Atomic structure

Atomic structure

1. The Atom

   • Atoms are made from protons, neutrons and electrons
     • Electrons
       • -1 charge
       • Whizz around the nucleus in orbitals
       • Take up most of the volume of an atom
     • Nucleus
       • Where you'll find the protons and neutrons.
         • Protons +1 charge
         • Neutrons 0 charge- neutral
       • The diameter of the nucleus is small compared to the whole atom
       • Most mass of an atom is concentrated in the nucleus
   • Nuclear symbols show numbers of subatomic particles
     • Mass number- tells you the total number of protons and neutrons in the nucleus
     • Atomic (proton) number- number of protons in the nucleus (identifies the atom)- all atoms of the same element have the same number of protons.
     • Neutral atoms with no overall charge the number of electrons is the same as the number of protons.
     • The number of neutrons is just the mass number minus the atomic number
   • Ions have different numbers of protons and electrons.
     • Negative ions have more electrons than protons.
     • Positive ions have fewer electrons than protons.
   • Isotopes are atoms of the same element with different number of neutrons.
     • Its the number and arrangement of electrons that decides the chemical properties of an element.
       • Isotopes have the same configuration of electrons so have the same chemical properties.
     • Isotopes of an element do have slightly different physical properties though such as different densities and rate of diffusion.
       • This is because physical properties tend to depend more on the mass of the atom.

2. Atomic models

   • The accepted model of the atom has changed throughout history
     • The one we need to learn is currently excepted but in the past completely different models were.
       • As new evidence is found models are modified.
         • 19th century John Dalton described atoms as solid spheres an dsaid that different spheres and made form different elements.
     • 1897 JJ Thompson -experiments concluded atoms weren't solid and indivisible.
       • Measurments of charge and mass show atoms must contain even smaller negatively charge particles (electrons)
         • Solid sphere idea of atomic structure had to be changed- became known as the PLUM PUDDING MODEL
   • RUTHERFORD showed that the plum pudding model was wrong.
     • 1909 Ernest Rutherford and his students Hans GREIGER and Ernest Marsden conducted the famous gold foil experiment.
       • Fired alpha particels at extremely thin sheets of gold.
       • From the plum pudding model they expected most of the alpha particles to be deflected very slightly by positive 'pudding' that made up most of the atom.
         • Most alpha particles passed straight through the gold and only a very small number were deflected backwards.
           • Plum pudding model couldn't be right.
       • Rutherford came up with a model that could explain the new evidence- nuclear model of an atom.
         • Tiny positively charges nucleus at centre surrounded by a cloud of negative electrons most of teh atom is empty space.
   • The refined BOHR model
     • Niels Bohr model got pretty close to the accepted one.
       • Has four basic principles.
         • Electrons can only exist in fixed orbits or shells and not anywhere in between
         • Each shell has a fixed energy
         • When an electron moves between shells electromagnetic radiation is emitted or absorbed
         • Energy of shells are fixed- radiation has a fixed frequency.
       • Wasn't 100% correct- had to refine it to include sub shells but showef radiation emitted and absorbed by atoms.
     • Bohr model also explained why some elements (noble gases) are inert
       • Shells of an atom can only hold fixed numbers of elctrons
       • Elements reactivity is due to its electrons
       • When an atom has full shells of electrons it is stable and does not react
   • More than one model pf atomic structure is used today.

3. Relative Mass

   • Relative masses are masses of atoms compared to carbon-12
     • The mass of an atom is very very tiny
     • The relative atomic mass is the average mass of an atom of an element on a scale where an atom of carbon-12 is 12.
     • Relative isotopic mass is the mass of an atom of an isotope of an element on a scale where an atom of cabon-12 is 12.
     • The relative molecular mas is the average mass of a molecule or formula unit on a scale where an atom of caron-12 is 12.
   • Relative masses can be measured using a mass spectrometer