Atoms

History Democritus of Abdera: 4th century B.C. suggested the existence of atoms 4th century B.C. suggested the existence of atoms No experimental support No experimental support Didn’t explain chemical behavior Didn’t explain chemical behavior

John Dalton 2000 years later 2000 years later Studied the ratio in which elements combined in chemical reactions Studied the ratio in which elements combined in chemical reactions Came up with a hypothesis which later became a theory based on his observations Came up with a hypothesis which later became a theory based on his observations

Atomic Theory John Dalton John Dalton Every element is made of tiny particles called atoms that cannot be subdivided* Every element is made of tiny particles called atoms that cannot be subdivided* Atoms of the same element are exactly alike Atoms of the same element are exactly alike Atoms of different elements can join to form molecules Atoms of different elements can join to form molecules

** Most of the theory is accepted today** One important change is atoms are now known to be divisible One important change is atoms are now known to be divisible Atoms can be broken down into subatomic particles Atoms can be broken down into subatomic particles Atom smashers break them apart Atom smashers break them apart

Subatomic Particles Neutrons Neutrons Neutral charge (no charge) Neutral charge (no charge) Found in nucleus Found in nucleus Protons Protons Positively charged ( ) Positively charged ( ) Found in the nucleus Found in the nucleus Electrons Electrons Negatively charged (  ) Negatively charged (  ) Found outside nucleus in an electron cloud Found outside nucleus in an electron cloud

Comparing Subatomic Particles ParticleCharge Mass (Kg) Location Proton x Nucleus Neutron0 Nucleus Electron 9.11 x Outside Nucleus

Nucleus Center of the atom Center of the atom Tiny Tiny CHARGES OF AN ATOM Atoms are neutral (# of protons = # of electrons) Atoms are neutral (# of protons = # of electrons) Nucleus is positively charged Nucleus is positively charged (Protons= + and Neutrons have no charge) (Protons= + and Neutrons have no charge) FYI: Like charges repel one another and opposite charges are attracted to one another (Watch this)

How big is an atom? 6VdY 6VdY 6VdY 6VdY

Electrons Electrons have energy and the further from the nucleus an electron is the more energy it has Electrons have energy and the further from the nucleus an electron is the more energy it has Found in electron cloud which has energy levels Found in electron cloud which has energy levels There are a maximum amount of electrons that can occupy each energy level. There are a maximum amount of electrons that can occupy each energy level. When an outer level is full the atom is the most stable When an outer level is full the atom is the most stable

J. ThomsonJ. J. Thomson's cathode ray experiment J. Thomson Discovered by J.J. Thompson in 1897 Discovered by J.J. Thompson in 1897 "cathode rays" pass from negative electrode towards positive electrode in an evacuated tube "cathode rays" pass from negative electrode towards positive electrode in an evacuated tube

J. ThomsonJ. J. Thomson's cathode ray experiment J. Thomson hypothesis: cathode rays are streams of electrons hypothesis: cathode rays are streams of electrons calculated mass to charge ratio for electrons by observing bending of cathode rays in electric and magnetic fields calculated mass to charge ratio for electrons by observing bending of cathode rays in electric and magnetic fields

observationshypothesis ray properties are independent of the cathode material... cathode ray stuff is a component of all materials cathode rays bend near magnets... magnets bend the paths of moving charged particles; maybe cathode rays are streams of moving charged particles rays bend towards a positively charged plate. rays impart a negative charge to objects they strike.... cathode rays are streams of negative charges Cathode rays don't bend around small obstacles, cast sharp shadows, can turn paddlewheels placed in their path, and travel in straight lines

Rutherford Scattering In the early part of the last century, the accepted model of the atom was proposed by J J Thompson in his plum pudding model. This consisted of a matrix of protons in which were embedded electrons. In the early part of the last century, the accepted model of the atom was proposed by J J Thompson in his plum pudding model. This consisted of a matrix of protons in which were embedded electrons. Ernest Rutherford (1871 – 1937) used alpha particles to study the nature of atomic structure with the following apparatus: Ernest Rutherford (1871 – 1937) used alpha particles to study the nature of atomic structure with the following apparatus:

Rutherford was using alpha particles (helium nuclei) as nuclear bullets to smash up the atoms; he wanted to see atoms bursting like watermelons. But… Rutherford was using alpha particles (helium nuclei) as nuclear bullets to smash up the atoms; he wanted to see atoms bursting like watermelons. But… His observations are best illustrated with this diagram His observations are best illustrated with this diagram

Instead of bits of atom, Rutherford found that a small proportion of the alpha particles were deflected, while an even smaller proportion bounced right back. From analysis of these observations he concluded: Instead of bits of atom, Rutherford found that a small proportion of the alpha particles were deflected, while an even smaller proportion bounced right back. From analysis of these observations he concluded:

Conclusion Most of the atom was empty space. Most of the atom was empty space. The positive charge was concentrated in a very small space The positive charge was concentrated in a very small space The radius of the nucleus was in the order of 3  m. The radius of the nucleus was in the order of 3  m. The alpha particles that were deflected back had to be travelling in a line with the nucleus. The alpha particles that were deflected back had to be travelling in a line with the nucleus.

Rutherford’s estimates were not far out. Later research has shown the nuclear radius to be in the order of 1.5  m. However the boundary is not sharp, but rather fuzzy, as the nucleus is a very dynamic entity. Rutherford’s estimates were not far out. Later research has shown the nuclear radius to be in the order of 1.5  m. However the boundary is not sharp, but rather fuzzy, as the nucleus is a very dynamic entity.

Composition of the Nucleus Composition of the Nucleus nuclei are composed of "nucleons": protons and neutrons nuclei are composed of "nucleons": protons and neutrons Measured in atomic mass units (amu) Measured in atomic mass units (amu)

Bohr’s Model Suggested that electrons in an atom move in set paths around the nucleus much like the planets orbit the sun Suggested that electrons in an atom move in set paths around the nucleus much like the planets orbit the sun Later evidence suggest they behave more like waves on a vibrating string Later evidence suggest they behave more like waves on a vibrating string

Finding electrons Like the fan, the likelihood of finding an electron in a given location is by shading. Like the fan, the likelihood of finding an electron in a given location is by shading. The darker the shade the better the chance of finding the electron The darker the shade the better the chance of finding the electron The whole region shaded is the electron cloud The whole region shaded is the electron cloud The regions they are found are called orbitals The regions they are found are called orbitals They may occupy four different kinds of orbitals They may occupy four different kinds of orbitals

Lewis Dot Structures G.N. Lewis used dots to represent the valence electrons in his teaching of chemical bonding. G.N. Lewis used dots to represent the valence electrons in his teaching of chemical bonding. He eventually published his theory of chemical bonding in He eventually published his theory of chemical bonding in He put dots around the symbols so that we see the valence electrons for the main group elements. He put dots around the symbols so that we see the valence electrons for the main group elements. Formation of chemical bonds to complete the requirement of eight electrons for the atom becomes a natural tendency. Formation of chemical bonds to complete the requirement of eight electrons for the atom becomes a natural tendency.