5.1 Atoms & Isotopes

• Atoms

  Building blocks of all matter

  Really small, radius of only 1 × 10-10 m
• Nucleus

  Atoms have a tiny, dense nucleus at their centre, with electrons orbiting around the nucleus

  Radius of nucleus -> smaller than the atom, but contains almost all of the mass of the atom
• Parts of the Atom

  The nucleus contains -> Protons : positively charged particles, mass of 1

  Neutrons : no charge, mass of 1

  Electrons -> negative charge, almost no mass
• Electron Structure

  Electrons in an atom orbit around the nucleus at a particular distance, known as energy levels

  A certain number of electrons can occupy each energy level

  Eg. only 2 electrons can orbit in the first energy level

  The higher the energy level, the further the distance of the electron from the nucleus, the greater potential energy
• Electrons & Protons

  Although, atoms contain particles of different charge, the total charge within an atom is zero -> the number of electrons = number of protons

  If an atom loses electrons, then it is said to be ionised

  Eg. Lithium

  
• The Absorption & Emission of EM Radiation

  Electrons in an atom are arranged in energy levels -> lower energy levels are closer to the nucleus, whilst higher energy levels are further away from the nucleus of the atom

  These electrons absorb energy

  This happens when waves of electromagnetic radiation (like light & heat) hit them

  When electrons absorb electromagnetic radiation, they move to a higher energy level

  
• Dark coloured objects are good absorbers of radiation -> they appear darker because they do not reflect the energy that hits them
• What happens to an electron when it moves up an energy level?

  It becomes unstable.
• Why does an electron eventually move back down to its original energy level?

  Because it is unstable at the higher energy level.
• What occurs when an electron moves back down to its original energy level?

  It emits a wave of electromagnetic radiation.
• What type of radiation is emitted when an electron moves down energy levels?

  Electromagnetic radiation.
• How are colors in the visible spectrum produced?

  • By electrons moving down energy levels
  • Emitting electromagnetic radiation
• What is the source of light waves in the context of electron energy levels?

  Electrons moving down energy levels and emitting electromagnetic radiation.
• Atomic Number

  The number of protons in an atom

  Elements in the periodic table are ordered by their atomic number

  Therefore, the number of protons determines which element an atom is

  Atomic number of an element is always the same

  eg. Hydrogen's atomic number is 1, it always has 1 proton

  Also, is equal to the number of electrons in an atom
• Atomic Mass Number 


  The total number of particles in the nucleus of an atom

  The mass number is the number of protons and neutrons in the atom

  Number of Neutron = Mass Number - Atomic Number

  For example if, sodium atom has a mass number of 23 and an atomic number of 11, then the number of neutrons would be 23 – 11 = 12
• Nuclear Notation

  The mass number & atomic number of an atom are shown by writing them with the atomic symbol called, nuclear notation

  E.g.

  
• Isotopes

  Although, the number of protons in an element is always the same, the number of neutrons can be different

  Isotopes are atoms of the same element that have an equal number of protons, but different number of neutrons

  Isotopes occur naturally, some are rare than others

  
• Difference between Isotopes

  The number of neutrons in an atom doesn't affect the chemical properties of an atom, like charge but only its mass

  This is because neutrons have no charge but do have mass

  Some isotopes are unstable, as they have an imbalance of protons and neutrons
• Example of Difference between Isotopes

  
• What can happen to electrons in the outer energy level of an atom?

  They can be knocked out from an atom.
• What are the ways electrons can be removed from an atom?

  • Rubbing objects together (friction)
  • Absorbing electromagnetic radiation
  • Chemical reactions
• What happens when electrons absorb electromagnetic radiation?

  They can gain enough energy to leave the atom.
• What occurs when one or more electrons are removed from an atom?

  The atom becomes positively charged.
• Why does an atom become positively charged when electrons are removed?

  Because an electron is negatively charged.
• What is formed when an atom loses one or more electrons?

  The atom becomes a positive ion.
• What is an ion?

  An ion is an atom or particle with a non-zero charge.
• When radiation passes close to atoms it can knock electrons out, leaving the atom with an overall positive charge

  Ions are more chemically reactive than atoms because of their positive charge

  
• Atomic Models

  Scientists discovery of atoms has changed over time

  Different models have been developed, and then replaced as new evidence from experiments is discovered
• Democritus Belief 


  That although objects could not be cut into smaller pieces, the smallest possible piece would be indivisible (it couldn't be cut further)
• Atoms were believed to be tiny spheres that could not be divided before the discovery of the electron

  Later models described the atom as small, solid spheres
• Alpha Scattering Experiment - Ernest Rutherford 


  They were directing a beam of alpha particles at a thin sheet of gold foil

  They expected the alpha particles to travel through the gold foil

  Instead they discovered :

  • most of the alpha particles passed straight through the foil
  • some of the alpha particles changed direction but continued through the foil
  • a few of the alpha particles bounced back off the gold foil
• Nuclear Model - Rutherford

  • nearly all the mass of the atom is concentrated in the centre of the atom (in the nucleus)
  • the nucleus is positively charged
  • negatively charged electrons orbit the nucleus at a distance
• The Nuclear Model replaced the Plum Pudding Model as it could better explain the observations of Rutherford's Scattering Experiment
• Bohr Model of the Atom 


  • electrons orbit the nucleus at different distances
  • the different orbit distances are called energy levels
  • up to 2 electrons orbit 1st energy level
  • up to 8 electrons orbit 2nd energy level
  • up to 8 electrons orbit 3rd energy level
• Successes of the Bohr Model 


  Became the accepted model because :

  It was able to explain findings from different experiments better than the nuclear model of the atom

  It was able to explain the processes of absorption and emission of electromagnetic radiation

  Theoretical calculations made using the Bohr model agreed with experimental results
• Worked Example : An alpha particle is the nucleus of a Helium atom (He4,2)

  Determine the charge of an alpha particle

  Step 1 : Determine number of protons in the particle

  Alpha particle has 2 protons

  Step 2 : Determine number of electrons

  Since alpha particle is a helium nucleus, no electrons

  Step 3 : Determine charge of alpha particle

  total charge = sum of protons - sum of electrons

  2-0=2 -> charge on alpha particle is +2
• Comparing Atomic Models

  Rutherford's alpha scattering experiment led to a change in the atomic model

  The Plum Pudding Model was replaced by the nuclear model

  The nuclear model could explain why the alpha particles bounced back from the gold foil
• Differences between Plum Pudding Model & Nuclear Model 


  Plum Pudding model - the atom is mostly positively charged 'dough' while Nuclear model - atom mostly empty space

  Plum Pudding model - electrons are distributed throughout the atom, while Nuclear model - electrons orbit the nucleus at a distance

  Plum Pudding model - mass of atom is evenly distributed, while Nuclear model - almost all of mass of atom is concentrated in the positively charged nucleus