atoms

• Robert Boyle defined the "element" as the simplest composition of matter that cannot be broken down further by any means
• Joseph Priestly isolated gas, which he called "dephlogisticated air" by heating mercury oxide (HgO)
• Antoine-Laurent Lavoisier learned about dephlogisticated air from Priestly and carried out further experiments on the gas
• Law of conservation of mass: silver nitrate + sodium chromate → silver chromate + sodium nitrate
• Joseph-Louis Proust stated the Law of definite proportions, which states that any sample of a given compound will always be composed of the same elements in the same proportion by mass
• John Dalton proposed the Law of multiple proportions, which states that for elements that can form different compounds, masses of the second element that can combine with a fixed mass of the first element are in a ratio of small whole numbers
• John Dalton's Atomic Theory: Matter is made up of extremely small indivisible particles called atoms
• Postulates of Atomic Theory:

  • Atoms of the same element are identical, and are different from those of other elements
  • Compounds are formed when atoms of different elements combine in certain whole-number ratios
  • Atoms rearrange only during a chemical reaction to form new compounds
• John Dalton proposed that all matter is made of atoms, which are too small to see, indivisible, and indestructible. All atoms of a given element are identical
• J.J. Thompson discovered the negative electron and predicted the existence of a positive particle to hold the electrons in place
• Ernest Rutherford discovered the nucleus of an atom and named the positive particles in the nucleus "protons". He concluded that electrons are scattered in empty space around the nucleus
• James Chadwick discovered that neutrons were also located in the nucleus of an atom and that they contain no charge
• Niels Bohr concluded that electrons are located in planet-like orbits around the nucleus in certain energy levels
• The Modern Atomic Theory states that electrons do not orbit the nucleus in neat planet-like orbits but move at high speeds in an electron cloud around the nucleus
• After the postulation of Dalton's atomic theory, scientists conceptualized the structure of the atom and discovered the subatomic particles - electrons, protons, and neutrons
• Joseph John Thomson discovered the electron while studying the nature of cathode rays
• The nucleus of an atom consists of protons and neutrons
• Wilhelm Conrad Roentgen discovered X-rays and Henri Becquerel discovered the radioactivity of uranium, leading to investigations on radiation
• In 1920, Rutherford discovered the proton and proposed the existence of another subatomic particle in the nucleus to hold the positive protons together despite having similar charges
• Atoms consist of an electron cloud and a nucleus
• Three subatomic particles make up every atom:

  • Proton: Positive (+) charge, located in the nucleus or "core"
  • Neutron: No charge (0), located in the nucleus or "core"
  • Electron: Negative (-) charge, located in the electron cloud
• Atoms of the same element can have different numbers of neutrons, resulting in different mass numbers but the same atomic number
• Isotopes are atoms of the same element with different numbers of neutrons, leading to different mass numbers but the same atomic number
• Sample Problem: Cobalt has an atomic number of 27 and a mass number of 60, with 33 neutrons
• Ions and Molecules: Neutral atoms can either exchange or share electrons depending on the nature of the interacting atoms