1. 12.1 Structure of the Atom
In order to understand atoms, we need to understand the idea of electric charge.
We know of two different kinds of electric charge and we call them positive and negative.

2. 12.1 Electric charge in matter
Scientists use the letter e to represent the elementary charge.
At the size of atoms, electric charge always comes in units of +e or –e.
Electric charge appears only in whole units of the elementary charge.

3. 12.1 An early model
In 1897 English physicist J. J. Thomson discovered that electricity passing through a gas caused the gas to give off particles that were too small to be atoms.
These negative particles were eventually called “electrons.”

4. 12.1 The nuclear model
In 1911, Ernest Rutherford, Hans Geiger, and Ernest Marsden did a clever experiment to test Thomson’s model.
We now know that every atom has a tiny nucleus, which contains more than 99% of the atom’s mass.

5. 12.1 Force inside atoms What holds the nucleus together?
There is another force that is even stronger than the electric force.
We call it the strong nuclear force.

6. 12.1 How atoms of various elements are different
The atoms of different elements contain different numbers of protons in the nucleus.
Because the number of protons is so important, it is called the atomic number.

7. 12.1 Atomic number and protons
Each element has a unique atomic number.
Atoms of the same element always have the same number of protons in the nucleus.

8. 12.1 Ions Complete atoms have a net zero charge.
Ions are atoms that have a different number of protons than electrons and so they have a positive or negative charge.

9. 12.1 How atoms of various elements are different
Isotopes are atoms of the same element that have different numbers of neutrons.
The mass number of an isotope tells you the number of protons plus the number of neutrons.
How are these carbon isotopes different?

10. 12.2 Bohr model of the atom
Danish physicist Neils Bohr proposed the concept of energy levels to explain the spectrum of hydrogen.
When an electron moves from a higher energy level to a lower one, the atom gives up the energy difference between the two levels.
The energy comes out as different colors of light.

11. 12.2 The quantum theory
Quantum theory says that when things get very small, like the size of an atom, matter and energy do not obey Newton’s laws or other laws of classical physics.

12. 12.2 The quantum theory
According to quantum theory, particles the size of electrons are fundamentally different.
An electron appears in a wave-like “cloud” and has no definite position.

13. 12.2 The quantum theory
The work of German physicist Werner Heisenberg (1901–1976) led to Heisenberg’s uncertainty principle.
The uncertainty principle explains why a particle’s position, momentum or energy can never be precisely determined.
The uncertainty principle exists because measuring any variable disturbs the others in an unpredictable way.

14. 12.2 Electrons and energy levels
In the current model of the atom, we think of the electrons as moving around the nucleus in an area called an electron cloud.
The energy levels occur because electrons in the cloud are at different average distances from the nucleus.

15. 12.2 Rules for energy levels
Inside an atom, electrons always obey these rules:
The energy of an electron must match one of the energy levels in the atom.
Each energy level can hold only a certain number of electrons, and no more.
As electrons are added to an atom, they settle into the lowest unfilled energy level.

16. 12.2 Models of energy levels
While Bohr’s model of electron energy levels explained atomic spectra and the periodic behavior of the elements, it was incomplete.
Energy levels are predicted by quantum mechanics, the branch of physics that deals with the microscopic world of atoms.

17. 12.2 Energy levels
In the Bohr model of the atom, the first energy level can accept up to two electrons.
The second and third energy levels hold up to eight electrons each.
The fourth and fifth energy levels hold 18 electrons.

18. 12.3 The Periodic Table
The periodic table organizes the elements according to how they combine with other elements (chemical properties).
The periodic table is organized in order of increasing atomic number.

19. 12.3 The Periodic Table
The periodic table is further divided into periods and groups.
Each horizontal row is called a period.
Each vertical column is called a group.

20. 12.3 Atomic Mass
The mass of individual atoms is so small that the numbers are difficult to work with.
To make calculations easier, scientists use the atomic mass unit (amu).
The atomic mass of any element is the average mass (in amu) of an atom of each element.

21. 12.3 Atomic Number
Remember, the atomic number is the number of protons all atoms of that element have in their nuclei.
If the atom is neutral, it will have the same number of electrons as protons.

22. 12.3 Groups of the periodic table
The first group is known as the alkali metals.
The alkali metals are soft and silvery in their pure form and are highly reactive.
This group includes the elements lithium (Li), sodium (Na), and potassium (K).

23. 12.3 Groups of the periodic table
The Group Two metals include beryllium (Be), magnesium (Mg), and calcium (Ca).
They also bond easily with oxygen.

24. 12.3 Halogens
The halogens tend to be toxic gases or liquids in their pure form.
Fluorine (F), chlorine (Cl), and bromine (Br) form salts when they bond with alkali metals.

25. 12.3 Noble Gases
The noble gases, including the elements helium (He), neon (Ne), and argon (Ar).
These elements do not naturally form chemical bonds with other atoms and are almost always found in their pure state.

26. 12.3 Transition metals
In the middle of the periodic table are the transition metals, including titanium (Ti), iron (Fe), and copper (Cu).
These elements are usually good conductors of heat and electricity.

27. 12.4 Properties of the elements
Most of the pure elements are solid at room temperature.
Only 11 naturally occurring elements are a gas.
Only 2 elements (Br and Hg) are liquid at room temperature.

28. 12.4 Periodic properties of elements
Periodicity means properties repeat each period (row) of the periodic table.

29. 12.4 Thermal and electrical conductivity
Electricity is the movement of electric charge, usually electrons.
Metals are good electrical conductors.
They allow electrons to flow easily through them.

30. 12.4 Thermal and electrical conductivity
Like copper, most metals are also good thermal conductors.
That is one reason pots and pans are made of metal.

31. 12.4 Thermal and electrical conductivity
Elements on the far right of the table are called non-metals.
Nonmetals make good insulators.
An insulator is a material which slows down or stops the flow of either heat or electricity.

32. 12.4 Metals and metal alloys
An alloy is a solid mixture of one or more elements.
Most metals are used as alloys and not in their pure elemental form.
Yellow brass is an alloy of 72% copper, 24% zinc, 3% lead, and 1% tin.

33. 12.4 Carbon and carbon-like elements
Almost all the molecules that make up plants and animals are constructed around carbon.
The chemistry of carbon is so important it has its own name, organic chemistry.

34. 12.4 Carbon and carbon-like elements
Pure carbon is found in nature as either graphite or diamond.
Silicon is the second most abundant element in the Earth’s crust, second only to oxygen.
Why are carbon and silicon important?

35. 12.4 Nitrogen, oxygen and phosphorus
Oxygen and nitrogen are crucial to living animals and plants.
For example, proteins and DNA both contain nitrogen.
Phosphorus is a key ingredient of DNA, the molecule responsible for carrying the genetic code in all living creatures.

36. 12.4 Nitrogen, oxygen and phosphorus
Proteins and DNA both contain oxygen and nitrogen, making these elements crucial to life.
46% of the mass of Earth’s crust is also oxygen bound up in rocks and minerals.

37. 12.4 Nitrogen, oxygen and phosphorus
Phosphorus is a key ingredient of DNA, the molecule responsible for carrying the genetic code in all living creatures.
When phosphorus atoms absorb light, they store energy, then release it in a greenish glow.