Download 8th Grade Science Chapter 11 Answers

8th Grade Science Chapter 11 Answers
1. An atom is the smallest particle into which an element can be divided and still be the same substance. P 280
2. All matter is made of elements or compounds, atoms are often called the building blocks of matter.
3. A theory is a unifying explanation for a broad range of hypotheses and observations that have been
supported by testing.
4. Democritus was a Greek philosopher who theorized that eventually you would end up with your smallest
particle of matter, he called this matter an atom. Greek philosopher Aristotle disagreed with him and so
Democritus’s theory was largely ignored for centuries. P 280-281
5. Dalton was a British chemist that theorized, 1. All substances are made of atoms and atoms cannot be
created, divided, or destroyed; 2. Atoms of the same elements are exactly alike, yet different to those
unique to other elements; 3. Atoms are able to join up with other atoms to form new substances. These
theories have all evolved over time as science has discovered more about the elements and atoms.
6. J. J. Thomson was a British scientist who experimented with a cathode-ray tube. He discovered that a
positively charged plate attracted a beam made of particles with a negative electric charge. This meant that
there were smaller particles inside the atom, thus, atoms could be divided into even smaller particles. P 282
7. Electrons are the small negatively charged particles found in all atoms. These are the “corpuscles” that
Thomson discovered in his later experiments.
8. Thomson knew that atoms had no overall charge, therefore he knew that there must be positively charged
particles to balance or cancel out the negative charges from electrons.
9. A model is a representation of an object or a system. Like a model of the solar system or a die-cast car.
10. Thomson’s plum-pudding model successfully showed the location of the negatively charged electrons.
Think of the “chocolate chip cookie ice cream example,” the chips are the negatively charged electrons
inside the positively charged vanilla ice cream.
11. Rutherford, deciding to test Thomson’s theory, designed an experiment to look at the structure of an atom.
In doing so he discovered that some particles deflected off of the gold foil he used, leading him to believe
that there was more to discover within the atom. P 284 – Look at FIGURE 5 on page 284
12. Rutherford proposed that the center of the atom contained a very small and dense positively charged region
called the nucleus. P 285 – Look at FIGURE 6 on page 285
13. Most of the mass of an atom is located within the nucleus.
14. Rutherford’s atom contained a small, dense, positively charged center or nucleus. Outside of the nucleus
was the empty space (vanilla ice cream) through which the negatively charged electrons (chocolate chips)
traveled.
15. Bohr proposed that electrons within an atom aren’t stationary nor move randomly about an atom. Instead,
electrons move about the nucleus in a definite path & on certain levels. He compared these levels to that of
a ladder. An electron can move from rung to rung on the ladder, but not rest between the rungs. P 286
16. Current theory on the atom is that electrons are likely to be found in regions called electron clouds.
Electron clouds are in the space surrounding the nucleus.
17. An atom consists of the electrons and nucleus, as well as the protons and neutrons which are found inside
the nucleus. P 288 – Make sure you look at the FIGURE 11 on p 288
18. A proton is a positively charged particle located in the nucleus of an atom. It has a mass of 1(one) amu
19. The atomic mass unit (amu) is the SI unit used to express the mass of the particles in the atom.
20. A neutron is the protons roommate. Both Protons the Positively charged particles and the Neutrons or the
NO Charge particles share the nucleus of an atom. Neutrons are larger than protons, but the difference is so
small that neutrons have been given a mass of 1 (one) amu as well.
21. Atoms are neutral because the electrons outside of the nucleus in the electron cloud contain a negative
charge. On the other hand the nucleus contains the neutrons (no charge) and the positively charged protons
that are equal in size to the electrons, therefore their charges cancel out. P 289
22. When an atom has a greater number of protons or electrons it becomes a “charged particle.” The atom with
a charge, either negative or positive, is called an ion. If the atom contains more protons, you have a
positively charged ion, if the electrons outnumber the protons then you have a negatively charged ion.
23. The mass of an electron is generally given the value of zero (0). This is because of the size of the electron.
It takes more than 1,800 electrons to equal the mass of just 1 proton.
