atomic structure

• • John Dalton came up with ‘atom’ from ‘atomos’ meaning indivisible

  his ideas included:

  • atoms couldnt be created or destroyed (✔️)
  • atoms of a given element were identical to each other (half true - isotopes)
  • cannot be broken down into anything simpler ✖️ (subatomic particles)
  • they were spherical and indestructible

  he created the solid sphere model

  
• J.J Thomson discovered the electron by experimenting with cathode ray tubes, found that all atoms had sub-atomic negatively charged particles

  he proposed the plum-pudding model:

  
• Ernest Rutherford discovered the nucleus and the idea that it was at the centre of the atom, and most of the atom’s mass was concentrated in the nucleus.

  • he used the gold foil experiment where a beam of positively charged alpha particles was aimed at foil, where most passed through but some were scattered back
  • this meant the nucleus was also positively charged
  • he believed electrons surrounded the nucleus

  he proposed the nuclear model:

  
• • in 1932, James Chadwick discovered the neutron
• as a result of these discoveries, we know:

  • atoms can be broken down into protons, neutrons, electrons
  • atoms of the same element have an identical number of protons and electrons but differing number of neutrons (called isotopes)
  • protons and neutrons are in the nucleus
  • electrons surround the nucleus in outer shells
  • the nucleus takes up most of the mass of a atom but not most of the SPACE
• structure of an atom:

  • nucleus in the centre of the atom, containing protons and neutrons
  • the neutrons stop the protons from being attracted to electrons and so keeping nucleus intact
  • the electrons orbit the nucleus in shells surrounding it
• proton:

  • relative mass = 1
  • relative charge = +1
• neutron:

  relative mass = 1

  relative charge = 0
• electron:

  • relative mass = negligible
  • relative charge = -1
• an atom contains equal numbers of protons and electrons because:

  • they have equal and opposite charges
  • which means that an atom’s overall charge is neutral because neutrons have a charge of 0
• • the nucleus is very small compared to the atom as a whole
  • the diameter of a nucleus is 100,000 times less than the diameter of an atom
  • most of an atom is empty space
• most of the mass of an atom is concentrated in the nucleus because the relative mass of electrons are much smaller than protons and neutrons, which make up the nucleus
• mass number = number of protons + number of neutrons in an atom of an element
• all atoms of a given element have the same number of protons in the nucleus and this number is unique to that element

  • atomic number = number of protons in the nucleus
• isotopes are different atoms of the same element containing the same number of protons but different numbers of neutrons in their nuclei

  • same atomic number
  • different mass number
• to calculate number of subatomic particles using atomic number and mass number:

  • number of protons = atomic number
  • number of electrons = atomic number (unless ion)
  • number of neutrons = mass number - atomic number
• relative atomic mass: the average mass of an atom of an element compared to 1/12th the mass of an atom of carbon-12
• the relative atomic mass of chlorine is 35.5 because:

  • chlorine naturally exists as two isotopes and so they have differing numbers of neutrons even though they have the same number of protons
  • chlorine-35 and chlorine-37
  • though chlorine-37 has a larger mass, chlorine-35 is more abundant
  • so the relative atomic mass is closer to 35 than 37
• calculating relative atomic mass:

  (mass number x abundance)/total abundance