Chemistry Chapter 4
Apply Concepts The law of conservation of mass was introduced in Chapter 2. Use Dalton's atomic theory to explain this law.
In the fifth statement of Dalton's atomic theory, he states that chemical reactions are the result of atoms being joined, separated, or rearranged. Atoms are neither created nor destroyed in when atoms are mixed or chemically combined, but rather "rearranged" differently than before, possibly with or without other atoms.
What is the atomic mass of an element?
The atomic mass of an element is a weighted average mass of the atoms in a naturally occurring sample of the element.
How is the atomic mass of an element calculated from isotope data?
The atomic mass of an element is calculated from isotope data because the percent of natural abundance, the atomic mass, and the number of stable isotopes are needed to calculate the atomic mass of an element.
Define electron
a negatively charged subatomic particle
Where are the protons and neutrons located in the nuclear atom?
in the nucleus
What is the equation for finding the number of neutrons?
mass number - atomic number
Would you expect two electrons to attract or repel each other?
repel each other because they're like forces
What did Democritus reason about atoms?
that they were indivisible and indestructible
What was Thomson's atomic model?
the "plum-pudding model"; electrons stuck into a lump of positive charge
What is the amu?
atomic mass unit
Who was the fifth atomic theorist? Describe his experiments and theory(s)
Eugen Goldstein; observed a cathode-ray and found rays traveling in the direction opposite to that of the cathode rays. He concluded that they were composed of positive particles (protons), with a mass 1840 times that of an electron
Do you know how you're supposed to calculate the percent of natural abundance?
because I don't; please tell me if you do
Millikan measured the quantity of charge carried by an electron. How did he then calculate the mass of an electron?
The mass is 1/1840 of the proton's mass, or it's 9.11 x 10^-28. He used the charge to mass ratio to then divide and get the electron mass.
The table shows some of the data collected by Rutherford and his colleagues during their gold-foil experiment. a. What percentage of the alpha particle deflections were 5º or less? b. What percentage of the deflections were 15º or less? c. What percentage of the deflections were 60º or greater?
A. about 92.9% B. about 99.8% C. about 9.930 x 10^-3%
What is the order of the five main atomic models?
Dalton, Thomson, Rutherford/nuclear atom, Bohr theory (what's this again?), quantum model
Who was the first to have an atomic theory? Describe this theory and what his experiment was
Democritus; atoms were indivisible and indestructible; despite agreeing with later scientific theory, they didn't explain chemical behavior and lacked experimental support because his approach wasn't based off the scientific method
How can there be more than 1000 different atoms when there are only about 100 different elements?
Isotopes of an element have different numbers of neutrons and thus have different mass numbers.
What makes isotopes of the same element chemically alike?
Isotopes of the same element share the same number of protons and electrons, which defines an atom's chemical reactivity, so they are chemically alike.
Who was the third atomic theorist? Describe his theory(s) and what his experiment(s) was
J.J. Thomson; discovered the electron; experiments involved passing electric currents through gases at low pressures. Sealed the gases in glass tubes fitted at both ends with metal disks/electrodes, connected to a source of electricity. One electrode was the anode (positive), while the other was a cathode (negative). The result was a cathode ray that traveled from the cathode to the anode. He hypothesized that a cathode ray is a stream of tiny negatively charged particles moving at high speed
You are standing on the top of a boron‑11 nucleus. Describe the numbers and kinds of subatomic particles you see looking down into the nucleus, and those you see looking out from the nucleus.
Looking down, there are 5 protons and 6 neutrons. Looking out, there are 5 electrons orbiting the atom.
Describe the location of the electrons in Thomson's "plum-pudding" model of the atom.
The electrons were thought to be evenly distributed throughout an tom filled uniformly with positively charged material.
What is the difference between the mass number and the atomic number of an atom?
The mass number of an atom is the sum of the total number of protons and neutrons in an atom, while the atomic number is the number of protons in the atom's nucleus.
What data must you know about the isotopes of an element to calculate the atomic mass of the element?
You must know the number of (stable) isotopes of the element, their atomic masses, and their natural percent of abundance
The four isotopes of lead are shown below, each with its percent by mass abundance and the composition of its nucleus. Using these data, calculate the approximate atomic mass of lead. (82 p+ and 122n^0)=1.4%; (82 p+ and 125 n^0)=22.1%; (82 p+ and 124 n^0)=24.1%; (82 p+ and 126 n^0)= 52.4%
[(82+122) x 0.014] + [(82+124) x 0.524] 207 amu
Define proton
a positively charged subatomic particle found in the nucleus of an atom
With which of these statements would John Dalton have agreed in the early 1800s? For each, explain why or why not. a. Atoms are the smallest particles of matter. b. The mass of an iron atom is different from the mass of a copper atom. c. Every atom of silver is identical to every other atom of silver. d. A compound is composed of atoms of two or more different elements.
a. He would agree because he thought all elements are composed of tiny indivisible particles called atoms. b. He would agree with this because he thought the atoms of any one element are different from those of any other element c. He would agree because he thought atoms of the same element are identical. d. He would probably agree because it's mentioned that he believed that atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds.
