Isotopes (review questions only)

Atoms that have the same atomic number, but different mass numbers due to the number of neutrons.

·         Define isotope.

·         Describe three isotopes of hydrogen.

·         State how isotopes are named.

·         Explain why many isotopes are radioactive.

Is this creature a space alien? It may look alien, but the sketch is actually just a scientist wearing a special suit to protect himself from harmful radiation. The scientist is working with radioactive chemicals called isotopes. Not all isotopes are radioactive, but many of them are. To understand why, you first need to know what isotopes are.

What Are Isotopes?

All atoms of the same element have the same number of protons, but some may have different numbers of neutrons. For example, all carbon atoms have six protons, and most have six neutrons as well. But some carbon atoms have seven or eight neutrons instead of the usual six. Atoms of the same element that differ in their numbers of neutrons are called isotopes. Many isotopes occur naturally. Usually one or two isotopes of an element are the most stable and common. Different isotopes of an element generally have the same physical and chemical properties. That’s because they have the same numbers of protons andelectrons. For a video explanation of isotopes, go to this URL:

http://www.youtube.com/watch?v=6w7raarHNA8 (5:23)

An Example: Hydrogen Isotopes

Hydrogen is an example of an element that has isotopes. Three isotopes of hydrogen are modeled in the Figure below. Most hydrogen atoms have just one proton and one electron and lack a neutron. These atoms are just called hydrogen. Some hydrogen atoms have one neutron as well. These atoms are the isotope named deuterium. Other hydrogen atoms have two neutrons. These atoms are the isotope named tritium. For animated versions of these hydrogen isotopes, go to this URL: http://www.s-cool.co.uk/a-level/physics/atomic-structure/revise-it/isotopes.

Q: The mass number of an atom is the sum of its protons and neutrons. What is the mass number of each isotope of hydrogen shown in the Figure above?

A: The mass numbers are: hydrogen = 1, deuterium = 2, and tritium = 3.

Naming Isotopes

For most elements other than hydrogen, isotopes are named for their mass number. For example, carbon atoms with the usual 6 neutrons have a mass number of 12 (6 protons + 6 neutrons = 12), so they are called carbon-12. Carbon atoms with 7 neutrons have an atomic mass of 13 (6 protons + 7 neutrons = 13). These atoms are the isotope called carbon-13.

Q: Some carbon atoms have 8 neutrons. What is the name of this isotope of carbon?

A: Carbon atoms with 8 neutrons have an atomic mass of 14 (6 protons + 8 neutrons = 14), so this isotope of carbon is named carbon-14.

Stability of Isotopes

Atoms need a certain ratio of neutrons to protons to have a stable nucleus. Having too many or too few neutrons relative to protons results in an unstable, or radioactive, nucleus that will sooner or later break down to a more stable form. This process is called radioactive decay. Many isotopes have radioactive nuclei, and these isotopes are referred to as radioisotopes. When they decay, they release particles that may be harmful. This is why radioactive isotopes are dangerous and why working with them requires special suits for protection. The isotope of carbon known as carbon-14 is an example of a radioisotope. In contrast, the carbon isotopes called carbon-12 and carbon-13 are stable.

Summary

·         Atoms of the same element that differ in their numbers of neutrons are called isotopes. Different isotopes of an element generally have the same physical and chemical properties because they have the same numbers of protons andelectrons.

·         Most hydrogen atoms lack a neutron and are just called hydrogen. Hydrogen atoms with one neutron are the isotope known as deuterium, and those with two neutrons are the isotope named tritium.

·         For most elements other than hydrogen, isotopes are named for their mass number, which is the number of protons plus neutrons. For example, carbon with a mass number of 14 is called carbon-14.

·         Atoms need a certain ratio of neutrons to protons to have a stable nucleus. If they have too many or too few neutrons relative to protons, they are radioactive and will decay to more stable forms. Isotopes with radioactive nuclei are called radioisotopes.

Explore More

At the following URL, watch the video about isotopes of carbon. Then answer the questions below.

Carbon Dating

1.    How does carbon-14 form?

2.    Carbon-14 slowly decays over time because it is radioactive. Why does the percent of carbon-14 remain the same in living organisms?

3.    How can the percent of carbon-14 in a dead organism be used to estimate the amount of time that has passed since the organism died?

Review

1.    What are isotopes?

2.    Why do different isotopes of an element generally have the same physical and chemical properties?

3.    Describe the three isotopes of hydrogen.

4.    Relate the concepts of isotope and mass number.

5.    All oxygen atoms have eight protons, and most have eight neutrons as well. What is the mass number of an oxygen isotope that has nine neutrons? What is the name of this isotope?

6.    Why are many isotopes radioactive?

Energy Level (review questions only)

Introduction to electron shells; fixed distances from the nucleus of an atom where electrons may be found.

·         Describe energy levels.

·         Relate energy levels to orbitals.

·         Explain the significance of electrons in the outermost energy level of an atom.

