1. Atomic Structure Unit 2
S3 Chemistry

2. Atomic Structure and the History of the Atom
Learning Outcomes:
I know the different models of the atom that were proposed through the years.
I know the different sub-particles in the atom
Success Criteria:
I can sort the different models of the atom into a timeline to show when in history they were proposed.
I can name the different sub-particles, their mass, charge and location.

3. Glossary New Word Meaning Atomic Number
Number of protons found in the nucleus of the atom.
Mass Number
The mass of an atom (no. of protons + no. of neutrons)
Isotope
Atoms with the same atomic number and different mass number
Relative Atomic Mass (amu)
Average mass of the isotopes present, taking their relative proportions into account.
Ionic Bond
An electrostatic force of attraction between a positively charged metal ion and a negatively charged non-metal ion.
Covalent Bond
Formed when two nuclei are held together by their common attraction for a shared pair of electrons
Valency
The number of bonds that an atom can form with other atoms
Diatomic Molecule
A molecule made up of two atoms such as O2 or CO

4. Elements
Materials which cannot be broken down into simpler substances are called elements.
There are 118 elements known to scientists at present.  About 90 occur naturally in the earth. Some naturally occurring elements are radioactive, for example, radium. Plutonium and the other elements are made in nuclear reactors.

5. Every substance on earth is made from these elements. 
Each element has a symbol, consisting of one or two letters, to represent its name.

6. Atoms Elements are made up of tiny particles called atoms.
All the atoms of one element are the same
Different elements have different atoms!

7. A single atom is so small, its mass cannot be measured by a balance.
There is a special scale to measure something so small...

8. The mass of an atom is measured in atomic mass units (amu).
Hydrogen has the lightest atoms; its mass is 1 amu.
Silver atoms have a mass of 108 amu.

9. Using your data book fill in the missing information....
Element
Symbol
Mass (amu)
Hydrogen H 1
Silver Ag
108
Gold Au
197
Oxygen O 16
Sulfur S 32
Carbon C 12
Nitrogen N 14
Potassium K 39
Chlorine Cl
35.5

10. Democritus (about 480 – 370 BC)
History of the Atom
Democritus (about 480 – 370 BC)
John Dalton (1766 – 1844)
Ernest Rutherford (1871 – 1937)

12. Democritus (about 480 – 370 BC)
The term atom was first used by the Greek philosopher called Democritus.
Democritus thought that if a substance is broken up into smaller and smaller pieces, eventually a stage would be reached where a tiny particle of the substance would exist that could not be broken down.
He called this tiny particle an ATOM. From the Greek word “atomos,” this means indivisible.
All matter is made up of tiny particles called atoms.

13. It took over 2000 years before the idea of matter and atoms was used again. Why?
Aristotle and Plato favored the earth, fire, air and water approach to the nature of matter. Their ideas held sway because of their eminence as philosophers. The atomos idea was buried for approximately 2000 years.

14. John Dalton (1766 – 1844)
In 1803, a scientist called John Dalton put forward his ideas on elements and atoms.
John Dalton’s ideas:
All substances are made of tiny particles called ATOMS.
All atoms of one element are the same. E.g. Gold contains gold atoms
Atoms of different elements join together to form compounds.
Atoms cannot be broken down into anything smaller.

15. .
This theory became one of the foundations of modern chemistry.

16. Thomson’s Plum Pudding Model
In 1897, the English scientist J.J. Thomson provided the first hint that an atom is made of even smaller particles.
He proposed a model of the atom that is sometimes called the “Plum Pudding” model.
Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding.
J. J. Thomson ( – )
Thomson studied the passage of an electric current through a gas.
As the current passed through the gas, it gave off rays of negatively charged particles.
This surprised Thomson, because the atoms of the gas were uncharged.
Where had the negative charges come from?

17. Thomson concluded that the negative charges came from within the atom.
A particle smaller than an atom had to exist.
The atom was divisible!
Thomson called the negatively charged “corpuscles,” today known as electrons.
Since the gas was known to be neutral, having no charge, he reasoned that there must be positively charged particles in the atom.
But he could never find them.

18. John Dalton thought an atom was like a tiny ball.
John Dalton’s model of the atom.
The discovery of radioactivity in the latter part of the 19th century showed that atoms were made of smaller particles.
Ernest Rutherford (1871 – 1937)
In 1911, a new model of the atom was put forward by a scientist called Ernest Rutherford.

19. Rutherford’s Gold Foil Experiment
In 1908, the English physicist Ernest Rutherford was hard at work on an experiment that seemed to have little to do with unraveling the mysteries of the atomic structure.
Rutherford’s experiment Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick)

20. Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all.
Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid. He knew that positive charges repel positive charges.

22. + Ernest Rutherford’s model of the atom was: Ernest Rutherford’s atom
The atom is mostly made of empty space.
At the centre of the atom is a tiny POSITIVELY charged particle called the NUCLEUS. +
Moving round the nucleus are even smaller particles called ELECTRONS.
Negatively charged electron moving round the nucleus
Positively charged nucleus
Electrons carry a NEGATIVE charge.
The positive charge of the nucleus comes from positively charged particles called protons.

23. Bohr Model
In 1913, the Danish scientist Niels Bohr proposed an improvement. In his model, he placed each electron in a specific energy level.
According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits, or energy levels, are located at certain distances from the nucleus.

24. Wave Model

25. The Wave Model
Today’s atomic model is based on the principles of wave mechanics.
According to the theory of wave mechanics, electrons do not move about an atom in a definite path, like the planets around the sun.
In fact, it is impossible to determine the exact location of an electron. The probable location of an electron is based on how much energy the electron has.
According to the modern atomic model, at atom has a small positively charged nucleus surrounded by a large region in which there are enough electrons to make an atom neutral.

26. Electron Cloud: A space in which electrons are likely to be found.
Electrons whirl about the nucleus billions of times in one second
They are not moving around in random patterns.
Location of electrons depends upon how much energy the electron has.

27. Electron Cloud:
Depending on their energy they are locked into a certain area in the cloud.
Electrons with the lowest energy are found in the energy level closest to the nucleus
Electrons with the highest energy are found in the outermost energy levels, farther from the nucleus.

29. Atomic Structure

30. Particle
Location
Mass
Charge
Proton
Nucleus
1 amu +1
Neutron
Electron
Energy levels -1

31. Atoms are neutral - the POSITIVE charge of the protons in the nucleus is CANCELLED OUT by the NEGATIVELY charged electrons.

32. Atoms of an element contain the same number of protons (positive charge) and electrons (negative charge) and so have no overall charge

33. Diatomic Elements: An element whose molecules contain two atoms, such as:
Fluorine, F2
Chlorine, Cl2
Oxygen, O2
Hydrogen, H2
Nitrogen, N2
Bromine, Br2
Iodine, I2
(H NOF and down)