Chemical foundations
chemistry . It is a word that is impossible to define concisely because the field so diverse and because its practitioners perform such a incredible variety of jobs. Chemistry deals mainly with sitations in which nature of a substance is changed by altering its composition so that entirely new substance are synthesized or properties of existing substaces are enhanced.
Dalton's Atomic Theory
In 1808 Dalton published A New System of Chemical Philosophy .
In which he presented his theory of atoms:
.Each element is made up of tiny particles called atoms.
The atoms of a given element are identical ; the atoms of different elements are different in some fundamental way or ways.
. Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.
Chemical reactions involve reorganization of atoms –change the way they bound together. The atoms themselves are not changed in a chemical reactions.
about John dalton ;
John Dalton(1766-1844),an Englishman,began teaching at a Quaker school when he was 12.his fascination with science includ an intense interest in meteorlogy,which led to an interest in gases of the air atoms.Dalton is best known for his atomic theory, in which he postulated that fundemental difference among atoms are their masses. He was the first to prepare a table fo relative atomic weights.
The Electron
the first important experiments that led to an understanding of the composition of the atom were done by the English physicist J.J Thomasonwho studied electrical discharges in partially evacuated tubes called CATHODE_RAY_TUBES during the peirod from 1898 to 1903.Thomson found that when high voltage was apllied to the tube a "ray" called CATHODE RAY (because it emanated from the nagative electrod or cathode) was produced.Because this ray was produced at the negative electrode and was repelled by negative pole of an applied electric field. Thomson postulated that the ray was a stream fo negatively charged particles.(now called electrons).from experiment in which he measured the deflection of the beam fo electrons in a magnetic filed , thomson determind the charge-to-mass ratio fo an electron: e/m = -1.76*10۸ c/g
One of Thomson s primary goals in his cathode-ray tube experiments was to gain an understanding of the structure of the atoms. He reasoned that since electrons could be produced from electrodes made , electrically neutral, Thomson further assumed that atoms also must contain some positive charge. Thomson postulated that an atom consisted of a diffuse cloud of positive charge with the negative electrons embedded randomly in it. is often called the plum pudding model because the electrons are like raisins dispersed in a pudding (the positive charge cloud), as in plum pudding, a favorite English dessert.
In 1909 Robert Millikan (1968-1953), working at the University of Chicago, performed very clever experiments involving charged oil drops. These experiments allowed him to determine the magnitude of the electron charge (see fig. 2.10). With this value and the charge-to-mass ratio determined by Thomson, Millikan was able to calculate the mass of the electron as 9.11 * 10-31 kilogram.
Radioactivity
In the late nineteenth century scientists discovered that certain elements produce high-energy radiation. For example, in 1896 the French scientist Henri Becquerel found accidentally that a piece of a mineral containing uranium could produce its image on a photographic plate in the absence of light. He attributed this phenomenon to a spontaneous emission of radiation by the uranium, which he called Radioactivity. Studies in the early twentieth century demonstrated three types of radioactive emission: gamma (γ) rays, beta (β) particles, and alpha (α) particles. A γ ray is high-energy "light": a β particle is a high speed electron; an α particle has a 2+ charge , that is ,a charge twice that of the electron and with the opposite sign. The mass of an α particle is 7300 times that of the electron .more modes of radioactivity are now known.