Thomson Plum Pudding Atom 1 3 The example essay topic

1.1 Dalton's Theory - "billiard atom" - Matter is made up of small dense particles called atoms. - Atoms are each element are identical. - Atoms are indestructible. Problems which arose: Because Dalton did not believe that elements could be diatomic, his theory could not explain Gay Lus sacs Law of combining volumes. A Avogadro had solved the problem of correctly determining atomic weights as early as 1811. He also realised that ultimate particles of gases could be diatomic and was the first chemist to write the formula of water correctly.

It was not until Avogadro's Law was accepted that polyatomic elements were said to exist and the difference between atoms and molecules was understood. 1.2 The electrical atom Thomson showed that identical cathode rays were produced by different elements. He suggested that these cathode rays were small negatively charged particles which he called electrons. Atoms are positively charged spheres containing negatively charged particles distributed through them. Thomson (1903) 'plum-pudding' atom 1.3 The nuclear atom Rutherford (1911) The discovery of radioactivity by Becquerel in 1896 provided an incentive for scientists to probe to atom.

As a result of Rutherford's gold foil experiment, where although almost all the positively charged particles passed though the foil, a very small number bounced back, he formulated the following theory. Most atoms are made up of empty space. The mass and positive charge of atoms is concentrated in a minute nucleus. The electrons move about the central nucleus like "planets round the sun". The nuclear atom In 1920 he suggested that some nuclei also contained another neutral particle but the neutron was not discovered until 1932 (by Chadwick). Problems which arose A rotating electron must radiate energy so it would spiral into the nucleus.

Atomic spectra of elements could not be explained. 1.4 Bohr's atom In 1900, Planck proposed that energy existed in 'parcels' which he called quanta and that when atoms or molecules absorb or emit energy they do so only in quanta. Bohr's model was based on Planck's Theory. The electron rotates around the nucleus in definite orbits having specific amounts of energy. The orbit closest to the nucleus has the lowest energy level. Only certain shells or energy levels can exist.

An electron can move from shell to another but cannot be found between shells. An electron moving a shell does not radiate energy. An electron can change orbit only by absorbing or releasing a quantum of energy suitable for that transition. Problems which arose The theory explained the spectrum of hydrogen but not the spectra of other elements. 1.5 Modern atomic theory The modification of Bohr's theory was made possible by de Broglie (1922) suggesting that small particles of matter could also have wave properties. Diffraction patterns produced by electrons were similar to those of light waves Schrodinger (1926) derived a wave equation to describe the movement of an electron in the hydrogen atom.

The Quantum mechanical theory Note: Although a large number of subatomic particles have been found in this course only protons, neutrons and electrons will be discussed. Properties of major sub atomic particles. Particle Symbol Charge Mass a. m. u Proton P +1 1.008 Neutron N 0 1.009 Electron e -1 1/1840 1.6 Key terms in atomic structure Atomic number of an atomic, the number of protons in that atom (symbol Z). An element is therefore a substance hose atoms have the same atomic number.

Mass number is the sum of the protons and neutrons present in the atom (symbol A). The number of neutrons present is therefore A-Z. Comprehensive Symbol of an element is: (Mass no.) A x (symbol of element) Z (Atomic no.) e.g. The element sodium is written as 23 Na. 11 2.6 Structure of matter Matter may be classified according to its structure this depends on both the nature of the particles present and the forces holding the particles together (bonding forces). Ionic bonds are formed when complete electron transfer occurs between elements.

Ionic bonds are formed between metals and non-metals e.g. NaCl Covalent bonds may be single, double or triple bonds. A single covalent bond is formed when two atoms share a pair of valence electrons. This usually allows each atom to reach noble gas configuration and co-ordinate covalent bond is a bond formed when both electrons of the shared pair are provided by one atom. Covalent bonds are formed between non-metal diatomic gases and compounds containing non-metals only. Some compounds contain both ionic and covalent bonds. e.g. Na SO, CaCO, NH Cl. 2 4 3 4 Radicals form either the anion or cation in this case.

Covalent molecular substances consist of small discrete molecules. They have low melting points and boiling points since the forces between molecules are weak (vander waals forces). e.g. O, CO 2 2 In covalent network substances each atom is covalently bonded to other atoms forming a giant molecule. e.g. C, SiO, SiC. 2 Covalent network substances have very high melting points and boiling points. Since the lattice is held together by strong covalent bonds. Substances may be classified into four main groups on the basis of structure and bonding.

Table 2.6 Structures and Bonding of Substances Explanation of bonding Type of bonding and examples Appearance and state at 25^0 C Melting point Conductivity Solid Molten 'sea' of electrons metallic, eg, Cu, Ng Shiny solids, malleable and ductile (Hg only liquid) High High High Positive and negative ions forming a lattice Ionic, NaCl, Car 2 Crystalline solids High Nil High Small discrete molecule Covalent molecular, eg, O, S, CO 2 8 2 SO 3 Soft, brittle solid or gases (Br only 2 Liquid) Low Nil Nil Large covalent network, "giant molecules" Covalent network solids e.g. C, SiO 2 SiC (semi metals and some of their covalent compounds) Very hard and brittle Very high Nil nil 2.7 Allotropes The physical properties of some elements may be correlated with their crystal structure. Allotropes are the forms of an element that have distinctly different physical properties. Examples 1. Elementary carbon has two allotropes, graphite and diamond.

Diamond is extremely hard, while graphite is used as a lubricant since the weak van der waal's forces between layers allow sliding movement. Graphite can conduct electricity. Since it contains some de localised electrons and its non-conductor. Other elements showing allotropy are: 2. Sulphur the most stable allotrope at room temperature is rhombic sulphur which is made up of an S (8) ring. On heating, it changes to mono clinic sulphur which is also made up of an S (8) ring but with differently shaped crystals.

3. Phosphorus the allotropes of phosphorus may be white, red or black. The dangerous white phosphorus polymerases to red phosphorus which is safely used in matches. Heat and pressure convert red phosphorus to black phosphorus. 4.

Tin exists in three solid forms Transition temperature 18^0 C 161^0 C 232^0 C Form alpha beta gamma (') Appearance grey white white 5. Oxygen exists in two forms, O (2) (g) and O (3) (g) which is ozone. 3.1 Types of formulae Learn the symbols for the common elements (see periodic table) Learn common ions and their charges. Identify types of formulae. Learn to name compounds correctly. Table 3.1 Types of Formulae Empirical Show the number ratio of atoms, e.g. CH (3) for ethane Molecular Show the actual number of each kind of atom in the molecule, e.g. C (2) H (6) for ethane Structural Show arrangement of atoms in the molecule: graphic Significant of abridged graphic, e.g. CH (3) - CH (3).