chemistry questions?

Question

few questions:
1. How were the elements arranged on Mendeleev's periodic table? How are they arranged today?

2. What are the most stable elements? Why are they stable?

3. How many electrons are in the highest energy level of each group?

4. Which are more reactive, alkali metals or alkaline earth metals? Which are harder and denser?

5. A salt is formed from a ________ and a ___________

6. Which elements were created by man? How was this done?

7. How were the natural elements formed?

8. Which elements are radioactive?

Answer 1

1) In order of increasing atomic mass and grouped by their reactions with other chemicals
2) Noble gases. They have an octet (complete set of 8) valence electrons
3) No more than 8. Due to overlapping of the orbitals you always start a new energy level before you fill the d or f orbitals.
4) Alkali are more reactive (react faster) and alkaline earth are more dense
5) Metal (cation) and nonmetal (anion)
6) Elements over 92 are not naturally found on earth. They are formed from high-speed collisions of nuclei in a partical accelerator. The nuclei fuse together to form a new element which quickly decomposes through nuclear decay.
7) Through nuclear fusion in stars.
8) The elements which will undergo nuclear decay and give off alpha, beta, or gamma radiation.

Answer 2

Your questions are very wide-ranging, so I'll try to provide simple answers which may not be entirely true:
Q-1 Mendeleev was a Russian Chemist who lived in the 19th Centruy.
Mendeleev's arranged the elements known at his time based on several similar properties. For example, lithium (atomic no. 3), sodium atomic no. 11), potassium (atomic no. 19) and other Class I elements are low-temperature melting metals, highly reactive with water, and form ionic compounds with apparent valance 1. On a periodic table chart, they appear in columns going from the most "metallic" on the left to the most "non-metallic" on the right. Heavier elements that most closely resemble lighter elements are in successive rows beneath the lighter elements.

The same concepts is generally used today. However, we know more about the configuration of the electrons about the nucleus and energy levels, something Mendeleev knew nothing about (in his time, it wasn't clear where electrons were or even if there were electrons and atomic numbers were conjecture). So going from left to right, columns represent the alkaline metals (Class I), then the alkaline earth metals, and then trending towards the non-metals (carbon starts Class IV; silicon is the next higher-weight member) and then to the non-metals (Class VII are the halogens and Class VIII are the 'noble' gases). Hydrogen is a special case, since the first level of electrons only allow for 2 elements; the second being the noble gas helium. Moreover, the so-call transition metals are listed separately. These metals have "d" and "f" sub-orbitals of lower order than the "s" and "p" sub-orbitals of elements that have lower atomic numbers. This sounds confusing, but consider the fourth row of elements. Potassium (AN=19) resembles sodium (AN=11) and calcium (AN=20) resembles magnesium (AN=12). However, if we make believe that we are God and can construct elements, the next element (AN=21) does not resemble aluminum (AN=13), which would be expected if the level 3 pattern was followed. Neither do elements up through AN=30). Why? Because the elements from AN=21 to AN=30 have 3d suborbitals available for electrons that are employed before the 4p suborbitals are used. Then element 31 resembles 13, 32 resembles 14 and so on until krypton (AN=36) resembles argon (AN=18).
Q-2
The most stable elements are those of AN=82 (lead) or lower, with a few exceptions. Those of AN=83 or higher seem to be inherently unstable. Physic people believe this is due to the inability of a large number of neutrons (each of which can be thought of as a proton and electron with some "glue") and protons to hold together in a nucleus. There are 2 elements below AN=82 that don't occur naturally, and the reason for this has not be definitely established. Moreover, most elements have isotopes with equal number of protons in their nucelus but a different number of neutrons. Some isotopes of an other stable element are unstable. The best example is carbon. Carbon-12 is stable, but carbon-14 disintergrates, a fact used to date items found by archeologists. Chlorine has two stable isotopes, chlorine-35 and chlorine-37. They occur in a ratio of about 4 to 1, so the atomic weight of chlorine is 35.5 or so.
Q-3
I'll skip on this one. I think you need to rephrase.
Q-4.
Alkali metals are more reactive, the alkaline earths are harder and denser.
Q-5
Back in the old days, salts were formed from ions of metals with non-metals. For example, if you added sodium hydroxide solution to a hydrochloric acid solution, and allowed water to evaporate, sodium chloride would remain. This was the result with the original definition of acids and bases (acid + base = salt + water)Then metallic ion groups and non-metallic ion groups came along, such as ammonium (+1) and nitrate(-1), and they got included in the definition. Then, it was realized that you didn't need to form water to have a salt formed (try blowing ammonia in water over a bottle of hydrogen chloride in water).
Q-6. Generally, the elements above AN=92 (uranium) have been created by man by bombarding their nucleus with sub-atomic particles. Originally, either neutrons or helium nuclei were used. Thus, you could get plutonium-239 from U-235 (U-235 + He-4 = Pu-239). As mentioned above, 2 elements below lead in atomic number are inherently unstable, but have been created by similar methods. The most important is Te-99 with atomic number 43. It is used in medicine.

More recently, the sensitivity of chemical analysis has allowed for detection of plutonium and neptunium in samples, which means that eons ago, they were formed as were other "natural" elements, but over billions of years, disintegrated to the levels they exist now.


Q-7.
Most scientists believe the elements were formed after the formation of the earliest stars from hydrogen. In stars, higher weight elements are fused to generate energy, starting with hydrogen fused to form helium. Apparently this energy-creating process ends with iron (AN=26); above AN-26, energy is required to create elements of higher atomic number. Exploding stars are thought to provide the elements through the universe; in such explosions, elements of atomic number above iron can be formed.
Q-8
I think this has been covered. All elements above AN=82 are radioactive, although their radioactivity may be very slow. For example, pepto-bismol contains bismuth (AN=83), which decays very slowly and each decay reaction is not very powerful. So, if you take pepto-bismol, you aren't zapping yourself with radioactivity to an appreciable extent. There are 2 elements below AN=82 that are inherently radioactive, and otherwise stable elements have isotopes that are radioactive.

Also, there are different ways that an unstable element disintegrates. Naturally, the most important are alpha decay and beta decay. In alpha decay, the nucleus kicks out a helium nucleus ( 2 protons, 2 neutrons). This process leads to an element of 2 atomic number and 4 atomic weight less than the original unstable element. In beta decay, one neutron apparently disintegrates, and an electron is kicked out. This process leads to an element of the same atomic weight and atomic number of 1 more than the original unstable element. All decays lead to creation of energy, which differs for different isotopes.
This energy is measured in electron-volts.