1. Mr. Conkey Physical Science Chapter 4
Atomic Structure
Mr. Conkey Physical Science Chapter 4

2. Ancient Greek Atomic Models
Philosopher Democritus – believed all matter was made up of extremely small particles that could not be divided
He called them atoms from the Greek word atomos meaning “uncut” or “indivisible”
He thought that atoms had specific shapes such as rough and prickly for solids and round and smooth for liquids

3. Dalton’s Atomic Theory
Dalton – gathered evidence for atom’s existence by measuring the masses of elements that combine when compounds form
He found that all compounds have a fixed composition
He proposed a theory that all matter is made up of individual particles called atoms, which cannot be divided

4. Dalton’s Theory: Main Points
All elements are composed of atoms
All atoms of the same element have the same masses; different elements = different masses
Compounds contain atoms of more than one element
Atoms of different elements always combine in the same way

5. Thompson’s Atomic Model
J.J. Thompson – experimented using an electric current connected to a positive and negative metal plate
He found that a glowing beam (stream of charged particles) was deflected (bent) due to it’s being repelled by the negatively charged plate and attracted by the positively charged one
He hypothesized that the negatively charged particles came from within atoms!
His experiments provided first evidence of atoms consisting of smaller particles

6. Thomson’s Model
Negative charges are evenly spaced throughout the atom filled with a positively charged mass

7. Rutherford’s Atomic Theory
Discovered that fast-moving positively charged particles are emitted from uranium; he called them alpha particles
He asked Marsden to find out what happens to the particles when they pass through a thin gold sheet
Marsden aimed a narrow beam of particles at the gold and found that some particles were deflected, some by more than 90 degrees
Rutherford then concluded that the positive charge is not spread throughout the atom, but is concentrated in a small central area he called the nucleus
Particles deflected more than 90 degrees came very close to a nucleus

8. Rutherford’s Model
Nucleus – a dense positively charged mass located in the center of the atom

9. The structure of an Atom

10. Atomic and Mass Numbers
Talk with your shoulder partner and try to determine the significance of the atomic number…
How about the atomic mass number?
Where do these numbers come from?

11. Atomic and Mass Numbers (cont.)
Atomic # = the number of protons in an atom of that element
Atomic Mass # = the sum of protons and neutrons in the nucleus of that atom
Number of neutrons = Mass # - Atomic #
Isotopes – atoms of the same element that have different numbers of neutrons and different mass #s
Isotopes of an element have the same atomic # but different mass #s due to differing #s of neutrons
Correction: Hydrogen-1 has no neutrons, hydrogen-2 has 1 neutron (heavy water), and hydrogen-3 has 2-neutrons!

12. Bohr’s Atomic Model
Niels Bohr – a Danish physicist who developed a model that had a similar nucleus to Rutherford’s model but also included
Bohr’s model focused on electrons moving around the nucleus in fixed orbits and at constant speeds
An atom can gain or lose energy, which can change the energy of an electron(e −)
Energy levels – possible energies that electrons in an atom can have and the levels in which they exist due to those energies.
Worked with Rutherford for a time

13. Bohr’s Model (cont.)
An electron can move up energy levels if energy is gained and down if energy is lost
Lowest level
Highest level

14. Energy Levels
Energy the electrons gained when moving up energy levels as well as the loss of energy when moving down them is measurable
Energy is released as different colors of light when electrons move back to lower energy levels after atoms gain energy from an external source
Electron cloud models – describe the possible locations of electrons around an atom’s nucleus
Draw nucleus with 4 rings around it. Then arrows up with one color = energy must be absorbed and arrows down another color for energy being released

15. Visual Representation of Electron Movement

16. Atomic Orbitals
Orbital – a region of space around an atom’s nucleus where electrons are likely to be found
An electron cloud is a good indicator of how electrons act in their orbitals

17. Atomic Orbitals (cont.)

18. Electron Configuration
Electron configuration – the arrangement of electrons in the orbitals of an atom
When electrons are in orbits with the lowest possible energies they are most stable
Ground state- when all electrons of an atom have the lowest possible energy
Excited state- when an electron is promoted (moves) to a higher energy level

19. First video of an electron (2008)

20. The End...or is it?