Search found 59 matches
- Thu Dec 05, 2019 10:54 pm
- Forum: Calculating the pH of Salt Solutions
- Topic: Question 6D.11
- Replies: 2
- Views: 282
Question 6D.11
For parts (e) and (f), in which small, highly charged cations are acting as Lewis acids and accepting electrons from water, how is it known that Al 3+ and Cu 2+ bind to 6 water molecules?
- Thu Dec 05, 2019 10:47 pm
- Forum: Shape, Structure, Coordination Number, Ligands
- Topic: Coordination #
- Replies: 6
- Views: 465
Re: Coordination #
Coordination number refers to the number of points to which ligands are attached to the central atom. For example, in [NiCl4]2-, the nickel is attached to 4 Cl- ligands, so the coordination number is 4.
- Thu Dec 05, 2019 10:42 pm
- Forum: Properties & Structures of Inorganic & Organic Acids
- Topic: H2CO3 vs H2GeO3 [ENDORSED]
- Replies: 3
- Views: 975
Re: H2CO3 vs H2GeO3 [ENDORSED]
For oxoacids with the same number of oxygens, look at electronegativity. Since C is more electronegative than Ge, it will pull electron density away from the O-H bond. This strengthens the polarity of the O-H bond, and also weakens the bond. The H+ is more likely to be removed.
- Thu Dec 05, 2019 10:39 pm
- Forum: Properties & Structures of Inorganic & Organic Acids
- Topic: HBrO2 vs HClO2
- Replies: 1
- Views: 179
Re: HBrO2 vs HClO2
Look at atomic radius for binary acids (like HBr vs HCl). Since Br is larger, the bond is longer and therefore weaker, so H is more likely to be removed. HBr would be the stronger acid. However, for oxoacids in which the number of oxygens are the same, look at electronegativity. Since Cl is more ele...
- Thu Dec 05, 2019 10:10 pm
- Forum: Properties & Structures of Inorganic & Organic Acids
- Topic: # of Oxygens and Acidity
- Replies: 1
- Views: 114
Re: # of Oxygens and Acidity
The more oxygens available, the more stable the resulting anion will be. The oxygens will pull electron density away from the hydrogen, making it easier to remove.
- Wed Dec 04, 2019 7:56 pm
- Forum: Biological Examples
- Topic: Porphyrin ligand
- Replies: 1
- Views: 166
Porphyrin ligand
What is the charge of a porphyrin ligand?
- Sun Dec 01, 2019 4:58 pm
- Forum: Shape, Structure, Coordination Number, Ligands
- Topic: Homework problem 9C #9d
- Replies: 1
- Views: 139
Re: Homework problem 9C #9d
edta refers to ethylenediaminetetraacetato. The ethylenediamine group contains two nitrogen atoms with lone pairs, and therefore two binding sites. The acetate regions each contain one binding site, or lone pair to donate, on one oxygen. Since there are 4 (tetra) acetate regions, this would make 4 b...
- Sun Dec 01, 2019 4:50 pm
- Forum: Shape, Structure, Coordination Number, Ligands
- Topic: polydentate
- Replies: 3
- Views: 172
Re: polydentate
When a ligand is polydentate, it can bind to more than one binding site simultaneously. For example, ethylenediamine, NH2CH2CH2NH2, has a nitrogen with a lone pair of electrons on each end. It can therefore bind to two sites on the central metal atom. Ethylenediamine is bidentate. The term polydenta...
- Sun Dec 01, 2019 4:28 pm
- Forum: Shape, Structure, Coordination Number, Ligands
- Topic: Oxidation Number
- Replies: 9
- Views: 731
Re: Oxidation Number
I would. It is important to specify the oxidation number of the metal, as many d-block elements can form a variety of charges. The added Roman numeral helps to specify the oxidation number of the particular metal atom, as well as the overall charge of the coordination compound.
- Sun Dec 01, 2019 4:26 pm
- Forum: Shape, Structure, Coordination Number, Ligands
- Topic: Hw problem 9C1.A
- Replies: 2
- Views: 175
Re: Hw problem 9C1.A
In this example, to determine oxidation number, it is important to know that -CH has a -1 charge. Since the overall charge is 4-, Fe must have a +2 charge, giving the name hexacyanoferrate (II) ion.
- Sun Dec 01, 2019 1:32 pm
- Forum: Bronsted Acids & Bases
- Topic: Ammonia NH3
- Replies: 1
- Views: 128
Re: Ammonia NH3
Yes, it is possible for ammonia to act as an acid. It is more common as a base, usually accepting a proton from water. However, in the reaction
2Na (l) + 2 NH3 (g) -----> 2NaNH2 (s) + H2 (g), ammonia acts as a proton donor, and therefore an acid, to produce sodium amide.
2Na (l) + 2 NH3 (g) -----> 2NaNH2 (s) + H2 (g), ammonia acts as a proton donor, and therefore an acid, to produce sodium amide.
- Sat Nov 30, 2019 4:21 pm
- Forum: Calculating pH or pOH for Strong & Weak Acids & Bases
- Topic: Question 6B.9
- Replies: 1
- Views: 250
Question 6B.9
In textbook question 6B.9, the concentration of [H30+] is given as 1.50. When calculating the pH, (-log[H3O+]= -log[1.50], I got the answer -0.176. Yet, the solution guide says 0.176. How can the pH be positive when the concentration is greater than 1?
Re: Oxidation
The oxidation number, in reference to coordination compounds and complexes, is the number of electrons gained or lost (like charge) on the metal. For example, in the ion [Cr (Cl)2 (NH3)4]+, the oxidation number is III, giving the name tetraamminedichloridochromium (III). This is because the ion over...
- Sun Nov 24, 2019 5:22 pm
- Forum: Naming
- Topic: oxidation state equation
- Replies: 2
- Views: 198
Re: oxidation state equation
Usually, the oxidation number of the metal can be calculated using the overall charge of the compound and the known charges of anions. For example, in the ion [FeCl(OH2)5]+, the overall charge is +1, water is neutral, and Cl has a charge of -1. So, Fe must have a charge of +2. The oxidation number i...
- Sun Nov 24, 2019 5:15 pm
- Forum: Naming
- Topic: Cobalt vs. Cobaltate
- Replies: 4
- Views: 381
Re: Cobalt vs. Cobaltate
The suffix -ate is added to the stem of the metal's name if the complex has an overall negative charge (an anionic complex). For example, the complex [Ni(CN)4] has a 2- charge, so the name of the ion would be hexacyanidoferrate (II) ion. Hope that helps!
Re: Prefixes
Use bis-, tris-, tetrakis-, etc. if the ligand already contains a Greek prefix or if it is polydentate (able to attach at more than one binding site simultaneously.) An example would be K2 [Cr(OH2)2(ox)2], which would be read as potassium diaquabis(oxalato)chromate (II).
- Sun Nov 24, 2019 3:01 pm
- Forum: Hybridization
- Topic: Why do we use Hybridization?
- Replies: 2
- Views: 190
Re: Why do we use Hybridization?
Hybridization of valence atomic orbitals is necessary in bond formation, and helps to explain the observed structure. For example, the ground state electron configuration of carbon is [He]2s2 2px1 2py1. However, due to hybridization, the experimentally determined structure is better explained by mix...
- Sun Nov 24, 2019 2:55 pm
- Forum: Naming
- Topic: 9C.1 - Ate ending
- Replies: 1
- Views: 122
Re: 9C.1 - Ate ending
If the complex has a negative charge, then -ate is added to the end of the metal name. Since parts a and c have a negative charge, -ate is added. Parts b and d do not have a negative charge.
- Sun Nov 17, 2019 4:33 pm
- Forum: Dipole Moments
- Topic: London Forces
- Replies: 4
- Views: 202
Re: London Forces
Yes, all molecules have London Dispersion interactions. This is because at any instant, the electron clouds of atoms and molecules are not uniform. One region can have a fleeting partial charge while another has a fleeting negative charge. These interactions can therefore occur between all molecules...
- Sun Nov 17, 2019 4:19 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: Clarification on 2E.5
- Replies: 1
- Views: 110
Re: Clarification on 2E.5
In the Lewis Structure, the central atom is Cl, which has a lone pair. The VSEPR formula is therefore AX2E, giving the molecule an angular shape. In the trigonal planar shape, all angles would be 120 degrees. However, in the angular shape, the lone pair on the central atom (in this case Cl) repels t...
- Sun Nov 17, 2019 4:14 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: 2E.7 help
- Replies: 1
- Views: 79
Re: 2E.7 help
The O-S-Cl angles are identical because the lone pair (the shape is trigonal pyramidal) repels the bonded electron pairs equally, and therefore all O-S-Cl bond angles are compressed equally.
- Sun Nov 17, 2019 3:57 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: replusion strength
- Replies: 2
- Views: 184
Re: replusion strength
This means that 2 lone pairs repel each other more than a lone pair and a bonding pair, which repel each other more than a bonding pair and another bonding pair. This relationship explains the distortion of angles when there are lone pairs present on the central atom.
- Sun Nov 17, 2019 3:54 pm
- Forum: Determining Molecular Shape (VSEPR)
- Topic: 2E.21
- Replies: 1
- Views: 102
Re: 2E.21
Yes, when there is more than one "central" atom, the bonding about each atom is treated independently. In the case of C2H4, the three bonding pairs about each carbon result in a trigonal planar shape, in which each H-C-H bond and each H-C-C bond are 120 degrees.
- Sun Nov 17, 2019 3:51 pm
- Forum: Bond Lengths & Energies
- Topic: Angle Distortion
- Replies: 4
- Views: 332
Re: Angle Distortion
For example, in the case of a trigonal pyramidal shape, in which there are 3 bonding pairs and one lone pair, the lone pair- bonding pair repulsion is stronger than a bonding pair- bonding pair repulsion. This condenses the angle to less than 109.5. The tetrahedral shape lacks this lone pair-bonding...
- Sun Nov 10, 2019 6:28 pm
- Forum: Formal Charge and Oxidation Numbers
- Topic: Partial Charge
- Replies: 2
- Views: 188
Re: Partial Charge
The formal charge of an atom indicates the gain or loss of electrons while forming a covalent bond. It is a comparison of the neutral atom to the atom in the molecule, covalently bonded to other atoms. Partial charge, on the other hand, is the result of differences in electronegativities of atoms th...
- Sun Nov 10, 2019 5:46 pm
- Forum: Dipole Moments
- Topic: Dipole Moment
- Replies: 6
- Views: 143
Re: Dipole Moment
A dipole moment is essentially the measurement of charge differences (when the molecule has a dipole). It can be calculated by multiplying the charge (q) times distance between atoms. Dipole moment therefore increases with larger distances and charges.
- Sun Nov 10, 2019 5:36 pm
- Forum: Ionic & Covalent Bonds
- Topic: London Dispersion
- Replies: 13
- Views: 636
Re: London Dispersion
London Dispersion forces can happen in all molecules because of the fluctuating electron distribution on atoms results in fluctuating dipoles. These interactions are very weak and are easily broken.
- Sun Nov 10, 2019 5:28 pm
- Forum: Dipole Moments
- Topic: Hydrogen bonding base pairs
- Replies: 4
- Views: 333
Re: Hydrogen bonding base pairs
AT is held together by two hydrogen bonds, while GC is held together by three.
- Sun Nov 10, 2019 5:22 pm
- Forum: Interionic and Intermolecular Forces (Ion-Ion, Ion-Dipole, Dipole-Dipole, Dipole-Induced Dipole, Dispersion/Induced Dipole-Induced Dipole/London Forces, Hydrogen Bonding)
- Topic: Hydrogen bonding
- Replies: 7
- Views: 390
Re: Hydrogen bonding
Hydrogen bonds are present in molecules with N, O, or F atoms. An H atom on one molecule is bonded to an electronegative atom, giving it a partial positive charge, and when it becomes close to another electronegative atom that has an available lone pair, a hydrogen bond forms. Hydrogen bonds are not...
- Sun Nov 03, 2019 6:30 pm
- Forum: Bond Lengths & Energies
- Topic: Does bond length have any effects?
- Replies: 5
- Views: 208
Re: Does bond length have any effects?
Yes, shorter bonds are stronger bonds and require more energy to break. Additional bonding electrons attract the two nuclei more strongly and pull the atoms closer together, strengthening the bond overall.
- Sun Nov 03, 2019 6:19 pm
- Forum: Electronegativity
- Topic: electronegativity and electron affinity
- Replies: 4
- Views: 182
Re: electronegativity and electron affinity
Electron affinity and electronegativity are related in that elements with high electron affinities and high ionization energies have high electronegativities. However, electronegativity refers to the electron-pulling power of an atom when it is part of a molecule. Electron affinity refers to a neutr...
- Sun Nov 03, 2019 6:14 pm
- Forum: Trends in The Periodic Table
- Topic: Ionization Energy
- Replies: 2
- Views: 100
Re: Ionization Energy
There is an exception for ionization in regards to oxygen. While ionization energy normally increases across a period, the ionization energy of oxygen is actually less than that of nitrogen. This is because nitrogen has one electron in each p orbital, which has half-filled stability. This configurat...
- Sun Nov 03, 2019 6:07 pm
- Forum: Polarisability of Anions, The Polarizing Power of Cations
- Topic: 2D. 9
- Replies: 1
- Views: 82
Re: 2D. 9
Polarizing power refers to the ability of atoms and ions to cause large distortions in the electron clouds of other atoms. Smaller cations that are highly charged have high polarizing power, as they can get very close to anions, and their positive charge can distort the electron cloud of the anion. ...
- Sun Nov 03, 2019 4:45 pm
- Forum: Coordinate Covalent Bonds
- Topic: Definition
- Replies: 17
- Views: 780
Re: Definition
A coordinate covalent bond is a bond in which both electrons come from one of the atoms. For example, when boron trifluoride reacts with ammonia, the lone pair on ammonia completes boron's octet. Both electrons needed to complete boron's octet come from ammonia, rather than some coming from boron an...
- Sun Nov 03, 2019 4:39 pm
- Forum: Quantum Numbers and The H-Atom
- Topic: Difference between n, l, and m...?
- Replies: 2
- Views: 412
Re: Difference between n, l, and m...?
N is the principal quantum number, referring to the shell in which an electron is located. "l" refers to the angular momentum, and describes the subshell, or shape of the orbital (s, p, d, or f orbital). ml, on the other hand, refers to the different orbitals within the subshells. For exam...
- Sun Oct 27, 2019 8:23 pm
- Forum: Electron Configurations for Multi-Electron Atoms
- Topic: 2A.15
- Replies: 3
- Views: 130
Re: 2A.15
In approaching a problem like this, it is best to determine the number of valence electrons to figure out the most likely charge for the ions instead of the entire electron configuration. For example, S has six valence electrons, and will most likely gain two electrons to fill the octet. Therefore, ...
- Sun Oct 27, 2019 8:18 pm
- Forum: Trends in The Periodic Table
- Topic: 1F.5
- Replies: 1
- Views: 71
Re: 1F.5
The trend for first ionization energy is as follows: first ionization energies typically decrease down a group, and increase across a period. They decrease down a group because the outermost electrons occupy shells farther and farther from the nucleus, and therefore are less tightly bound and easier...
- Sun Oct 27, 2019 7:24 pm
- Forum: Lewis Structures
- Topic: 2A.19
- Replies: 3
- Views: 119
Re: 2A.19
In the example of Sb 3+, the electron configuration would be [Kr] 4d^10 5s^2. Since every orbital is filled with two electrons, there are no unpaired electrons. However, in the case of Ni 2+, the electron configuration would be [Ar] 3d^8. Since there are 5 orbitals in the 3d-orbital, this means that...
- Sun Oct 27, 2019 6:35 pm
- Forum: Ionic & Covalent Bonds
- Topic: 2A 5
- Replies: 2
- Views: 106
Re: 2A 5
For a copper atom to have a positive charge, it must lose an electron, becoming an anion. Therefore, Cu+ contains 28 electrons instead of the 29 of Cu. The electron configuration for Cu is [Ar]3d^10 4s^1. After losing an electron and becoming Cu +, the electron configuration would be [Ar]3d^10.
- Sun Oct 27, 2019 4:44 pm
- Forum: Electron Configurations for Multi-Electron Atoms
- Topic: Orbital Energies
- Replies: 1
- Views: 114
Re: Orbital Energies
P-electrons penetrate much less than s-electrons because the orbital angular momentum prevents it from approaching close to nucleus. An s-electron is much closer to nucleus (the probability density is nonzero at the nucleus), and can therefore penetrate more. Therefore, since p-electrons penetrate l...
- Sun Oct 27, 2019 3:24 pm
- Forum: Ionic & Covalent Bonds
- Topic: ionization
- Replies: 5
- Views: 299
Re: ionization
Ionization energy refers to the energy needed to remove an electron from an atom. When looking at a periodic table, ionization energies decrease down a group, as the value of n increases and electrons are further from the nucleus. Ionization energies also increase across a period, as the electrons a...
- Sun Oct 20, 2019 6:28 pm
- Forum: Properties of Light
- Topic: 1B. 5
- Replies: 2
- Views: 95
Re: 1B. 5
First, it is necessary to convert 140.511 keV into Joules. Since there are 1.6022 x 10^-19 J per eV, multiply (140.511 x 10^3 eV) x ( 1.6022 x 10^-19 J/eV) to obtain 2.2513 x 10^-14 J. Combining the formulas E=hv and c= v x lambda, solve for lambda= hc/E. Using Planck's constant, the speed of light,...
- Sun Oct 20, 2019 4:46 pm
- Forum: Wave Functions and s-, p-, d-, f- Orbitals
- Topic: 1E.5
- Replies: 1
- Views: 113
Re: 1E.5
Part c is false, because electrons become less able to penetrate the nucleus as l increases. l=1 refers to the p-orbital, whereas l=2 refers to the d-orbital. Electrons in the p-orbital are held more tightly to the nucleus than those in the d-orbital and therefore can penetrate the nucleus and shiel...
- Sun Oct 20, 2019 4:29 pm
- Forum: Properties of Electrons
- Topic: Hund's Rule
- Replies: 5
- Views: 197
Re: Hund's Rule
Hund's Rule states that due to electron repulsion, electrons in the same subshell occupy different orbitals with parallel spin. It is used, along with the Pauli Exclusion Principle to obtain ground state electron configurations.
- Sun Oct 20, 2019 4:15 pm
- Forum: Electron Configurations for Multi-Electron Atoms
- Topic: 1E.11
- Replies: 2
- Views: 110
Re: 1E.11
To obtain the ground state electron configuration of an atom, apply both the Pauli exclusion principle (no more than 2 e- per orbital) and Hund's Rule (e- in the same subshell occupy different orbitals with parallel spin). This will result in the lowest energy state. For a sodium atom, the ground st...
- Sun Oct 20, 2019 3:56 pm
- Forum: Heisenberg Indeterminacy (Uncertainty) Equation
- Topic: h bar formula [ENDORSED]
- Replies: 4
- Views: 358
Re: h bar formula [ENDORSED]
The formula given during lecture for Heisenberg's Indeterminacy Equation utilized h rather than h bar. It read (delta p)(delta x) is greater than or equal to (h/ 4pi). In the textbook, instead of using (h/4pi), they described the equation in terms of (h bar/2) which is actually equivalent since h ba...
- Sun Oct 20, 2019 3:39 pm
- Forum: Bohr Frequency Condition, H-Atom , Atomic Spectroscopy
- Topic: 1D. 25
- Replies: 4
- Views: 173
Re: 1D. 25
Possible values of l include 0, 1, 2..... n-1. Therefore, when n=2, l can have the values of only 0 and 1. "d" corresponds to an l value of 2, so therefore 2d cannot exist. With n=4, l can have the values of 0, 1, 2, and 3. Therefore, 4d can exist, but 4g cannot (l=4). With n=6, l can have...
- Sun Oct 13, 2019 9:20 pm
- Forum: Photoelectric Effect
- Topic: Equation for threshold energy
- Replies: 2
- Views: 181
Re: Equation for threshold energy
The equation involving threshold energy is as follows: E(photon) - threshold energy (or work function) = Kinetic energy (or excess energy). If the energy of the photon is known as well as the excess energy, the threshold energy can be found. Threshold energy also follows the equation E(threshold ene...
- Sat Oct 12, 2019 5:27 pm
- Forum: Properties of Electrons
- Topic: Electron Excitation
- Replies: 6
- Views: 305
Re: Electron Excitation
When electrons are excited, this refers to their position at a higher energy level than is necessary. On the other hand, ground state refers to when all of the atom's electrons are in their lowest possible orbitals. Ultimately, electrons get excited when they absorb photons.
