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Atomic hydrogen constitutes about 75% of the baryonic mass of the universe. In everyday life on Earth, isolated hydrogen atoms (called 'atomic hydrogen') are extremely rare. Instead, a hydrogen atom tends to combine with other atoms in compounds, or with another hydrogen atom to form ordinary hydrogen gas, H 2. 'Atomic hydrogen' and 'hydrogen. The atomic mass is useful in chemistry when it is paired with the mole concept: the atomic mass of an element, measured in amu, is the same as the mass in grams of one mole of an element. Thus, since the atomic mass of iron is 55.847 amu, one mole of iron atoms would weigh 55.847 grams. The number of atoms or molecules (n) in a mass (m) of a pure material having atomic or molecular weight (M) is easily computed from the following equation using Avogadro's number (NA = 6.022×10 23 atoms or molecules per gram-mole): M mN n A (1) In some situations, the atomic number density (N), which is the concentration of atoms or molecules per. Name: Hydrogen Symbol: H Atomic Number: 1 Atomic Mass: 1.00794 amu Melting Point:-259.14 °C (14.009985 K, -434.45203 °F) Boiling Point:-252.87 °C (20.280005 K, -423.166 °F) Number of Protons/Electrons: 1 Number of Neutrons: 0 Classification: Non-metal Crystal Structure: Hexagonal Density @ 293 K: 0.08988 g/cm 3 Color: colorless Atomic Structure. Calculate the average atomic mass of hydrogen using the following data Isotope% Natural abundance Molar mass `.^(1)H 99.985 1` `.^(2)H 0.015 2`.
Atomic and Molecular Weights
The subscripts in chemical formulas, and the coefficients in chemical equations represent exact quantities. (ce{H_2O}), for example, indicates that a water molecule comprises exactly two atoms of hydrogen and one atom of oxygen. The following equation:
[ ce{C3H8(g) + 5O2(g) rightarrow 3CO2(g) + 4H2O(l)} label{Eq1}]
not only tells us that propane reacts with oxygen to produce carbon dioxide and water, but that 1 molecule of propane reacts with 5 molecules of oxygen to produce 3 molecules of carbon dioxide and 4 molecules of water. Since counting individual atoms or molecules is a little difficult, quantitative aspects of chemistry rely on knowing the masses of the compounds involved.
Atoms of different elements have different masses. Early work on the separation of water into its constituent elements (hydrogen and oxygen) indicated that 100 grams of water contained 11.1 grams of hydrogen and 88.9 grams of oxygen:
[text{100 grams Water} rightarrow text{11.1 grams Hydrogen} + text{88.9 grams Oxygen} label{Eq2}]
Later, scientists discovered that water was composed of two atoms of hydrogen for each atom of oxygen. Therefore, in the above analysis, in the 11.1 grams of hydrogen there were twice as many atoms as in the 88.9 grams of oxygen. Therefore, an oxygen atom must weigh about 16 times as much as a hydrogen atom:
[ dfrac{dfrac{88.9;g;Oxygen}{1; atom}}{dfrac{111;g;Hydrogen}{2;atoms}} = 16 label{Eq3}]
Hydrogen, the lightest element, was assigned a relative mass of '1', and the other elements were assigned 'atomic masses' relative to this value for hydrogen. Thus, oxygen was assigned an atomic mass of 16. We now know that a hydrogen atom has a mass of 1.6735 x 10-24 grams, and that the oxygen atom has a mass of 2.6561 X 10-23 grams. As we saw earlier, it is convenient to use a reference unit when dealing with such small numbers: the atomic mass unit. The atomic mass unit (amu) was not standardized against hydrogen, but rather, against the 12C isotope of carbon (amu = 12).
Thus, the mass of the hydrogen atom (1H) is 1.0080 amu, and the mass of an oxygen atom (16O) is 15.995 amu. Once the masses of atoms were determined, the amu could be assigned an actual value:
1 amu = 1.66054 x 10-24grams conversely: 1 gram = 6.02214 x 1023amu
All matter, including mineral crystals, is made up of atoms, and all atoms are made up of three main particles: protons, neutrons, and electrons. As summarized in Table 2.1, protons are positively charged, neutrons are uncharged and electrons are negatively charged. The negative charge of one electron balances the positive charge of one proton. Both protons and neutrons have a mass of 1, while electrons have almost no mass.
Elementary Particle | Charge | Mass |
---|---|---|
Proton | +1 | 1 |
Neutron | 0 | 1 |
Electron | −1 | ~0 |
The element hydrogen has the simplest atoms, each with just one proton and one electron. The proton forms the nucleus, while the electron orbits around it. All other elements have neutrons as well as protons in their nucleus, such as helium, which is depicted in Figure 2.2. The positively charged protons tend to repel each other, and the neutrons help to hold the nucleus together. The number of protons is the atomic number, and the number of protons plus neutrons is the atomic mass. For hydrogen, the atomic mass is 1 because there is one proton and no neutrons. For helium, it is 4: two protons and two neutrons.
For most of the 16 lightest elements (up to oxygen) the number of neutrons is equal to the number of protons. For most of the remaining elements, there are more neutrons than protons, because extra neutrons are needed to keep the nucleus together by overcoming the mutual repulsion of the increasing numbers of protons concentrated in a very small space. For example, silicon has 14 protons and 14 neutrons. Its atomic number is 14 and its atomic mass is 28. The most common isotope of uranium has 92 protons and 146 neutrons. Its atomic number is 92 and its atomic mass is 238 (92 + 146).
The dot in the middle is the nucleus, and the surrounding cloud represents where the two electrons might be at any time. The darker the shade, the more likely that an electron will be there. An angstrom (Å) is 10-10m . A femtometre (fm) is 10-15m. In other words, a helium atom’s electron cloud is about 100,000 times bigger than its nucleus.
Electrons orbiting around the nucleus of an atom are arranged in shells — also known as “energy levels.” The first shell can hold only two electrons, while the next shell holds up to eight electrons. Subsequent shells can hold more electrons, but the outermost shell of any atom holds no more than eight electrons. The electrons in the outermost shell play an important role in bonding between atoms. Elements that have a full outer shell are inert in that they do not react with other elements to form compounds. They all appear in the far-right column of the periodic table: helium, neon, argon, etc. For elements that do not have a full outer shell, the outermost electrons can interact with the outermost electrons of nearby atoms to create chemical bonds. The electron shell configurations for 29 of the first 36 elements are listed in Table 2.2.
Number of Electrons in Each Shell | ||||||
---|---|---|---|---|---|---|
Element | Symbol | Atomic No. | First | Second | Third | Fourth |
Hydrogen | H | 1 | 1 | |||
Helium | He | 2 | 2 | |||
Lithium | Li | 3 | 2 | 1 | ||
Beryllium | Be | 4 | 2 | 2 | ||
Boron | B | 5 | 2 | 3 | ||
Carbon | C | 6 | 2 | 4 | ||
Nitrogen | N | 7 | 2 | 5 | ||
Oxygen | O | 8 | 2 | 6 | ||
Fluorine | F | 9 | 2 | 7 | ||
Neon | Ne | 10 | 2 | 8 | ||
Sodium | Na | 11 | 2 | 8 | 1 | |
Magnesium | Mg | 12 | 2 | 8 | 2 | |
Aluminum | Al | 13 | 2 | 8 | 3 | |
Silicon | Si | 14 | 2 | 8 | 4 | |
Phosphorus | P | 15 | 2 | 8 | 5 | |
Sulphur | S | 16 | 2 | 8 | 6 | |
Chlorine | Cl | 17 | 2 | 8 | 7 | |
Argon | Ar | 18 | 2 | 8 | 8 | |
Potassium | K | 19 | 2 | 8 | 8 | 1 |
Calcium | Ca | 20 | 2 | 8 | 8 | 2 |
Scandium | Sc | 21 | 2 | 8 | 9 | 2 |
Titanium | Ti | 22 | 2 | 8 | 10 | 2 |
Vanadium | V | 23 | 2 | 8 | 11 | 2 |
Chromium | Cr | 24 | 2 | 8 | 13 | 1 |
Manganese | Mn | 25 | 2 | 8 | 13 | 2 |
Iron | Fe | 26 | 2 | 8 | 14 | 2 |
. | . | . | . | . | . | . |
Selenium | Se | 34 | 2 | 8 | 18 | 6 |
Bromine | Br | 35 | 2 | 8 | 18 | 7 |
Krypton | Kr | 36 | 2 | 8 | 18 | 8 |
Attributions
Atomic Mass Of Hydrogen-3
Figure 2.2
Helium Atom by Yzmo is under CC-BY-SA-3.0
Helium Atom by Yzmo is under CC-BY-SA-3.0