gen chem

• An ionic compound results from an attraction between a cation and an anion. The attraction between oppositely charged sodium and chloride ions form the ionic compound, sodium chloride. An ionic compound assumes a crystalline structure, a three-dimensional well-ordered arrangement of cations and anions.
• When a neutral atom gains or loses one or more electrons, it becomes an electrically charged particle called ions.
• Metals tend to lose electrons and become positively charged cations.
• Nonmetals, on the other hand, gain electrons and become negatively charged anions.
• The number of electrons lost or gained is the charged number. For example, the sodium atom can lose one electron, forming an atom with a charge of +1. 
• An ion that consist of only one atom is referred to as a monatomic 
• Different atoms can also combine and form polyatomic ions.
• • When atoms of nonmetal shares electrons, they form an electrically neutral aggregate called molecule.
• In 1808, John Dalton published his book A New System of Chemical Philosophy, where he proposed an atomic theory of matter that can explain chemical observations as predicted by the three fundamental laws. The atomic theory comprised the following postulates:
• After the postulation of Dalton's atomic theory, numerous experiments and inferences allowed scientists to conceptualize the structure of the atom and discover the subatomic particles :

  1. electrons
  2. protons
  3. neutrons
• in 1897, British physicist Joseph John Thomson (1856-1940) discovered the electron while studying the nature of cathode rays. He observed that cathode rays in an evacuated tube, called cathode ray tube (CRT), are deflected by a negatively charged plate and attracted by a positively charged
• • In 1910, American physicist Robert Millikan_(1868-1953) published the result of his oil-drop experiment through which he determined the actual charge of the electron to be 1.592×10-" coulomb, just slightly lower than the presently accepted value of 1.602x10-1 coulomb.
• Rutherford, who was a former student of Thomson, performed the gold foil experiment in 1909
• The discovery of protons is credited to Ernest Rutherford (1871-1937). In 1911, he bombarded nitrogen gas with alpha particles from a radioactive source and found that some of these particles were scattered at large angles. This led him to conclude that there must be small positive centers within the nucleus of the atom.
• • British physicist James _Chadwick (1891-1974), a former student of Rutherford who later became his co-researcher, discovered the other type of particle in the nucleus the neutron.
  • The nucleus at the center consists of protons and neutrons, collectively known
• in 1913, British physicist Henry Moseley (1887-1915) found a relation between the X-ray spectra of certain elements and their atomic numbers.
• Atoms of the same element, however, can have different number of neutrons.

  This means that they have different mass numbers but the same atomic number.

  These atoms are referred to as isotopes,
• The emergence of chemistry as a discipline of science began when Anglo-Irish philosopher Robert Boyle (1627-1691) published The Sceptical Chymist in 1661. In this book, he defined the "element" as the simplest composition of matter that cannot be broken down further by any chemical means.

  He also suggested that atoms of elements combine to form different "compounds."
• interest on the concept of elements and compounds heightened when English scientist Joseph Priestley (1733-1804) isolated oxygen gas, which he called "dephlogisticated air" by heating mercury oxide (HgO). French chemist Antoine-Laurent Lavoisier (1743-1794) learned about this dephlogisticated air from Priestley and carried out further experiments on the gas. He found out that the gas, which he already called

  "oxygen," is involved in combustion and
• In 1774, Lavoisier formulated the law of conservation of mass, which states that in a chemical reaction, the mass of the substances produced is equal to the mass of the substances reacted.