ATOMIC STRUCTUREModern theory of matter
LET’S REVIEW
Dalton’s atomic theory – Elements are made of atoms; all atoms of an element
are identical, atoms of one element are different from atoms of other elements, atoms can combine to form compounds. Compounds always have same relative numbers and types of atoms. Atoms are indivisible in chemical processes. Chemical reactions simply rearrange atoms.
Questions arose from Dalton’s theory: What causes atoms to “stick together” to form
compounds? What is an atom like? What is its internal
structure?
SCIENTISTS NEEDED EVIDENCE TO HELP THEM UNDERSTAND ATOM’S INTERNAL STRUCTURE
JJ Thomson – 1890s Cathode ray tube Evidence: any
element can be made to emit negatively charged particles
Conclusion: all types of atoms must contain these negative particles.
He called them electrons
JJ THOMSON – OTHER CONCLUSIONS
Thomson knew that atoms were neutral – they didn’t have a charge
This meant that there also had to be a positive particle in the atom to counterbalance the negative charge of the electron
PLUM PUDDING (OR CHOCOLATE CHIP COOKIE) MODEL
Diffuse positive “pudding” (or cookie dough0
Negative charges inside like raisins (or chocolate chips)
NO MORE PLUM PUDDING! Ernest Rutherford Shot positively
charged α-particles at thin gold foil
Evidence: most alpha particles went through foil, but some got deflected straight back
Conclusion: something concentrated and positive in center of atom – rest of atom mostly empty space
THE NUCLEAR ATOM
Atom with dense center of positive charge (the nucleus) around which tiny electrons move in a space that is otherwise empty
BOHR MODEL
Electrons revolve around positive nucleus (which also contains neutrons)
Electron charge (-) same same magnitude (but opposite) of proton charge (+)
MORE CURRENT VIEW
Tiny nucleus (10-13 cm diameter)
Electrons move about nucleus (10-8 cm or so away)
Neutron and proton have similar mass
Electron about 2000 times smaller than neutron and protons
Electrons “intermingle” to bond to other atoms
NUMBER OF ELECTRONS AN ATOM HAS AFFECTS HOW IT INTERACTS WITH OTHER ATOMSAtoms of different elements, with different numbers of electrons, behave differently
ISOTOPES
Neutral atom has equal numbers of protons and electrons
All atoms of particular element contain same number of protons and electrons
E.g. Na (Sodium) – 11 protons, 11 electrons Some atoms of Na contain 12 neutrons; some
contain 13 neutrons ISOTOPES = same type of atoms – same
protons and neutrons – but different number of neutrons
HOW TO SPECIFY ISOTOPES
For Example:
€
Atomic number (number of protons)Mass Number (sum of protons and neutrons)ElementSymbol
€
1123Na
€
1124Na
12 neutrons 13 neutrons
HOW TO DETERMINE NUMBER OF NEUTRONS IN AN ATOM
Mass number – Atomic Number = # Neutrons
12 – 6 = 6 Neutrons
Mass number (number of protons + number of neutrons)
Atomic Number (number of protons)
€
612C
YOU DO IT
Mass number – Atomic Number = # Neutrons
Mass number
Atomic Number
€
613C
13 – 6 = 7 Neutrons
YOU DO IT
Mass number – Atomic Number = # Neutrons
Mass number
Atomic Number
€
80201Hg
201 – 80 = 121 Neutrons
THE END