1. Chemistry Unit – Part 2

2. Bohr Models Bohr models are used to predict reactivity in elements.
Reactivity refers to how likely an element is to form a compound with another element.
When looking at Bohr models, we look at its valence electrons (the electrons on the last energy level) to determine reactivity.

3. Bohr Models
There are two parts to a Bohr model. The nucleus and the orbits which contain electrons
Nucleus
A nucleus is made of protons and neutrons which must be indicated on a Bohr model
If an atom were the size of a football stadium, the nucleus would be the size of a marble.

4. Bohr Models
Orbits
Electrons travel around the nucleus in specific paths called shells or orbits.

5. Drawing Bohr Models Draw the nucleus.
Write the number of neutrons and the number of protons in the nucleus. (p+ , n0)
Draw the first energy level. (First orbit for e- )
Draw the electrons in the energy levels according to the rules below. Make sure you draw the electrons spaced out evenly (possible locations are top, bottom, or side)
- If more than 4 electrons, draw in pairs.

6. Rules for Energy Levels
Level 1 (closest to the nucleus) can hold a maximum of 2e.
Level 2 can hold a max of 8e.
Level 3 can hold a max of 8e.
You must fill one level before going on to draw the next level!

8. Guided Practice
In order to draw Bohr models of these elements, you must first determine the number of protons, neutrons, and electrons. 6 C
Carbon
12.011

9. Guided Practice 16 S
Sulfur
32.066

10. Guided Practice 3 Li
Lithium
6.941

11. Guided Practice 10 Ne
Neon
20.180

12. Complete Bohr Periodic Table

13. Reactivity and Stability
Atoms are stable or not depending on their electron structure
Noble gases are unreactive
Alkaline Earth Metals and Chalcogens are moderately reactive
Alkali metals and halogens are very reactive
WHY!

14. Chemical reactivity is related to the number of valence electrons that need to be lost or gained so that the atom has a completely filled outer electron shell.
The fewer the number of electrons needed, the greater the reactivity of the element.
Lets look at an example:

15. Draw Bohr models for
Sodium Beryllium Carbon Helium

16. Noble gas – has a full valence shell
Noble gas – has a full valence shell. It does not require any further electrons. It is STABLE
Alkaline Earth Metals – need to lose 2 electrons
Chalcogens – need to gain 2 electrons
These are both moderately reactive
Once they react, they will have a similar structure to their nearest noble gas

17. Beryllium after losing 2 electrons Helium

18. Alkali metals – need to lose 1 electron
Halogens – need to gain 1 electron
These are both VERY reactive, because they only need to gain/lose ONE more electron to become stable and have a similar structure to their nearest noble gas.

19. Sodium Fluorine Neon
Missing 1 electron gaining 1 electron

20. So what happens during a chemical reaction?
During a chemical reaction, one element combines with another to form a new substance.
There must be some movement of electrons so that both atoms can become stable (complete their valence shell)

21. Who will do what?
You have one atom of the elements below. Match up the atoms which you think will react together. Draw a line to connect them. Think about which will lose electrons, or gain. As well as how many! Li Cl Ar Xe Mg O

22. Chemical reactions
There are three activities that can provide an atom with a completely filled valence electron shell.
1. The atom can lose/give up one or more electrons
Alkali metals only have 1 electron, and alkaline earth metals have 2.
It is simpler for them to give away their extra electrons than find enough to fill their outer shells

23. 2. The atom can gain more electrons Halogens and chalcogens need to gain only 1 or 2 more electrons. It is simpler for them to find the remaining electrons than to lose all of their valence electrons 3. The atom can share electrons with another

24. Complete the Table In the table indicate if the elements listed are :
Reactivity high, moderate, zero
Electrons gained or lost - gained, lost
# of electrons number (0, 1, 2)
Family name the family
Answer questions 2 and 3

25. Elements, Atoms and Molecules
Elements and compounds are both PURE SUBSTANCES
Some substances are made of one element.
This smallest particle of an element is called an atom.
It is impossible to break down gold into anything simpler than gold.
Some elements (such as oxygen and chlorine) will be made of two atoms bonding together to form a molecule (O2 is the formula for oxygen gas, so it is 2 atoms combined).
These elements are called diatomic elements.

27. ***Elements can be formed from atoms OR molecules
Most substances, however, are compounds made by combining two or more elements.
Water, steel, rubber, nylon, and sugar are all compounds, made from elements that are joined together in precise ways.

28. Compounds and molecules
These are NOT elements – they have different atoms together.
A compound has totally different characteristics from the elements that formed the compound.

29. Compounds and Molecules
A compound is the simplest form of a material that contains two or more elements in a fixed proportion (it is made up of different elements).
A molecule is the smallest particle of a compound (it is made of atoms).
Compounds are pure substances, because all their molecules are the same

30. Mixtures
These are NOT pure substances. They are a combination of different types of molecules and atoms
They can be separated
Filtration or distillation for example
**Compounds CAN also be separated, but it is more difficult and involves electrolysis – adding energy to break apart the molecules

32. For each, state whether you have :
Pure Substance or mixture
Element Compound or Neither
Atoms or Molecules or Both

33. 1.

34. 2. He He

35. Complete elements, molecules practice sheets

36. Chemical Formulas Chemical formulas tell you:
The elements that make up a compound
The number of each element in the compound
For example: H2O.
H = symbol for hydrogen. The number following H indicates the number of hydrogen atoms present.
O = symbol for oxygen. The number following O indicates the number of oxygen atoms present. When no value is shown, it is treated as 1.

37. Compound Structure Elements Number of Atoms Total CO2 CuSO4 Carbon 1 3
Oxygen 2
CuSO4
Copper 6
Sulfur 4

38. Compound Elements Number of Atoms Total Na2CO3 NH4C2H3O2 Sodium 2 6
Carbon 1
oxygen 3
NH4C2H3O2
Nitrogen 12
Hydrogen 7

39. Placing a number in front of a molecule tells you HOW MANY of that molecule you have!
Compound
Elements
Number of Atoms
Total
2 CaCl2
Calcium 2 6
Chlorine 4
3 CH3
Carbon 3 12
Hydrogen 9