History of the Periodic Table
In the early 1800s they could only go by Atomic Mass
There were two ways to categorise elements:
- Their physical and chemical properties
- Their Relative Atomic Mass
They had no knowledge of atomic structure or of protons and neutrons so there was no such thing as an atomic number.
They could only measure Relative Atomic Mass and so the known elements were arranged in order of atomic mass. When this was done, a periodic pattern was noticed in the properties of the elements therefore leading to the Periodic Table.
Newland's Law of Octaves
In 1864 he noticed that every eighth element had similar properties and so he listed the known elements in rows of 7. These sets of 8 were called Newland's Octaves. However, the pattern broke down on the third row with transition metals like Titanium and Iron messing up the pattern.
History of the Periodic Table.2
Dmitri Mendeleev Left Gaps and Predicted New Elements
In 1869, Dmitri Mendeleev in Russia arranged around 50 known elements into his Table of Elements with various gaps.
He put the elements in order of atomic mass, but he found that he had to leave gaps in order to keep elements with similar properties in the same vertical columns ( known as groups).
He left very big gaps in the first two rows before the transition metals come in on the third row.
The gaps predicted the properties of the so far undiscovered elements.
When they were found, they fitted the pattern.
The Modern Periodic Table
At the time whe the periodic table was first released, there wasn't much evidence to suggest the elements did fit together in that way.
After Mendeleev released his work, newly discovered elements fitted into the gaps he left. This was convincing evidence in favour of the periodic table. Once there was more evidence, many more scientists realised that the periodic table could be a useful tool for predicting properties of elements.
In the late 19th century, scientists discovered protons, neutrons and electrons. The periodic table matches up very well to whats been discovered about the structure of the atom. Scientists now accept that it's a very important and useful summary of the structure of atoms.
- The Modern Periodic Table is Based on Electronic Structure When electrons, protons and neutrons were first discovered, the periodic table was arranged in order of atomic number. All elements were put into groups.
- The elements in the periodic table can be seen as being arranged by their electronic structure. Using the electron arrangement, you can predict the element's chemical properties.Electrons in an atom are set out in shells which each correspond to an energy level.
The Modern Periodic Table.2
Apart from the transition metals, elements in the same group have the same number of electron in the highest occupied energy level (outer shell).
The group number is equal to the number of electrons in he highest occupied energy level- e.g. Group 6 all have 6 electrons in the highest energy level.
The positive charge of the nucleus attracts electrons and holds them in place. The further from the nucleus the electron is, the less the attraction
The attraction of the nucleus is even less when there are a lot of inner electrons. Inner electrons get in the way of the nuclear charge, reducing the attraction. The effect is known as shielding.
The combination of increased distance and increased shielding means that an electron in a higher energy level is more easily lot because there's less attraction from the nucleus holding it in place. That's why Group 1 metals get more reactive as you go down the group.
Increased distance and shielding also means that a higher energy level is less likely to gain an electron- there's less attraction from the nucleus pulling electrons into the atom. That's why Group 7 elements get less reactive going down the group
Group 1- The Alkali Metals
- The alkali metals are silvery solids that have to be stored in oil and handled with forceps ( they burn the skin).
- As you go down Group 1, the alkali metals become more reactive because its further from the nucleus. They also have lower melting and boiling points as you go down Group 1.
- The alkali metals have low density; the first three Lithium (Li), Sodium (Na), and Potassium (K), are less dense than water.
The alkali metals are Lithium (Li), Sodium (Na), Potassium (K), Rubidium (Rb), Caesium (Cs) and Francium (Fr)
The alkali metals all have one outer electron. This makes them very reactive and they all have similar properties.
The alkali metals form Ionic Compounds with Non-Metals. They want to lose their one outer electron to form a 1+ ion. They never share electrons so covalent bonding doesn't take place with the alkali metals. So they always form ionic bonds and they produce white compounds that dissolve in water to form colourless solutions.
Group 1- The Alkali Metals.2
When alkali metals react with Water, they produce hydrogen gas
Metal + water → metal hydroxide + hydrogen
When lithium, sodium or potassium are put in water, they react vigorously.
They float and move around the surface of the water fizzing vibrantly.
They produce hydrogen. Potassium gets hot enough to ignite it. A lighted splint will indicate hydrogen by producing a squeaky pop as the hydrogen ignites.
