1answer.
Ask question
Login Signup
Ask question
All categories
  • English
  • Mathematics
  • Social Studies
  • Business
  • History
  • Health
  • Geography
  • Biology
  • Physics
  • Chemistry
  • Computers and Technology
  • Arts
  • World Languages
  • Spanish
  • French
  • German
  • Advanced Placement (AP)
  • SAT
  • Medicine
  • Law
  • Engineering
inessss [21]
3 years ago
6

When did oxygen first appear in earth's atmosphere??

Chemistry
1 answer:
ad-work [718]3 years ago
7 0
<span> the atmosphere holds about 21 per cent oxygen. Over the Earth’s 4.6 billion year history, oxygen did not appear in the atmosphere until perhaps about 2.5 billion years ago. Since then, oxygen levels have fluctuated in tandem with global geological and biological events, such as mass extinctions.</span>
You might be interested in
1s^2 2s^2 2p^6 3s^2 3p^6 how many unpaired electrons are in the atom represented by the electron configuration above?
Sedbober [7]
It's a combination of factors:
Less electrons paired in the same orbital
More electrons with parallel spins in separate orbitals
Pertinent valence orbitals NOT close enough in energy for electron pairing to be stabilized enough by large orbital size
DISCLAIMER: Long answer, but it's a complicated issue, so... :)
A lot of people want to say that it's because a "half-filled subshell" increases stability, which is a reason, but not necessarily the only reason. However, for chromium, it's the significant reason.
It's also worth mentioning that these reasons are after-the-fact; chromium doesn't know the reasons we come up with; the reasons just have to be, well, reasonable.
The reasons I can think of are:
Minimization of coulombic repulsion energy
Maximization of exchange energy
Lack of significant reduction of pairing energy overall in comparison to an atom with larger occupied orbitals
COULOMBIC REPULSION ENERGY
Coulombic repulsion energy is the increased energy due to opposite-spin electron pairing, in a context where there are only two electrons of nearly-degenerate energies.
So, for example...
↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−− is higher in energy than
↑
↓
−−−−−

↓
↑
−−−−−

↑
↓
−−−−−
To make it easier on us, we can crudely "measure" the repulsion energy with the symbol
Π
c
. We'd just say that for every electron pair in the same orbital, it adds one
Π
c
unit of destabilization.
When you have something like this with parallel electron spins...
↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−
It becomes important to incorporate the exchange energy.
EXCHANGE ENERGY
Exchange energy is the reduction in energy due to the number of parallel-spin electron pairs in different orbitals.
It's a quantum mechanical argument where the parallel-spin electrons can exchange with each other due to their indistinguishability (you can't tell for sure if it's electron 1 that's in orbital 1, or electron 2 that's in orbital 1, etc), reducing the energy of the configuration.
For example...
↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−− is lower in energy than
↑
↓
−−−−−

↓
↑
−−−−−

↑
↓
−−−−−
To make it easier for us, a crude way to "measure" exchange energy is to say that it's equal to
Π
e
for each pair that can exchange.
So for the first configuration above, it would be stabilized by
Π
e
(
1
↔
2
), but the second configuration would have a
0
Π
e
stabilization (opposite spins; can't exchange).
PAIRING ENERGY
Pairing energy is just the combination of both the repulsion and exchange energy. We call it
Π
, so:
Π
=
Π
c
+
Π
e

Inorganic Chemistry, Miessler et al.
Inorganic Chemistry, Miessler et al.
Basically, the pairing energy is:
higher when repulsion energy is high (i.e. many electrons paired), meaning pairing is unfavorable
lower when exchange energy is high (i.e. many electrons parallel and unpaired), meaning pairing is favorable
So, when it comes to putting it together for chromium... (
4
s
and
3
d
orbitals)
↑
↓
−−−−−
↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−
compared to
↑
↓
−−−−−
↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−

↑
↓
−−−−−
is more stable.
For simplicity, if we assume the
4
s
and
3
d
electrons aren't close enough in energy to be considered "nearly-degenerate":
The first configuration has
Π
=
10
Π
e
.
(Exchanges:
1
↔
2
,
1
↔
3
,
1
↔
4
,
1
↔
5
,
2
↔
3
,