How did Thomson test his hypothesis of cathode rays being a stream of negatively charged particles moving at high speed?
he set up an experiment to measure the ratio of a n electron's charge to its mass. He found the ratio to be constant. The charge-to-mass ratio of electrons didn't depend on the kinds of gas in the tube or metal used for the electrodes. He concluded that electrons are a component of the atoms of all elements
What was the startling results of Rutherford's experiment?
most alpha particles went straight through the gold foil, or were slightly deflected. A small fraction of the alpha particles bounced off the gold foil at very large angles, some even bounced straight back toward the sources
How do you calculate the atomic mass of an element?
multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products
Who was the fourth atomic theorist? Describe his theory(s) and his experiment(s)
Robert A. Millikan; carried out experiments to find the quantity of an electron's charge. His oil-drop experiment= suspended negatively charged oil droplets between two charged plates. He changed the voltage on the plates to see how this affected the droplets' rate of fall. He found that the charge on each oil droplet was a multiple of 1.60 x 10^-19 oculomb, meaning this isan electron's charge
Infer: Diamond and graphite are both composed of carbon atoms. The density of diamond is 3.52 g/cm3. The density of graphite is 2.25 g/cm3. In 1955, scientists successfully made diamond from graphite. Using the relative densities, consider what happens at the atomic level when this change occurs. Then suggest how this synthesis may have been accomplished.
Since graphite and diamond are made up of the same type of particles, we can assume that in diamonds atoms of carbon are closer to each other than in graphite. Therefore, if the atoms of graphite were forced to come closer to each other, graphite would become diamond.
Compare the size and density of an atom with its nucleus.
The nucleus is tiny and densely packed compared with the atom as a whole. The nucleus contains almost all the mass and would be the size of a marble if the atmo was the size of a football stadium.
What is the charge, positive or negative, of the nucleus of every atom?
The nucleus of every atom has a positive charge (no charge+positive=positive)
If you know the atomic number and mass number of an atom of an element, how can you determine the number of protons, neutrons, and electrons in that atom?
The number of protons is the atomic number. The difference of the mass number and the atomic number is the number of neutrons. The number of electrons should be equal to the number of protons or the atomic number.
In the Rutherford atomic model, which subatomic particles are located in the nucleus?
The protons and neutrons are located at the positively charged nucleus.
Characterize the size of an atom.
The radii of most atoms fall within the range of 5 x 10^-11m to 2 x 10^-10m. If you could line up 100, 000, 000 copper atoms side by side, they would produce a line only 1cm long. Atoms are very tiny and miniscule.
Explain This chapter illustrates the scientific method in action. What happens when new experimental results cannot be explained by the existing theory?
The results are re-experimented on. If the results are constant, the existing theory is either revised or replaced by a new theory to explain for the new evidence.
If isotopes are chemically alike, but physically different, propose which subatomic particles are responsible for determining an element's chemical reactivity.
The subatomic particles of protons and electrons are what determines an element's chemical reactivity.
How is an average mass different from a weighted average mass?
The weighted average mass of an element takes into account the natural percent of abundance, while the average mass just finds the arithmetic mean of the masses of the isotopes.
Explain: Rutherford's atomic theory proposed a dense nucleus surrounded by very small electrons. This structure implies that atoms are composed mainly of empty space. If all matter is mainly empty space, why is it impossible to walk through walls or pass your hand through your desk?
This is impossible because of the electromagnetic repulsion of the electron cloud. Also, the space in an individual atom is large relative to an object the size of a hand.
Dalton's atomic theory was not correct in every detail. Should this be taken as a criticism of Dalton as a scientist? Explain.
This should never be taken as a criticism of someone being a scientist. Humans will most likely never know the truth in every detail. Scientists are constantly learning from others mistakes and making mistakes of their own. Science will never be 100% accurate. A scientist is limited by their amount/quality of knowledge and their technology, so they should never be criticized for not being completely accurate.
What characteristics of cathode rays led Thomson to conclude that the rays consisted of negatively charged particles?
Thomson found that cathode rays are deflected by electrically charged metal plates. Since the negatively charged plate repels it, and like charges repel, Thomson concluded that the rays consisted of negatively charged particles (electrons).
