Atomic Structure
• Atom = indivisible (in Greek)
• Epicurus explained that matter can be broken down into very small invisible particles called atoms.
• John Dalton proposed the atomic theory and explained the law of chemical combination.
• Atom is the smallest possible amount of matter which still retains its identity as a chemical element.
Introduction
• Atom is the smallest possible amount of matter which still retains its identity as a chemical element, consisting of a nucleus surrounded by electrons.
• An atom is composed of a positively charged centre termed as
“nucleus” and the central nucleus is surrounded by negatively
charged electrons.
Dalton’s Atomic law
• All matter is made of atoms, and atoms are indivisible.
• All atoms of a given element are identical in mass and properties. The atoms of different elements vary in size and mass.
• Compounds are combinations of two or more different types of atoms.
• A chemical reaction is a rearrangement of atoms.
Dalton based his theory on;
• the law of conservation of mass
• the law of constant composition
• The law of conservation of mass says that matter is not created or destroyed in a closed system.
• The law of constant composition says that a pure compound will always have the same proportion of the same elements.
For example,
• Carbon may produce CO and CO2 with oxygen.
• 1 mol of CO includes 12 g carbon and 16 g oxygen.
• 1 mol of CO2 includes 12 g carbon and 32 g oxygen.
• There is a ratio of 32/16=2 between the oxygen amounts that react with carbon.
Electron
• J.J Thompson proposed that an atom constitutes of at least one negatively charged particle called electron.
• The charge of an electron is the negative charge of 1.6×10-
19 coulombs.
• The relative mass of an electron is considered as 0.
Proton
• Rutherford discovered protons with his gold foil experiment.
• The absolute charge of a proton is the positive charge of 1.6×10-
19 coulomb.
• The mass of a proton is 1.6×10 -24 g and is considered 1 that is mass of a hydrogen atom.
Neutron
• Neutron was discovered by James Chadwick.
• Neutron is represented by “n” and is considered a neutral particle.
• The mass of a neutron is measured to be 1.6 x 10 -24 g.
Millikan oil-drop experiment
• Millikan determined the size of the charge on an electron in 1909 with an experiment called «oil-drop experiment»
• Millikan created an electric field in a chamber and sprayed droplets of oil. Some oil drops became electrically charged and rise in spite of the gravity.
• m.g = E.q
• Since the mass (m), gravity (g), and (E) is known, the charge of an electron (q) can be calculated.
Atomic number
• The number of protons in the nucleus of the atom is equal to the atomic number (Z).
• Henry Moseley discovered the atomic number of each element using x-rays, which led to more accurate organization of the periodic table