2
↔
4
,
2
↔
5
,
3
↔
4
,
3
↔
5
,
4
↔
5
)
The second configuration has
Π
=
Π
c
+
6
Π
e
.
(Exchanges:
1
↔
2
,
1
↔
3
,
1
↔
4
,
2
↔
3
,
2
↔
4
,
3
↔
4
)
Technically, they are about
3.29 eV
apart (Appendix B.9), which means it takes about
3.29 V
to transfer a single electron from the
3
d
up to the
4
s
.
We could also say that since the
3
d
orbitals are lower in energy, transferring one electron to a lower-energy orbital is helpful anyways from a less quantitative perspective.
COMPLICATIONS DUE TO ORBITAL SIZE
Note that for example,
W
has a configuration of
[
X
e
]
5
d
4
6
s
2
, which seems to contradict the reasoning we had for
Cr
, since the pairing occurred in the higher-energy orbital.
But, we should also recognize that
5
d
orbitals are larger than
3
d
orbitals, which means the electron density can be more spread out for
W
than for
Cr
, thus reducing the pairing energy
Π
.
That is,
Π
W
5 0
2 years ago
Read 2 more answers
Which component of a plant makes the cell walls around plant cells tough and allows branches, stems, and leaves to be strong
Vsevolod [243]
<h3><u>Answer;</u></h3>

Cellulose

<h3><u>Explanation</u>;</h3>
  • Cellulose is a polysaccharide and the most abundant organic compound on the Earth's surface.
  • <em><u>It  is an important organic molecule due to its strong structure which provides a wide variety of functions. </u></em>
  • <em><u>Cellulose is a major component of tough cell walls that surround plant cells and is what makes plant stems, leaves, and branches very  strong.</u></em>
  • The molecules of cellulose are arranged such that they are parallel to each other joined by hydrogen bond. this arrangement forms long structures that combine with other cellulose molecules producing a strong support structure.
8 0
3 years ago
Compare and contrast the functions of each of the following types of flasks: filter, Erlenmeyer, volumetric. (Site 2)
gogolik [260]

Filter flasks are also known as vacuum, suction or the Buchner flasks. They have thick walls and also have a short glass tube. The thick walls are designed to enable the filter withstand high pressures of vacuum applied to filter substances. Generally this is used for filtering.

While the Erlenmeyer flask also called as a conical flask, is a titration flask which consists of a conical body, a flat bottom, and round neck. This is used for used for general uses such as mixing, titrations, preparation of cultures, for recrystallization, and for supporting filter funnels.

<span>Lastly, the Volumetric flasks are graduated flasks which having markings for different volumes. They are calibrated accurately for a specific amount of liquid that can be contained in it hence this is specially used for storing precise amounts of liquid. </span>
7 0
3 years ago
____H3PO4 + ____ KOH --&gt; ______K3PO4 + ____H2O can someone please balance that chemical equation?
Murrr4er [49]

Answer:

H3PO4 + 3KOH ----> K3PO4 + 3H2O

Explanation:

The valency of K element is + 1 while that of PO4 compound is -3

Hence, at least 3 K atoms are needed to combine with PO4 to form K3PO4 compound.

Hence, the revised equation will be

H3PO4 + 3KOH ----> K3PO4 + 3H2O

Now, the number of atoms and charges of each element is a given equation are equal on both the left and right hand side.

6 0
2 years ago
What types of fields does moving electricity produce?
Lena [83]

The electric and magnetic fields are generated by moving electric charges, the electric and magnetic fields interact with each other, the electric and magnetic fields produce forces on electric charges, the electric charges move in space.

<h3> I hope it'll help you....</h3>
3 0
2 years ago
Other questions:
  • Given the formula: what is a chemical name of this compound? propane propanal propanol propanone
    12·1 answer
  • For the following, identify the most likely value for x.a. BHx b. CHx c. NHx d. CH2Clx
    15·1 answer
  • Why greenhouse affect referred to as a "blanket" around the atmosphere?
    7·1 answer
  • Heating gas to create plasma can yield
    15·1 answer
  • a scientist uses 68 grams of CaCo3 to prepare 1.5 liters of solution. what is the molarity of this solution?
    14·1 answer
  • Wind is related to the movement of warm and cold air masses. Which kind of heat transfer does this represent?
    15·1 answer
  • What is the mass of 42.0 mL of a liquid with a density of 1.65 g/mL?
    14·1 answer
  • Find the pH of a 0.0015 M HCl solution
    12·1 answer
  • 25 L of a gas is collected at 115 kPa. If the pressure increases to 300 kPa, what is the new volume?
    9·1 answer
  • A girl and her dog are running away from their house. The dog has a mass of 20kg. The girl has a mass of 60 kg.
    9·1 answer
Add answer
Login
Not registered? Fast signup
Signup
Login Signup
Ask question!