1. Unit 4
Atomic Structure

2. History of the Atomic Theory
Atomic Theory: states that atoms are the smallest units and the building blocks of all matter
Contribution of numerous scientists

3. John Dalton Each element is composed of particles called atoms
atom: smallest particle of an element that retains the chemical identity of that element
All atoms of a given element are identical, but differ from those of any other element
Element: substance that can’t be broken down into simpler substances by a chemical change
Atoms are neither created nor destroyed in any chemical reaction- they can be REARRANGED
compounds always have the same relative numbers and kinds of atoms
Ex: water is always two hydrogens combined with an oxygen (H2O)

5. J.J. Thomson
Showed that an atom was actually made up of even smaller parts!
Experiments showed that atoms contained negatively charged particles called electrons
Also contains positively charged particles called protons
“chocolate chip cookie model”

7. Ernest Rutherford
Scientists had assumed that the atom resembled a chocolate chip cookie, with the positive and negative charges evenly distributed
Famous “Gold Foil Experiment”
He aimed a beam of alpha particles at the gold foil and observed that most of the particles went straight through
However, some were deflected and bounced back

10. This experiment proved the Plum Pudding Model wrong- the positive charge of the atom was concentrated in the middle
Conclusions:
The atom is mostly space
The atom contains a positively charged, dense center called the nucleus
Negatively charged electrons move about this nucleus

12. Niels Bohr James Chadwick
proposed a model of the atom showing a central dense nucleus with electrons found in surrounding orbits
Called “planetary model”
James Chadwick
Discovered neutral particles called neutrons in the nucleus

14. The Present Day Model of the Atom
WAVE MECHANICAL MODEL
Central nucleus
Clouds of electrons around the nucleus
Electrons are found in orbitals (most probable location of an electron)

16. The Atom Composed of three subatomic particles
Proton: positively charged with a mass of 1
Electron: negatively charged with a mass of 0 (1/1836 of a proton)
Neutron: neutral particle with a mass of 1
Nucleus contains protons and neutrons
Electrons move around the nucleus
# of protons = # of electrons
The number of protons in an atom is called the atomic number  identifies the element!!!

17. Atomic Number The number of protons in an element
No two elements have the same atomic number
Ex: What is the atomic number of Mg?
Ex: An element has an atomic number of 28. What is the element?

18. Atomic Mass and Mass Number
The atomic mass is determined by the number of protons and the number of neutrons
The atomic mass reported on the table is the average atomic mass
The mass number is the average atomic mass adjusted to a whole number
To find the number of neutrons, the atomic number is subtracted from the atomic mass number

19. Finding the number of neutrons
What is the mass number of oxygen?
How many P? How many E? How many N?
What is the mass number of Manganese?
What is the mass number of hydrogen?

20. Isotopes
Atoms that have the same number of protons but different number of neutrons are called isotopes
To identify an isotope, the mass number of the element is added at the end of the symbol
Mass # = protons + neutrons
Ex: Cl-35 and Cl-37

21. Average Atomic Mass
Atomic Mass: the weighted average masses of all the naturally occurring isotopes of that element
To calculate:
Convert given percentages to decimals
Multiply the mass of each isotope by its percentage
Add resulting numbers

22. 1. Element “X” has three isotopes: X-35 occurs 28
1. Element “X” has three isotopes: X-35 occurs 28.2% of the time, X-34 occurs 13.6% of the time, and X-36 occurs the remainder of the time. What is the average atomic mass of element X?

23. Ions Neutral elements have the same number of protons and electrons
Ions are charged atoms that result when an atom either loses or gains electrons
When an atom loses electrons, it has a + charge
When an atom gains electrons, it has a – charge

24. Determine how many protons and electrons are in the following ions:
Al3+
Sr2+
As3-

25. Electrons
Principal Energy Levels (PEL) show the distance of the electron from the nucleus
Up to 7 PEL’s (given the symbol “n” – quantum number)
As you move further away from the nucleus, energy increases
Each PEL is divided further into sublevels- orbitals

26. Electron Configuration:
The distribution of electrons among the orbitals of an atom
Bottom left hand corner of each element box
It describes where the electrons are found and what energies they possess
Electrons occupy the lowest energy levels before filling up the next energy level

27. Rules for Electron Configuration
electrons are added one at a time, to the lowest energy levels until they are filled up
PELs can hold different numbers of electrons- smaller ones hold fewer
1st PEL- holds up to 2
2nd PEL – holds up to 8
3rd PEL – holds up to 18 (full with 8)
If the outermost level- VALENCE level- has 8, the atom is stable

28. Ground State vs. Excited State
Ground state: the lowest energy configuration possible
Periodic table shows ground state configurations
Excited state: an atom becomes “excited” when one or more of its electrons move to a higher energy level without fully filling up the lower level (absorbs energy)
Unstable; eventually the electrons will fall back down to ground state (emits energy)

29. Ground State vs. Excited State
Which element is this?
Ground state: 2-8-6
Excited state: 1-8-7

30. Bright Line Spectrum
When electrons fall from an excited state to the ground state, they release the same amount of energy they absorbed when they moved up
This energy is released in the form of light and forms a spectrum of different colors
Each element has its own unique spectrum

32. Valence Electrons Electrons in the outermost PEL
Valence electrons participate in chemical bonding
No element has more than 8 valence electrons!!!!!!!!!!!!!!!

33. Lewis Dot Diagrams
Lewis Dot Diagrams depict valence electrons