This phenomenon is of four types:. For example, atomic numer of Sodium is 11, so its electronic configuration: K 2 L 8 M 1. The outermost shell of Sodium atom contain only one electron, so numer of valence electron in Na is 1.
By this method an atom of any elementcan be represented. The symbol of the element represents nucleus and electrons in inner shell. The dots on the symbol represents the number of valence electrons in that atom. Valency is the number of valence electrons of an element which actually take part in any chemical reaction.
The valency of an element is:. Such elements are having variable valence. The chemical reactivity of an element depends upon its electronic configuration. The noble gases — Helium, Neon, Argon etc do not show any chemical reactivity. Therefore, it can be said that elements having incomplete outermost shell are reactive. Molecules of an atom are made up of the atoms of the same type.
Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. In the Laboratory, scientists are able to make other isotopes or varieties of Helium but these are often unstable and they decay into something else. This makes them radioactive. These are extremely short lived isotopes varieties. Helium 5 has 3 neutrons and has a half-life of 0.
So Helium 5 does not stick around very long. The half-life is the measured prediction of the time it takes for half of the atoms in a sample to decay and turn into something else in a radioactive process. Helium 6 has 4 neutrons and is somewhat more stable than Helium 5. Half of Helium 6 will decay in 0.
To figure how many protons are in He 8 is easy, we just use the atomic number, 2 protons. The term nuclide generally refers to atoms of different elements. We already know what isotopes are. The term isotope generally refers to atoms of the same element, having the same number of protons. So when we talk of isotopes we are referring to varieties of a specific element, varieties of Carbon for example are isotopes of Carbon.
Now when we talk of nuclides , we are referring to varieties of atoms of all types. So we use the word nuclides when referring to varieties of elements. Below is a small nuclide chart. I am using an old chart from the Knolls Atomic Power Laboratory to keep the chart small for teaching purposes. There is a more up to date nuclide chart available on the internet from Japan. Lets look at the chart.
The names of the stable atoms are red and the names of all the radioactive elements that breakdown are blue. In each box are two pieces of information. The top symbol indicates the name of the nuclide. The bottom symbol for the stable elements indicate the percent of natural occurrence. If we had a sample of Lithium, we can see that it would be made of 7. The bottom symbol for the radioactively unstable elements gives the half-life for that nuclide. Looking at Carbon 15, we can see that it has a half-life of 2.
If you had a sample of Carbon 15, it would take 2. It would be changing right before your eyes! Notice that the stable elements are organized into a line where there are approximately equal numbers of protons and neutrons. On either side of this line are the unstable nuclides.
As a general rule, the half-life gets shorter the further a nuclide is away from the stable group of nuclides. The atoms with the longest half-lives are those nuclides that are next to the stable elements. These atoms are more stable however they still break down. There are exceptions, looking at the chart, Helium 5 is one of the most glaring exceptions.
It has one of the shortest half-life of all the nuclides on the chart, yet it is right next to the stable group of nuclides He 4 and Li 6 but also there are other Helium isotopes further away from the stable nuclides with longer half-lives. This chart to the left is like the chart above except that each nuclide is reduced to a dot. No information for each nuclide can be given, we just want to see the pattern or distribution of stable nuclides.
So, only the stable nuclides are indicated on this chart. There are some very interesting things to see and notice in this chart. Only nuclides with certain characteristics are stable. Other nuclides that do not match these characteristics are not stable. Let's note some of these characteristics to try to understand what it is that make these atoms stable. There are around known nuclides but only are stable nuclides. Bismuth with 83 protons is the last stable nuclide. All nuclides with more than 83 protons are unstable.
Tin with 50 protons has the highest number of stable isotopes. There are 10 isotopes of tin. You can see that the ratio of neutrons to protons changes as we look at larger and still larger atoms. H 1 has no neutrons at all. Up to about Ca 40 20 protons the ratio is about 1 to 1. However even then we can see the line start to shift away from a strict 1 to 1 ratio. Beyond Calcium, the ratio strays away from the 1 to 1 ratio to maybe 1 to 1.
Bismuth has a ratio of 1 to 1. From the chart we can see that nuclides are more likely to have even numbers of protons and neutrons. It is extremely rare to have odd numbers of both protons and neutrons in a stable nuclide. Also elements that have an odd atomic number usually have only one or two stable isotopes.
Elements having an even atomic number have more stable isotopes. In addition nuclides having 2, 8, 20, 50, 82 protons or neutrons are usually more stable than other nuclides. For example, there are five stable calcium isotopes that have 20 protons. There are only two stable isotopes of potassium having 19 protons, and only one stable scandium nuclide having 21 protons.
We can also see the same kind of phenomena with neutrons. There are four stable nuclides that have 20 neutrons. However there are no stable nuclides that have either 19 or 21 neutrons. Tin Sn having 50 protons is even more dramatic with 10 stable isotopes. So, why are some nuclides stable and others, the majority, are not stable? Why are all the super large nuclides above 83 protons radioactive? Most of us know that opposite charges attract and that like charges repel.
From the start it might seem that the nucleus of the atom might have problems with stability since there are all these positively charged protons. What is it that keeps the nucleus from flying apart.
Many think that neutrons help to keep the nucleus together by providing a nuclear force that is able to keep the protons and neutrons together in the nucleus.
Without the neutrons, the positive charged protons would cause a repulsion force that would result in the nucleus flying apart. For small nuclides it takes approximately equal numbers of neutrons and protons for it to be stable. Apparently, having too many neutrons is just as unstable as not having enough neutrons.
When we look at larger and larger nuclides having more and more positive charge within the nucleus, we see that a higher percentage of neutrons are needed.
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