1. Chapter 4
Atomic Structure

2. Pre-Test True/False: Compounds have fixed compositions
What is an atom?
Which of the following units is a unit of mass? mL ˚C g cm

3. Pre-Test continued Volume is:
The straight-line distance between two points
The quantity of matter in an object
The amount of space taken up by an object
A representation of an object or event

4. Pre-Test continued What is density?
Which two of the following events can take place when a liquid absorbs energy?
The average kinetic energy of the particles in the liquid increase
The temperature decreases
The liquid freezes
The liquid changes to a gas

5. 4.1 Studying Atoms
Studying and understanding atoms can be difficult--we cannot see atoms with our naked eye
So we have to use indirect evidence to prove our hypotheses

6. Ancient Greek Atom Models
Democritus-believed that all matter consisted of extremely small particles that could not be divided
Called these particles atoms
Comes from the Greek word atomos, which means uncut or indivisible
Thought there were different types of atoms with specific sets of properties
Ex: liquid atoms were smooth and round; solid atoms were rough and prickly

7. Ancient Greek Atom Models
Aristotle- did not think there was limit to the number of times matter could be divided
Thought that all substances were built up from only four elements: earth, air, water, and fire
These elements were a combination of four qualities: hot, cold, dry, and wet

8. Dalton’s Atomic Theory
John Dalton born in England in 1766
Teacher, in spare time did scientific experiments
Was interested in weather, so he studied the behavior of gases
He concluded (correctly) that gas consists of individual particles, based on the way gases exert pressure

9. Dalton-Evidence for Atoms
Massed elements before he mixed them, and then massed the compound that formed after the reaction took place
Gathered evidence for atoms
Concluded that no matter the size of the sample of a particular compound, the ratio of the elements that make it up is always the same, therefore the composition of compounds is fixed

10. Dalton’s Theory Dalton proposed the following theory:
All matter is made up of individual particles called atoms, which cannot be divided.

11. Dalton’s Theory Dalton’s theory has four main pts:
All elements are composed of atoms
All atoms of the same element have the same mass, and atoms of different elements have different masses
Compounds contain atoms of more than one element
In a particular compound, atoms of different elements always combine in the same way

12. Thomson’s Model of the Atom
Thomson found evidence for subatomic particles
Thomson’s experiments provided the first evidence that atoms are made of even smaller particles
Atoms overall are neutral in charge even though there are charged particles inside the atom
Protons (+1), Neutrons (no charge), Electrons (-1)

13. Rutherford’s Atomic Theory
In 1899 Rutherford discovered that uranium emits fast moving particles that have a positive charge
Alpha particles
Gold Foil Experiment
A narrow beam of alpha particles at a thin sheet of gold foil
The screen around the foil was made of a material that flashed when struck by a fast-moving alpha particle

14. Gold Foil Experiment continued
Marsden (a student of Rutherford’s) noticed that some of the alpha particles were reflected and some went right through the foil
This lead to the notion of the nucleus of an atom and it being positively charged
Nucleus- a dense, positively charged mass located in the center of the atom
According to Rutherford’s model, all of an atom’s positive charge is concentrated in its nucleus

15. 4.2 The Structure of an Atom
Properties of Subatomic Particles
1920: Rutherford saw evidence for the existence of 2 subatomic particles and predicted a 3rd
Protons, Electrons, and Neutrons are subatomic particles

16. Protons
Proton- a positively charged subatomic particle that is found in the nucleus of an atom
Each proton has a +1 charge
Can be abbreviated: p+
Found in the nucleus of an atom

17. Electrons
Electron- a negatively charged subatomic particle that is found in the space outside the nucleus
Each electron has a -1 charge
Can be abbreviated: e-
Found in the space outside of the nucleus

18. Neutrons
Neutron- a neutral subatomic particle that is found in the nucleus of an atom
No charge
Has a mass almost identical to that of a proton
Was proven to exist in 1932 by James Chadwick
Can be abbreviated: n
Found in the nucleus of an atom

19. Properties of Subatomic Particles
Symbol
Relative Charge
Relative Mass (proton=1)
Actual Mass
Electron e- 1-
1/1836
9.11 x 10-28
Proton p+ 1+ 1
1.674 x 10-24
Neutron n
1.675 x 10-24

20. Comparing Subatomic Particles
Protons, electrons, and neutrons can be distinguished by mass, charge, and location in an atom

21. Atomic Number
Atomic Number- equals the number of protons in an atom of a particular element
Atoms of different elements have different numbers of protons
Each positive charge of a proton is balanced by one negative charge of an electron--> therefore for every proton there is an electron
So, the atomic number also tells us how many electrons are in an atom

22. Mass Number
Mass number- the sum of the protons and neutrons in a nucleus of an atom
To figure out the number of neutrons in an atom you do the following calculation:
Number of neutrons = Mass number - Atomic number

23. Isotopes
Isotopes- atoms of the same element that have different mass numbers
Isotopes of an element have the same atomic numbers, but different mass numbers because they have different numbers of neutrons

24. 4.3 Modern Atomic Theory Bohr’s Model of the Atom
Niels Bohr a Danish physicist worked with Rutherford for a while
Bohr expanded on Rutherford’s model by including information about electrons
Bohr included a description of the arrangement of electrons in an atom
This is now the centerpiece of the modern atomic model

25. Bohr’s Model Energy Levels
e-’s move with constant speed in fixed orbitals around the nucleus
Each e- has the a specific amount of energy
If an atom gains or loses energy, the energy of an electron can change
Energy levels- are the possible energies that e-’s in an atom can have
e-’s cannot exist between energy levels
No two elements have the same set of energy levels
An e- in an atom can move form one energy level to another when the atom gains or loses energy

26. Bohr’s Model Evidence for energy levels
Scientists can measure the energy gained when e-’s absorb energy and move to a higher energy level
They can measure the energy released when the e- returns to a lower energy level

27. Electron Cloud Model
Bohr’s model was improved as new discoveries were/are made
There are energy levels, but they do not move in orbitals like planets
e-’s move in much less predictable ways
Scientists must deal with probability when trying to predict the location of e-’s
Electron Cloud- a visual model of the most likely locations for e-’s in an atom
The denser the cloud the more probable it is to find an e- there

28. Energy Levels, Orbitals, & Electrons
Number of Orbitals
Maximum Number of Electrons 1 2 4 8 3 9 18 16 32

29. Atomic Orbitals
Orbital- a region of space around the nucleus where an electron is likely to be found

30. Electron Configuration
Electron Configuration- the arrangement of electrons in the orbitals of an atom
The most stable e- configuration is the one in which the electrons are in orbitals with the lowest possible energies
Ground state- when all e-’s in an atom have the lowest possible energies