54. fluorine
55. oxygen
56. they have the same number of protons
57. nucleus 3 
Explanation:
54. The element represented by the nucleus 1 have 9 protons and 9 neutrons. The atomic number of the element is equal to the number of protons = 9, so the element is fluorine.
55. The element represented by the nucleus 2 have 8 protons and 10 neutrons. The atomic number of the element is equal to the number of protons = 8, so the element is oxygen.
56. nucleus 2 contains 8 protons and 10 neutrons
nucleus 4 contains 8 protons and 11 neutrons.
The atoms that have the same number of protons but different number of neutrons are called isotopes.
57. nucleus 3 have 10 protons and 10 neutrons and correspond to neon. Neon have 10 electrons (equal to the number of protons) and have the electronic configuration 1s² 2s² 2p⁸. Because all the electrons shells are filled with electrons, neon have a stable electron configuration.
Learn more about:
subatomic particles
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Density = Mass / Volume
V = 1.00 * 4.00 * 2.50 = 10 cm³ 
22.57 g/cm³ = Mass / 10 cm³
M = 22.57 g/cm³ * 10 cm³
M = 225.7 g
Answer: The mass of the block of osmium is 225.7 g.
        
                    
             
        
        
        
A mixture can be separated.  Everything in a mixture keeps it's own properties and are not chemically joined together.  I am not completely sure about the compound.  Although with the cake example, the ingredients have been mixed and kind of "fused" together upon baking. Hope this helps a little. (P.S. trail mix is a good example of a mixture.)
        
             
        
        
        
I believe it is the Great Plains in Nebraska.