Define Cathode Ray
a stream of electrons produced at the negative electrode (cathode) of a tube containing a gas at low pressure
Define neutron
a subatomic particle with no charge and a mass of 1 amu; found in the nucleus of an atom
How many protons are in the nuclei of the following atoms? a. phosphorus (P) b. molybdenum (Mo) c. aluminum (Al) d. cadmium (Cd) e. chromium (Cr) f. lead (Pb)
a. 15 b. 42 c. 13 d. 48 e. 24 f. 82
Complete the following table. Atomic # Mass # # of Protons # of Neutrons 9 A. B. 10 C. D. 14 15 E. 47 F. 25 G. 55 25 H.
a. 19 h. 30 b. 9 c. 14 d. 19 e. 22 f. 22 g. 25
What experimental evidence did Thomson have for each statement? a. Electrons have a negative charge. b. Atoms of all elements contain electrons.
a. The cathode rays would bend or repel away from the negatively charged plate, indicating that the cathode ray is a stream of tiny negatively charged particles moving at a high speed. (remember that like charges repel) Also, magnets bent the stream b. This is a conclusion drawn from his experiments to measure the ratio of an electron's charge to its mass. The electron ratio of charge-to-mass was independent of the type of gas in the tube or the metal used for the electrode.
Interpret Diagrams The diagram below shows gold atoms being bombarded with fast-moving alpha particles. a. The large yellow spheres represent gold atoms. What do the small gray spheres represent? b. List at least two characteristics of the small gray spheres. c. Which subatomic particle cannot be found in the area represented by the gray spheres?
a. The nuclei b. Positive charge, placed in the center of the atoms c. electrons
How are elements different?
because they contain different numbers of protons
Why do isotopes of an element have different mass numbers?
because they have different numbers of neutrons
How can scientists observe individual atoms?
by using instruments such as scanning electron microscopes
Millikan used his discovered electron charge value and Thomson's charge-to-mass ratio of an electron to do what?
calculate an electron's mass. His values for electron charge and mass are similar to those accepted today
Evaluate and Revise How could you modify Rutherford's experimental procedure to determine the relative sizes of different nuclei?
change the metal used as a target and account for differences in deflection
Define isotope
each of two or more forms of the same element that contain equal numbers of protons but different numbers of neutrons in their nuclei, and hence differ in relative atomic mass but no in chemical properties; in particular, a radioactive form of an element
What si the relative and actual mass of electrons protons, and neutrons?
electron=1/1840; 9.11 x 10^-28 protons= 1; 1.67 x 10^-24] neutrons= 1; 1.67 x 10^-24
What are the three kinds of subatomic particles?
electrons, protons, and neutrons
Describe Rutherford's atomic model related to his experiemnt
nuclear atom; the atom is mostly empty space, thus explaining the lack of deflection of most alpha particles. All the positive charge and almost all the mass are concentrated in a small region that has enough positive charge to account for the great deflection of some of the alpha particles (nucleus). The nucleus is tiny and densely packed compared with the atom as a whole.
How do you write the shorthand/ chemical symbol for an element?
superscript=mass number subscript=atomic number
What were J.J. Thomson's finding from his experiment?
that a cathode ray is deflected by electrically charged metal plates. A positively charged plate attracts the cathode ray, while a negatively charged late repels it.
Define Dalton's Atomic Theory
the first theory to relate chemical changes to events at the atomic level
Define atomic number
the number of protons in the nucleus of an atom of an element
What does the atomic number of each atom represent?
the number of protons in the nucleus of that element.
Define atom
the smallest particle of an element that retains its identity in a chemical reaction
What is an atom?
the smallest particle of an element that retains its identity in a chemical reaction.
Define nucleus
the tiny, dense central portion of an atom, composed of protons and neutrons
Define mass number
the total number of protons and neutrons in the nucleus of an atom
Define atomic mass
the weighted average of the masses of the isotopes of an element
Describe the electrons of a nuclear atom
they are distributed around the nucleus and occupy almost all the volume of the atom
How did Dalton transform Democritus's ideas on atoms into a scientific theory?
through experimental methods
Lithium has two isotopes, lithium-6 (atomic mass = 6.015, relative abundance = 7.5%) and lithium-7 (atomic mass = 7.016, relative abundance = 92.5%). Calculate the atomic mass of lithium.
(0.45 amu)+(6.49 amu)= atomic mass of lithium is 6.94 amu
Using the data for nitrogen listed in the table, calculate the weighted average atomic mass of nitrogen. Show your work.
(14 7 N): 14.003 amu; 99.63% (15 7 N): 15 amu; 0.37% 4.003 x 99.63% + 15 x 0.37%= Average atomic mass= 14.01 amu
What are the four statements of Dalton's atomic theory?
