Dalton
He proposed that atoms were the fundamental particles of nature
- All matter consists of indivisible atoms (we cannot divide a single atom) correct
- Atoms of a particular element are identical in mass and have identical properties (no, isotopes exist which have different masses and have different properties)
- Atoms of each element have a unique mass
- Atoms are neither created or destroyed in reactions (not true, they can be in nuclear reactions)
- Compounds are formed from the combination of the atoms of two or more elements
- The proportion and kind of atoms is fixed in a given compounds
- Atoms combine in numerical ratios
- The most table compounds of two elements contain atoms in a one-one ratio
The periodic table
Berzelius
He calculated the relative atomic masses of many elements, these masses were based on assigning 100 atomic mass units to oxygen.
Mendeleev
He realised that chemical properties vary periodically with increasing atomic mass. He was also aware that elements that from ions of the same charge exhibited similar chemical properties. He also recognised the alkali metals lithium, sodium, potassium, rubidium and caesium as a group with similar chemical properties despite their dissimilar atomic masses.
He constructed a periodic table by
- Arranging elements with similar chemical properties into vertical groups
- Arranging the elements in order of increasing atomic mass into horizontal periods
Mendeleev’s first periodic table clearly revealed that as atomic mass increased across a period, there was a gradual change in chemical properties but only 62 elements were known.
Using periodic trends and chemical properties of elements next to gaps in his table, Mendeleev predicted the properties of undiscovered elements. He predicted germanium to be a density of 5.5 g/ml and a high melting temperature. When germanium was discovered it had density of 5.4g/ml and melting temp of 958 degrees.
Discovery of noble gases
The presence of relatively un-reactive gas, nitrogen and reactive gas, oxygen in air had been discovered. Henry Cavendish discovered that a spark through the air caused N → N oxides, which could then be extracted by chemical reaction from the remaining gases. By adding more oxygen to sparked air, he tried to completely remove nitrogen gas. Whilst all the nitrogen could be removed, one percent would not react. He suggested that this might represent a new gas.
Lord Rayleigh discovered that the density he measured for samples of nitrogen obtained from air was 1.257 g/l whereas 1.251 g/l for nitrogen produced by chemical reactions. He was assisted by William Ramsey who isolated the gas that remained when N and O were removed from air and examined the spectrum produced when it was heated. Because spectrums are unique for the purpose of identification, the fact that it could not be identified rendered it new. It was named argon.
Ramsay recognised that he has isolated one of a new group of un-reactive elements. By fractional distillation of Argon, he found Neon, Krypton and Xenon. Nearly 20 years before Ramsay’s discovery, helium the first member of noble elements was found to exist in the sun by French astronomer Pierre Janssen who observed a new yellow spectral line in light emitted from the sun during solar eclipse. Helium gas was first found on earth by Ramsay in 1985 present in uranium ores. Spectroscopy was again used to conform its identity. Natural gas wells produce gas (7.5 % helium). The Helium from these wells is believed to result from decay of radioactive minerals such as uranium compounds. Whilst Helium is relatively scarce on earth, it is second only to hydrogen in its abundance.
Noble gases
- Generally un-reactive
- Unlike other gases which consist of molecules (N2, 02, C02) there is no strong bonding between atoms in noble gases. Elements exist as single atoms
- Attraction between atoms are weak so elements have low melting and boiling temperature and are all gases at room temp (Helium boils at 4K)
- The lack of reactivity results from arrangement of electrons in their atoms. In general, an atom is most stable when the outer shell electrons contains eight electron. Noble gases have outer shell containing 8 electrons (Helium 2). Presence of filled shell confers stability to the helium atom in a similar manner to that of the other noble gases
- These days you can react them however (Xenon di-fluoride, Krypton fluoride)
Discovery of polonium and radium
Antoine-Henri Becquerel noticed that if a uranium compound was placed on a photographic plate, the plate become exposed. A new type of penetrating radiation had been discovered and it was quickly shown to be emanating from the uranium atoms.
Marie Curie measured amount of radiation emitted by uranium, she and her husband Pierre found that uranium ore produced far more radiation than could be accounted for in terms of uranium content. They suspected the presence of an uncovered element much more radioactive than uranium. They processed the tonnes of uranium ore to extract a compound of the new element, which was named polonium.
Despite the high radioactivity of polonium, Curies found that the presence of this in uranium still did not account for the levels of radioactivity measured for ore samples. The existence of another element was suspected. Curies managed to extract one gram of a new, more radioactive element. They called it radium. The elements polonium and radium are radioactive because their atoms are unstable and break down into smaller particles The source of polonium and radium is elements such as uranium which are continually breaking down in multi-step nuclear reactions to form lead. Polonium and radium are just two of the several elements formed during the course of this process and are therefore always present in uranium ores.