ATOMIC STRUCTURE Modern theory of matter. LET’S REVIEW Dalton’s atomic theory – Elements are made of atoms ; all atoms of an element are identical, atoms.

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