1. All elements are composed of tiny indivisible particles called atoms 2. Atoms of the same element are identical. The atoms of any one element are different from those of any other element 3. Atoms of different elements can physically mix together or can chemically combine in simple whole-number ratios to form compounds 4. Chemical reactions occur when atoms are separated from each other, joined, or rearranged in a different combination. Atoms of one element, however, are never changed into atoms of another element as a result of a chemical reaction
What parts of Dalton's atomic theory no longer agree with the current picture of the atom?
1. Atoms are indivisible 2. Elements of the same element aren't identical. An example is that isotopes are still atoms of their elements, but they vary in their number of neutrons. (?)4. Partially correct; Atoms can't change into other elements as a result of chemical reactions. Chemical reactions occur when atoms are separated, joined, or are rearranged
Four things to remember when thinking of what's left once an electron is lost?
1. atoms have no net electric charge; they're electrically neutral 2. electric charges are carried by particles of matter 3. electric charges always exist in whole-number multiples of a single basic unit; there are no fractions of charges 4. when a given number of negatively charged particles combines with an equal number of positively charged particles, an electrically neutral particle is formed
Calculate: When the masses of the particles that make up an atom are added together, the sum is always larger than the actual mass of the atom. The missing mass, called the mass defect, represents the matter converted into energy when the nucleus was formed from its component protons and neutrons. Calculate the mass defect of a chlorine-35 atom by using the data in the table. The actual mass of a chlorine-35 atom is 5.81 × 10^−23 g.
17(1.67 x 10^-24 g) + 18(1.67 x 10^-24 g) + 17(9.11 x 10^-28 g) = 5.857 x 10^-23 g 5.857 x 10^-23 g - (5.81 x 10^-23 g) =4.70 x 10^-23 g
Calculate: Lithium has two naturally occurring isotopes. Lithium-6 has an atomic mass of 6.015 amu; lithium-7 has an atomic mass of 7.016 amu. The atomic mass of lithium is 6.941 amu. What is the percentage of naturally occurring lithium-7?
6.941 = [6.015 x (1-A)] + (7.016 x A) 6.941 = 6.015 - 6.015A + 7.016A 6.941 - 6.015 = 7.016A - 6.015A A = 0.926/1.001 = 0.925 = 92.5%
How do the charge and mass of a neutron compare to the charge and mass of a proton?
A neutron has no charge, having a mass nearly equal to a proton's. The proton is positively charged, having a mass of about 1840 times that of an electron.
Why does it make sense that if an atom loses electrons, it is left with a positive charge?
A proton has a positive charge, and an electron has a negative charge. When these subatomic particles are even in an atom, their opposing charges cancel each other out. If an atom lost an electron, however, their would be more protons than electrons and thus a positive charge.
Why is an atom electrically neutral?
Because you're not shocked every time you touch matter. The negative charge of the electrons cancel out the positive charge of the protons, if their numbers are equal.
What were the limitations of Democritus's ideas about atoms?
Democritus's ideas were limited by the lack of experimental support due to not following the scientific method and not explaining chemical behavior.
How did the results of Rutherford's gold-foil experiment differ from his expectations?
Due to the existing theory, he expected the alpha particles to pass easily through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms. Instead, most alpha particles went straight through the gold foil, were slightly deflected, some even bounced straight back toward the source, and a small fraction bounced off the gold foil at very large angles. He didn't expect the alpha particles to deflect at large angles.
Use Dalton's atomic theory to describe how atoms interact during a chemical reaction.
During a chemical reaction, atoms are separated, joined, or arranged in different combinations. This can never result in an atom becoming a different element.
Who was the seventh atomic theorist? Describe his experiments and theory(s)
Ernest Rutherford; tested Thomson's atomic model by devising the gold-foil experiment. A narrow beam of alpha particles were directed at a very thin sheet of gold foil. They should've passed through the gold, with only a slight deflection due to the positive charge thought to be spread out in the gold atoms
Cathode Ray
Is a stream of electrons observed in vacuum tubes. If an evacuated glass tube is equipped with two electrodes and a voltage is applied, the glass opposite of the negative electrode is observed to glow, due to electrons emitted from and traveling perpendicular to the cathode.
Name two ways that isotopes of an element differ.
Isotopes of an element have different numbers of neutrons and thus have different mass numbers.
Who was the sixth atomic theorist? Describe his experiments and theory(s)
James Chadwick; confirmed the existence of the neutron.
Who was the second atomic theorist? Describe his theory(s) and what his experiment(s) was
John Dalton; he studied the ratios in which elements combine in chemical reactions; Dalton's Atomic Theory (4 statements) was the result of observations from his experimental methods.