24. A hydrogen atom is the simplest of all atoms, containing 1 proton and 1 electron.
25. Neutrons are in the nucleus of an atom to keep the protons from moving apart. In the case of helium there
are 2 neutrons added to the nucleus, but in the case of oxygen you have 8 protons and 9 neutrons. As you
see the number of neutrons added Does NOT = the number of protons. P 290
26. The atomic number is determined by looking at the protons (positively charged particles) in the atom. The
atomic number = the number of protons in the nucleus.
27. The atomic number (# of protons present) determines what element is present. Each element has a unique
atomic number: no two elements have the same number of protons. The atomic number is then used to
categorize the elements on the periodic table. The 1st element has 1 proton, the 8th (Oxygen) has 8 protons.
28. An isotope is an atom that has the same number of protons but has a different number of electrons. These
isotopes are still the same element because they still share the same number of protons. So an isotope of
Hydrogen that has 1 proton, 1 neutron and 1 electron is still Hydrogen, even if it is not the element itself.
29. Isotopes vary, but there are a limited number that occur naturally. Isotopes can be unstable, meaning the
nucleus can change its composition. When this occurs it is said to be a radioactive isotope. P 291
30. Yes, isotopes share most of the same physical and chemical properties as the element they are related to.
31. The mass number of an element is the sum of the protons and neutron in an atom.
32. By looking at an isotopes mass number you can determine which isotope you have. Remember in isotopes
the protons do NOT equal the neutrons, this is why they are called isotopes.
33. Remember, electrons have a mass so small that they have little effect on the atoms total mass, therefore
they are not included in the elements mass number.
34. Isotopes are written with the name of the element – mass number of the isotope. Example, Carbon with a
mass number of 12 is called Carbon –12. P 292
35. Let’s look at our Carbon-12 example from above. We know that Carbon – 12 has a mass number of 12. We
can look up the atomic number on the Periodic Chart and determine the atomic number = 6 (# of protons in
the nucleus), therefore we can take 12 – 6 = 6. There are 6 neutrons in Carbon –12.
36. Aluminum-27 has a mass of 27 and its atomic number is 13, therefore
27-13=14, there are 14 neutrons in
Aluminum-27.
37. The atomic mass is the weighted average of the masses of all of the naturally occurring isotopes of a
particular element.
38. To calculate the atomic mass you multiply the mass number of each isotope by its percentage abundance in
decimal form.
39. Here is the Statue of Liberty example. The statue is made of Copper – 63 and Copper -65. The breakdown
is 69% Copper-63 and 31% Copper-65. So, multiply 0.69 (% in decimal form) by the mass number and
add the two values together.
0.69 x 63 + 0.31 x 65 = 43.47 + 20.15 = 63.62, the atomic mass number
is 63.6 amu. See MATH BREAKS on page 292
40. Gravity – reacts on the atom, remember that gravity between objects is determined by both there mass and
distance. The force of gravity within atoms is very small.
P 293
Electromagnetic Force – objects with the same charge repel each other. On the other hand objects with
opposite charges are attracted to one another. This electromagnetic charge is what holds the electrons
around the nucleus because the electrons and the protons have opposite charges.
Strong Force – this explains why a nucleus actually stays together. Remember we said that objects with like
charges repel each other. If this is the case then there must be something that keeps the nucleus and all of
its positively charged protons together. Because the protons are so close together, this strong force is
greater than the electromagnetic force, thus keeping the nucleus and all of its “repelling” protons together.
Weak Force – This force is important in radioactive atoms. In radioactive or “unstable” atoms, a neutron
can change into a proton or an electron. The weak force plays a key role in this ability to change.
41. 1. Electrons are found in the electron cloud around the nucleus. 2. All atoms of the same element contain
the same number of protons. 3. The neutrons have no electric charge. 4. The mass number of an element is
the number of protons and neutrons in the nucleus. The atomic mass is the average of the amsses of all the
naturally occurring isotopes of an element. \
42. The electron
43. 28, take
51-23 = 28
44. An atom has no overall charge if it contains an equal number of protons and electrons.