They form hydroxides that dissolve in water to give alkaline solutions. Here is Sodium + Water:
2Na + 2H2O → 2NaOH + H2
Its the same with Potassium (K), just replace Na with K.
Group 7- The Halogens
As you go down Group 7, the Halogens have the following properties:
They become less reactive because it is harder to gain an electron, because the outer shell is further from the nucleus. As you go down the melting and boiling points get higher.
The Halogens are all Non-Metals with coloured vapours
Flourine is a very reactive, poisonous yellow gas. Chlorine is a fairly reactive, poisonous dense green gas Bromine is a dense, poisonous, red-brown volatile liquid Iodine is a dark grey crystalline solid or a purple vapour
They all exist as molecules which are pairs of atoms: F (Flourine) 2, Cl (Chlorine) 2, Br (Bromine) 2, I (Iodine) 2
Group 7- The Halogens.2
The Halogens form Ionic Bonds with Metals
The halogens form 1- ions called halides ( F-, Cl-, Br- and I-) when they bond with metals e.g.
(Na+)Cl- or (Fe3+)Br-
More reactive halogens will displace less reactive ones
A more reactive halogen can displace a less reactive halogen from an aqueous solution of its salt. E.g. Chlorine can displace Bromine and Iodine from an aqueous solution of its salt ( a bromide or iodide). Bromine will also displace Iodine because of the trend in reactivity.
Cl2(g) + 2KI(aq) 2KCl(aq) + I2(s)
To use Potassium Bromide instead of Potassium Iodide, replace the I with Br
Transition elements are the typical metals: They are good conductors of heat and electricity. They are very dense, strong and shiny. Transition metals are much less reactive than Group 1 metals- they don't react as vigorously with water or oxygen for example. They are also much denser, stronger and harder than the Group 1 metals, and have much higher melting points (except for mercury, which is a liquid at room temperature) e.g. Iron melts at 1500C, Copper at 1100C and Zinc at 400C
- Transition metals often have more than one Ion e.g. Fe 2+, Fe 3+ Two other examples are copper: Cu+, and Cu 2+... and Chromium Cr2+, Cr3+
- The different ions usually form different coloured compounds too: Fe2+ ions usually give green compounds, whereas Fe3+ ions usually from red-brown compounds e.g. rust. The compounds are colourful due to the transition metal ion they contain e.g. Potassium Chromate (VI) is yellow, Potassium Manganate (VII) is purple, Copper (II) Sulfate is blue. The colours in gemstones, like blue sapphires and green emeralds and the colours in pottery glazes are all due to transition metals. And weathered copper is green.
- Iron is the catalyst used in the Haber process for making ammonia. Manganese(IV) is a good catalyst for the decomposition of hydrogen peroxide. Nickel is used for turning oils into fats for making margarine.
- they give out energy to the surroundings, usually in the form of heat and usually shown by an increase in temperature
- the energy released in bond formation is greater than the energy used in breaking bonds
- they take in heat from the surroundings, usually in the form of heat and usually shown by a fall in temperature.
- the energy required to break old bonds is greater than the energy released when new bonds are formed.
Energy must always be supplied to break bonds and energy is always released when bonds form.
Moles and Titrations
Concentration: the more solute you dissolve, the more crowded the solute molecules are and the more concentrated the solution is. It is measured in moles per decimetre cubed.
1 litre = 1000 cm cubed = 1 dm cubed
Titrations: used to find out concentrations, they allow you to find out exactly how much acid is needed to neutralise a quantity of alkali ( or vice versa)
- You put some alkali in a flask with some indicator- phenolphthalein or methyl orange for a definite colour change. A universal indicator would change the colour gradually.
- Add the acid a bit at a time using a burette, swirl the flask regularly. Add some acid slowly especially when you think the acid is almost neutralised
- The indicator changes colour when all the alkali has been neutralised- phenolphthalein is pink in alkali but colourless in acids, and methyl orange is yellow in alkali but red in acids
- Record the amount of acid used to neutralise the alkali. Repeat to test reproducibility
- Then take a mean of the results.
Titration Calculations & Bond Energies
If they ask for concentration is MOLES PER DM CUBED:
number of moles divided by concentration (mol/dm cubed) X volume (dm cubed)
If they ask for concentration in GRAMS PER DM CUBED:
mass (grams) divided by number of moles X relative formula mass
In exothermic reactions the energy change is negative
In endothermic reactions the energy change is positive