electrons. The electrons are negatively charged l. The mass of an electron is so small that it can often be ignored (negligible).
In an atom, the number if electrons equals the number of protons in the nucleus (proton number). This means that the total negative charge is equal to the positive charge and so the overall atom is uncharged.
Salts are compounds of metals and non-metals.
Salts such as sodium chloride and calcium fluoride are crystalline. Their crystals are shaped like cubes.
Salts have a much higher melting and boiling points like chemicals such as chlorine and bromine which are made of smaller molecules.
Sodium chloride is soluble in water. They are several other soluble salts including most of the compounds of the alkali metals (group 1) and the halogens (group 7).
Some salts are soluble in water such as lithium fluoride which is only slightly soluble in water. Fluorite (CaF2), Galena (PbS) and Pyrite a.k.a. Fool's gold (FeS2) aren't soluble in water either.
Salts can conduct electricity or produce electric current when they become liquid. Electrolysis occurs when the salts are in liquid form.
The same element can take different chemical forms with distinct properties. The different forms are referred to as species.
Chlorine has three simple species:
There are only two simple species of sodium: