Science atom

• Around 400 BC, Greek philosopher Democritus proposed the theory that everything in the world was made of tiny indestructible particles called "atomos", meaning "uncuttable"
• Democritus believed that the properties of materials depended on the type of atomos they were composed of, with sour or sharp-tasting things made of particles with pointy edges, sweet stuff made of more rounded or smooth atoms, and metals composed of hard atoms
• Aristotle later discredited Democritus' atomic theory, proposing that everything on Earth was made of four elements: earth, fire, water, and air
• John Dalton, a British chemist, proposed the theory of atomism, suggesting that everything in the world was made up of atoms—tiny indestructible solid spheres unique for every element
• Dalton's experiments led to the law of multiple proportions and the theory that atoms of different elements combine to form different compounds and are rearranged during chemical reactions
• J.J. Thompson's experiments with cathode rays led to the "plum pudding" model of the atom, where an atom is composed of a positively charged mass (the pudding) with tiny negative charges embedded in it (like plums)
• Ernest Rutherford's gold-foil experiment disproved the plum pudding model and proposed an atomic structure where most of the atom's mass was concentrated in a positively charged center (the nucleus) around which electrons orbited like planets around the sun
• Niels Bohr introduced the concept of quantized energy and proposed that electrons moved around the nucleus in fixed orbits or shells, with lower energy closer to the nucleus and higher energy farther away
• Erwin Schrödinger's quantum mechanical model of an atom described electrons not as revolving around the nucleus in circular orbits, but as electron clouds in an atomic orbital, where the probability of finding an electron is highest
• Schrödinger-wave equations help accurately calculate the energy levels of electrons in an atom in the quantum mechanical model
• Atoms are made up of protons, electrons, and neutrons
• There are 118 different kinds of atoms, each being the smallest part of an element
• The number of protons inside an atom determines the element it is
• The number of protons in an atom is also called its atomic number
• Electrons are much smaller than protons and zip around protons at lightning speed, forming an electron cloud around the atom
• Electrons have a negative electrical charge, which is attracted to the positive electrical charge of protons, creating the electric force
• The electric force is the pull between the positive protons and the negative electrons
• Neutrons are needed in atoms to prevent the positively charged protons from pushing each other out of the atom
• The strong nuclear force, or strong force, between neutrons and protons acts like glue to hold the nucleus together
• Proton experiment

  Proton moves towards the negative plate in an electric field
• The mass of an electron is approximately 0 amu (atomic mass unit)
• Subatomic particles of an atom


  • Proton
  • Electron
  • Neutron
• Electron experiment

  Electron moves towards the positive plate in an electric field
• The mass of a neutron is 1 amu (atomic mass unit)
• Shells


  The circular path around the nucleus where subatomic particles like protons and neutrons stay
• How do we know the charges of subatomic particles

  1. Electron experiment: Placing electron in an electric field, it moves towards the positive plate due to its negative charge

  2. Proton experiment: Placing proton in an electric field, it moves towards the negative plate due to its positive charge
• Nucleus


  The small central and dense region of an atom
• Some pieces of paper fell off while others stuck on
• Rubbing off negative charge

  1. Negative charge is transferred to the balloon

  2. Positive charges are left on the pieces of paper
• Negative charge on balloon

  Attracts positive charges on the pieces of paper
• Positive and negative charges attract
• All matter is made up of charges - positive and negative charges
• Schrodinger's quantum mechanical model


  • Most accurate model for electrons of an atom
  • Also known as the electron cloud model
• Electron was discovered by J. J. Thomson in 1897 when he was studying the properties of cathode ray
• The proton was discovered by Ernest Rutherford in 1909 during his famous gold foil experiment
• Johannes Wilhelm Geiger was born on September 30, 1882, and died on September 24, 1945. His greatest accomplishment was being the co-inventor of the Geiger counter, a device used for detecting and counting atomic particles
• James Chadwick (1891 - 1974) discovered the neutron in 1932. His discovery was directly influential in the discovery of atomic fission and development of the atom bomb
• Rutherford's model of the atom was rejected because it failed to obey the laws of physics as they were known at that time
• Cathode ray is a stream of particles coming out from the negatively charged electrode (cathode) to the positively charged electrode (anode)
• Properties of cathode ray particles: They travel in straight lines, are independent of the material composition of the cathode, and applying an electric field in the path of cathode ray deflects the ray towards a positively charged plate