- Sat Oct 12, 2019 4:36 pm
- Forum: Photoelectric Effect
- Topic: Threshold energy
- Replies: 6
- Views: 136
Re: Threshold energy
Yes, the work function and threshold energy both refer to the energy required to eject an electron from a metal surface. When looking at the equation E(photon)-E(required to remove electron)=E(excess), the energy required to remove the electron refers to the "work" that must be put in to a...
- Sat Oct 12, 2019 3:39 pm
- Forum: Properties of Light
- Topic: Wavelength
- Replies: 4
- Views: 155
Re: Wavelength
Wavelength and frequency are inversely related. Therefore, with a shorter wavelength, there would be a higher frequency, and the wave completes more cycles per second (in Hz.) On the other hand, frequency and energy are directly related. As frequency increases, so will energy, as modeled by the equa...
- Sat Oct 12, 2019 3:25 pm
- Forum: Properties of Electrons
- Topic: Measuring wavelike properties
- Replies: 3
- Views: 130
Re: Measuring wavelike properties
Usually, a particle that has a De Broglie wavelength of less than 10^-15 m does not have detectable wavelike properties. This usually occurs with a greater mass, which will have a smaller wavelength. It will still have particle-like behavior, but not wavelike properties.
- Sat Oct 12, 2019 12:56 pm
- Forum: Einstein Equation
- Topic: Planck's Constant
- Replies: 3
- Views: 290
Re: Planck's Constant
It's also helpful to think of Planck's constant in terms of a graph. If you were to plot the equation for energy of a photon, E=hv, E would be on the y-axis and frequency on the x-axis. In one experiment, measure the energy of light and in another measure the frequency. This would result in a linear...
- Sun Oct 06, 2019 11:34 pm
- Forum: SI Units, Unit Conversions
- Topic: Si Units
- Replies: 3
- Views: 179
Re: Si Units
Giga and mega are often used when describing digital information, for example, gigabytes and megabytes. Pico- is often used to describe the radius of atoms (typically 25 picometers to 260 picometers).
- Thu Oct 03, 2019 6:55 pm
- Forum: Limiting Reactant Calculations
- Topic: Percent yields/error
- Replies: 3
- Views: 1799
Re: Percent yields/error
The percent yield in chemistry, is the fraction of (actual yield/ theoretical yield) multiplied by 100%. That is, the percentage yield reveals what was actually obtained in ratio to the maximum yield possible. On the other hand, percent error is a measure of discrepancy between an observed value and...
- Thu Oct 03, 2019 6:37 pm
- Forum: Accuracy, Precision, Mole, Other Definitions
- Topic: Actual Yield vs Theoretical Yield
- Replies: 6
- Views: 521
Re: Actual Yield vs Theoretical Yield
The actual yield can differ from the theoretical yield for a variety of reasons. For example, some of the material might stick to the sides of the beakers or there may be impurities in the substances. Additionally, side reactions may cause the actual yield to be less from the theoretical yield. The ...
- Thu Oct 03, 2019 1:11 pm
- Forum: Accuracy, Precision, Mole, Other Definitions
- Topic: Fundamentals E.15
- Replies: 3
- Views: 270
Fundamentals E.15
The question reads: "The molar mass of the metal hydroxide M(OH)2 is 74.10 g/mol. What is the molar mass of the sulfide of this metal?" What does the question mean by sulfide?
- Thu Oct 03, 2019 1:05 pm
- Forum: Accuracy, Precision, Mole, Other Definitions
- Topic: differentiating between different types of masses
- Replies: 2
- Views: 250
Re: differentiating between different types of masses
Molar mass (M) refers to the mass per mole of particles (g/mol). This could mean that the molar mass of an element would be the mass per mole of its atoms, the molar mass of a molecular compound is the mass per mole of its molecules and the molar mass of an ionic compound is the mass per mole of its...
- Tue Oct 01, 2019 10:18 am
- Forum: Accuracy, Precision, Mole, Other Definitions
- Topic: Aqueous Solution
- Replies: 6
- Views: 130
Re: Aqueous Solution
To expand on the example of NaCl dissolving in water, the reason why the oxygen atom in water has a slight negative charge is due to its stronger electronegativity. Water is a polar molecule, and therefore, oxygen has a slight negative charge since it attracts electrons more strongly, and hydrogen a...