1. Atomic Structure

2. Atoms
True or False:
An atom is the smallest particle of an element that retains its identity in a chemical reaction.
True
False

3. Good Job!!!
That is correct!!!

4. Sorry.
That is incorrect.

5. Atoms Atoms – the smallest particle of an element
- retains its identity in a chemical reaction

6. Early Models of the Atom
Democritus’s Atomic Philosophy
- was one of the first to suggest existence of atoms
- believed that atoms were indivisible and indestructible
- ideas challenged by Plato and Aristotle

7. Dalton’s Atomic Theory
Dalton transformed Democritus’s ideas on atoms into a scientific theory
- studied ratios in which elements combine in chemical reactions

8. Dalton’s Atomic Theory
All elements composed of tiny indivisible particles called atoms.
Atoms of same element are identical. (Each element has a different type of atom.)
Atoms of different elements can physically mix together or form compounds in whole-number ratios.
Chemical reactions occur when atoms are separated, joined, or rearranged. Atoms of one element never change into atoms of another element due to chemical reactions.

9. Sizing up the Atom Radii of most atoms fall within range of
5 X 10-11m to 2 X 10-10m
Atoms can still be observed
- by scanning tunneling microscopes
- atomic scale = “nanoscale”

10. Sizing up the Atom

11. Subatomic Particles
Most of Dalton’s atomic theory is still accepted today
- the major change is that atoms are known to be divisible
- divide into electrons, protons and neutrons

12. Subatomic Particles

13. Electrons Electron – negatively charged subatomic particles
- discovered by J.J Thomson in 1897
- passed electric current through gas at low pressure
- he sealed gas in tubes with electrodes at both ends
- anode = positive charge
- cathode = negative charge
- cathode ray - glowing beam that traveled from cathode to anode

14. Electrons Thomson noted that cathode ray is deflected by a magnet
Positive charge = attracts cathode ray
Negative charge = repels
Opposite charges attract and like repel
Hypothesized cathode ray is a stream of tiny negatively charged particles
- originally called corpuscles
- now called electrons

15. J.J. Thomson and cathode ray

16. Robert A. Millikan Discovered electron has one unit of negative charge
- also discovered mass is of electron is 1/1840 of that of a Hydrogen atom

17. Protons and Neutrons
If cathode rays are electrons given off by atoms, what remains of atoms that have lost the electrons? What is left?
Atoms have no net electric charge
Electric charges carried by particles of matter
Electric charges always exist in whole-number multiples of single base unit
Neutral particles occur when equal number of positive charges combine with equal numbers of negative charges

18. Protons Protons – positively charged subatomic particles
- has a mass of 1840 times that of electron
- discovered by Eugen Goldstein after observing a cathode-ray tube
- found rays traveling in direction opposite to that of cathode rays
- concluded these to be positive

19. Neutrons Neutrons – subatomic particles with no charge
- mass nearly equal to that of a proton
- discovered by English physicist James Chadwick in 1932

20. Protons and Neutrons

21. Atomic Nucleus Thomson’s model was “plum pudding model”
- thought electrons were evenly distributed with positive charge particles
- model was short lived

22. Ernest Rutherford Ernest Rutherford – disproved “plum pudding model”
- used “gold-foil experiment

23. Atomic Nucleus “Gold-foil experiment”
- directed beam of positively charged alpha particles at thin gold foil
- most particles passed through
- only a few deflected
- discovered positive charges in dense core of atom
- nucleus – tiny central core of an atom that is composed of protons and neutrons
- electrons surround nucleus in electron cloud
- 10,000 times small than radius of overall atom

24. Gold-foil Experiment

25. Atomic Number
Elements differ because they contain different numbers of protons
Atomic Number – number of protons in nucleus of an atom of that element
- identifies an element
- number of protons = number of electrons
- electrically neutral

26. Atomic Number
Atomic Number

27. Mass Number Most of an atoms mass is concentrated in its nucleus
Mass Number – total number of protons and neutrons in an atom
Number of neutrons in an atom is difference between mass number and atomic number
# neutrons=mass number - atomic number

28. Number of Neutrons Example: Carbon: - mass number = 12
- atomic number = 6
Number of neutrons=
12 – 6 = 6 neutrons
Mass Number

29. Examples Atomic # Mass # Beryllium (Be) 4 9 Neon (Ne) 10 20
Sodium (Na)
How many electrons, protons, and neutrons in each?
Be = 4e, 4p, 5n
Ne = 10e, 10p, 10n
Na = 11e, 11p, 12n

30. Isotopes
Isotopes – atoms that have same number of protons but different numbers of neutrons
- also have different mass numbers
Example:
Carbon -12, Carbon-13, and Carbon-14

31. Isotopes

32. Atomic Mass Unit (amu) Carbon -12 is a standard reference isotope
Atomic Mass Unit (amu) – one twelfth of the mass of a carbon-12 atom
Carbon-12 atoms have 6 protons and 6 neutrons in it’s nucleus
- set as 12 amu

33. Atomic Mass Unit

34. Atomic Mass
Mass of atoms depends on number of protons and neutrons in nucleus
- in most cases atomic mass is not a whole number
- most elements occur as a mixture of two or more isotopes
Atomic Mass – weighted average mass of atoms in a naturally occurring sample of the element
- reflects both mass and relative abundance of isotopes as occur in nature

35. Calculating Average Atomic Mass
To calculate amu:
multiply mass of each isotope by its natural abundance (expressed as a decimal)
Add the products

36. Example of Average Atomic Mass calculation
Carbon -12 has natural abundance of 98.89%
Carbon-13 has a natural abundance of 1.11%
Carbon -12 mass is amu
Carbon-13 is amu
Atomic mass of carbon = ( amu X ) + ( amu X ) = amu

37. Practice calculating atomic mass
Copper has 2 isotopes Copper-63 and Copper-65
Copper-63 has an abundance of 69.2% and an amu of 62.93
Copper-65 has an abundance of 30.8% and an amu of 64.93
Calculate the average atomic mass
Answer = amu

38. Periodic Table Preview
Periodic Table – arrangement of elements in which elements
- separated into groups based on set of repeating properties
- allows you to compare properties of one element to another
Period – each horizontal row
Group – each vertical column

39. Periodic Table of Elements