Atomic History and Structure
Atomic Timeline
• Dalton (Indivisible Spheres)
• Thompson (Plum Pudding Model)
• Rutherford (Nuclear Model)
• Bohr (Planetary Model)
• Schrodinger/Heisenberg (Quantum Mechanical Model)
Atomic Structure
Subatomic Particles
Nucleus: Protons - decides identity of atom
Neutrons - hold protons in nucleus together
Protons and Neutrons are a type of subatomic particle called a Hadron. Hadrons are composed of
quarks (elementary particles/fermions that make up matter) and gluons (elementary
particles/fermions that hold quarks together). Quarks in hadrons are either up or down quarks and
determine the charge of particles.
Electron Cloud: Electrons - determines chemical properties
Electrons are a type of Lepton which is an elementary
particle/fermion (like quarks and gluons), which are very small.
All fermions (including electrons) cannot occupy the same
space at the same time, so they spread far apart.
Forces in Atoms
Electrostatic Force
• The force between particles due to their charge
• Like charges repel each other
• Opposite charges attract each other
• Coulombic force
• Coulomb's law (the charges of the particles and the distance
between them tells how strong the force is).
Electrostatic force in the atom: electrons are attracted to protons in the nucleus of the atom, which
keeps them attached to the atom. However, the electrons repel each other, which causes them to
buzz around in its own region/orbit, causing the electron cloud to take up a LOT of space. Protons
also repel each other, so...
• Some of the mass of protons and neutrons is converted to energy to hold the nucleus together
Strong Nuclear Force:
• An attractive force between all nuclear particles at short distances
• The nuclear force dominates the repulsive Coulomb force
• Nuclear force is due to the binding energy of the nucleus. This energy comes from the Mass
Defect in the nucleus (when you add up the mass of individual protons and neutrons in an atom,
the number is usually a little more than the atom actually weights. This lost mass is the mass
defect)
• E=mc2 tells us how much binding energy an atom has based on the mass defect. The mass
that is lost can be multiplied times the speed of light (c) squared to find the binding energy of an
atom.
*Neutral charge of neutrons also acts as both a buffer/glue holding the nucleus together
Atomic Radii
Atomic Radius is a measure of the distance from the edge to
the center an atom – the radius of the “spherical” atom.
Different elements have different atomic radii due to
electrostatic forces in the atom.
Nuclear Charge is is the positive attractive charge the nucleus
exhibits on electrons and increases as the number of protons increases.
Electron Sheilding is the repulsive effect that electrons in
lower energy levels have on outer electrons.
Effective Nuclear Charge is the results force between electrons due to both nuclear charge and
electron sheilding. In other words, electrons are both pulled into the nucleus and blocked from it by
other electrons at the same time. This force depends on the location of the electron.
Atomic Radii Trends
As the number of protons in an atom increase, the
atomic radius gets smaller.
The periodic table is organized by increasing atomic
number (this is the number of protons in an atom).
So as you go across a row/period, the atomic radius
decreases.
However, we know that electrons are added to the
atom at different energy levels - we can think of
these orbits as layers on an atom. When there are
more layers, the atom gets bigger. The atomic
radius also increases because the electron layers
are repelling each other. So, when you go down a
row/group, the atomic radius increases.