Atoms

The History of an Atom The structure of atoms is real simple. The Greeks had the right idea – they knew about atoms. After that, all kinds of crazy ideas went around until, in 1804, John Dalton reintroduced the idea of atoms. But it was Ernest Rutherford who really sorted it all out. Dalton reintroduced the idea of atoms

Models of the atom Experimental data have been the impetus behind the creation and dismissal of physical models of the atom. Rutherford’s model, in which electrons move around a tightly packed, positively charged nucleus, successfully explained the results of scattering experiments, but was unable to explain discrete atomic emission—that is, why atoms emit only certain wavelengths of light.

Models of the atom Bohr began with Rutherford’s model, but then postulated further that electrons can move only in certain quantized orbits; this model was able to explain certain qualities of discrete emission for hydrogen, but failed for other elements.

Models of the atom Schrödinger’s model, in which an electron is described not in terms of definite paths but in terms of the likelihood of finding the electron in a particular region, can explain certain qualities of emission spectra for all elements; however, further refinements of the model, made throughout the 20th century, have been needed to explain further spectral phenomena.

The Nucleus 1)It’s in the middle of the atom. 2)It contains protons and neutrons. 3)It has a positive charge because of the protons. 4)Almost the whole mass of the atom is concentrated in the nucleus. 5)But size-wise it’s tiny compared to the rest of the atom.

The Electrons 1)Move around the nucleus. 2)They’re negatively charged. 3)They’re tiny, but they cover a lot of space. 4)The volume their orbits occupy determines how big the atom is. 5)They have virtually no mass. 6)They occupy shells around the nucleus. 7)These shells explain the whole of Chemistry.

Sizes Atoms are really tiny, don’t forget. They’re too small to see with an ordinary microscope. The mass of a proton is × kg, or approximately 1,836 times that of an electron. The mass of a neutron is × kg, about one eighth of one per cent heavier than the proton.

No. of Protons = No. of Electrons 1)Neutral atoms have no charge overall. 2)The charge on the electrons is the same size as the charge on the protons but opposite. 3)This means the number of protons always equals the number of electrons in a neutral atom. 4)If some electrons are added or removed, the atom becomes charged and is then an ion. 5)The number of neutrons isn’t fixed but is usually just a bit higher than the number of protons.

Know Your Particles PROTONS are HEAVY and POSITIVELY CHARGED NEUTRONS are HEAVY and NEUTRAL ELECTRONS are TINY and NEGATIVELY CHARGED PARTICLEMASSCHARGE Proton1+1 Neutron10 Electron1/2000

Proton Number and Mass Number Total of Protons and Neutrons Number of Protons (Sometimes called atomic number) THE MASS NUMBER (A) THE PROTON NUMBER (Z) Na 23 11

Points to Note 1)The proton number (or atomic number) tells you how many protons there are (oddly enough). 2)This also tells you how many electrons there are. 3)The proton number is what distinguishes one particular element from another. 4)To get the number of neutrons – just subtract the proton number from the mass number. 5)The mass number is always the biggest number. It tells you the relative mass of the atom. 6)The mass number is roughly double the proton number. 7)Which means there’s about the same number of protons as neutrons in any nucleus.

Isotopes are the same except for an extra neutron or two “ISOTOPES ARE… Different atomic forms of the same element, which have the SAME number of PROTONS but a DIFFERENT number of NEUTRONS.

The upshot is: isotopes must have the same proton number but different mass numbers. If they had different proton numbers, they’d be different elements altogether. A very popular pair of isotopes are carbon-12 and carbon-14.

The number of electrons decides the chemistry of the element. If the proton number is the same (that is, the number of protons is the same) then the number of electrons must be the same, so the chemistry is the same. The different number of neutrons in the nucleus doesn’t affect the chemical behaviour at all.

Electrolysis Electrolysis is the process by which ionic substances are broken down into simpler substances using electricity. During electrolysis, metals and gases may form at the electrodes.ionic substanceselectrodes

Examples Aluminium is extracted from purified aluminium ore by electrolysis. The raw materials for aluminium extraction are aluminium ore (bauxite) and a compound of aluminium called cryolite, which allows the process to work at a lower temperature. Aluminium forms at the negative electrode and oxygen forms at the positive electrode. The positive electrodes burn away and need to be replaced regularly.orecompoundelectrode

Examples Copper is purified by electrolysis using copper electrodes and copper sulphate solution. The negative electrode is pure copper and the positive electrode is impure copper.

In order to understand electrolysis, you need to know what an ionic substance is. Ionic substances form when a metal reacts with a non-metal. They contain charged particles called ions. For example, aluminium oxide forms when aluminium reacts with oxygen. It contains positively-charged aluminium ions and negatively-charged oxide ions. Ionic substances can be broken down by electricity.ions

Electrolysis is the process by which ionic substances are decomposed (broken down) into simpler substances when an electric current is passed through them.

For electrolysis to work, the ions must be free to move. Ions are free to move when an ionic substance is dissolved in water or is molten. For example, if electricity is passed through copper chloride solution, the copper chloride is broken down to form copper metal and chlorine gas.molten

Electrolysis takes place in an electrolytic cell, the simplest form of which is shown below: The components which make contact with the electrolyte are called ELECTRODES. The electrode which is attached to the negative pole of the battery, and which supplies electrons to the electrolyte, is called the CATHODE. Reduction takes place at the cathode.

The electrode which is attached to the positive pole of the battery, and which accepts electrons from the electrolyte, is called the ANODE. Oxidation takes place at the anode.

When a direct electric current is passed through an ELECTROLYTE (such as a molten salt or an aqueous solution of a salt, acid or base), chemical reactions take place at the contacts between the circuit and the solution. This process is called ELECTROLYSIS

Various reactions take place at the electrodes during electrolysis. In general, reduction takes place at the cathode, and oxidation takes place at the anode.

Electrolysis of Water Water may be electrolyzed in the apparatus shown below. Pure water is however a very poor conductor of electricity, and one has to add dilute sulphuric acid in order to have a significant current flow.

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