Fireworks are a great way to celebrate happy events. Do you know what causes the brilliant, colored lights of fireworks? The lights are bursts of energy given off by atoms in the fireworks. What do you suppose causes these bursts of light? The answer has to do with energy levels of atoms.

What Are Energy Levels?

Energy levels (also called electron shells) are fixed distances from the nucleus of an atom where electrons may be found. Electrons are tiny, negatively charged particles in an atom that move around the positive nucleus at the center. Energy levels are a little like the steps of a staircase. You can stand on one step or another but not in between the steps. The same goes for electrons. They can occupy one energy level or another but not the space between energy levels.

The model in the Figure below shows the first four energy levels of an atom.Electrons in energy level I (also called energy level K) have the least amount of energy. As you go farther from the nucleus, electrons at higher levels have more energy, and their energy increases by a fixed, discrete amount. Electrons can jump from a lower to the next higher energy level if they absorb this amount of energy. Conversely, if electrons jump from a higher to a lower energy level, they give off energy, often in the form of light. This explains the fireworks pictured above. When the fireworks explode, electrons gain energy and jump to higher energy levels. When they jump back to their original energy levels, they release the energy as light. Different atoms have different arrangements of electrons, so they give off light of different colors. You can see an animation of electrons jumping from one energy level to another at this URL:http://cas.sdss.org/dr6/en/proj/advanced/spectraltypes/energylevels.asp.

Q: In the atomic model Figure above, where would you find electrons that have the most energy?

A: Electrons with the most energy would be found in energy level IV.

Energy Levels and Orbitals

The smallest atoms are hydrogen atoms. They have just one electron orbiting the nucleus. That one electron is in the first energy level. Bigger atoms have more electrons. Electrons are always added to the lowest energy level first until it has the maximum number of electrons possible. Then electrons are added to the next higher energy level until that level is full, and so on.

How many electrons can a given energy level hold? The maximum numbers of electrons possible for the first four energy levels are shown in the Figure above. For example, energy level I can hold a maximum of two electrons, and energy level II can hold a maximum of eight electrons. The maximum number depends on the number of orbitals at a given energy level. An orbital is a volume of space within an atom where an electron is most likely to be found. As you can see by the images in the Figure below, some orbitals are shaped like spheres (S orbitals) and some are shaped like dumbbells (P orbitals). There are other types of orbitals as well.

Regardless of its shape, each orbital can hold a maximum of two electrons. Energy level I has just one orbital, so two electrons will fill this energy level. Energy level II has four orbitals, so it takes eight electrons to fill this energy level. The animation below shows the electrons in the first two energy levels for an atom of lithium, which has a total of three electrons.

http://web.visionlearning.com/custom/chemistry/animations/CHE1.3-an-lithium.shtml

Q: Energy level III can hold a maximum of 18 electrons. How many orbitals does this energy level have?

A: At two electrons per orbital, this energy level must have nine orbitals.

The Outermost Level

Electrons in the outermost energy level of an atom have a special significance. These electrons are called valence electrons, and they determine many of the properties of an atom. An atom is most stable if its outermost energy level contains as many electrons as it can hold. For example, helium has two electrons, both in the first energy level. This energy level can hold only two electrons, so helium’s only energy level is full. This makes helium a very stable element. In other words, its atoms are unlikely to react with other atoms.

Consider the elements fluorine and lithium, modeled in the Figure below. Fluorine has seven of eight possible electrons in its outermost energy level, which is energy level II. It would be more stable if it had one more electron because this would fill its outermost energy level. Lithium, on the other hand, has just one of eight possible electrons in its outermost energy level (also energy level II). It would be more stable if it had one less electron because it would have a full outer energy level (now energy level I).

Both fluorine and lithium are highly reactive elements because of their number of valence electrons. Fluorine will readily gain one electron and lithium will just as readily give up one electron to become more stable. In fact, lithium and fluorine will react together as shown in the Figure below. When the two elements react, lithium transfers its one “extra” electron to fluorine.

Q: A neon atom has ten electrons. How many electrons does it have in its outermost energy level? How stable do you think a neon atom is?

A: A neon atom has two electrons in energy level I and its remaining eight electrons in energy level II, which can hold only eight electrons. This means that is outermost energy level is full. Therefore, a neon atom is very stable.

Summary

·         Energy levels (also called electron shells) are fixed distances from the nucleus of an atom where electrons may be found. As you go farther from the nucleus, electrons at higher energy levels have more energy.

·         Electrons are always added to the lowest energy level first until it has the maximum number of electrons possible, and then electrons are added to the next higher energy level until that level is full, and so on. The maximum number of electrons at a given energy level depends on its number of orbitals. There are at most two electrons per orbital.

·         Electrons in the outermost energy level of an atom are called valence electrons. They determine many of the properties of an atom, including how reactive it is.

Explore More

Watch the video about electrons at the following URL, and then answer the questions below.

http://www.ehow.com/video_5234848_electrons_.html

1.    How many energy levels exist in atoms?

2.    What is the maximum possible number of electrons at the fifth energy level?

3.    Assume an energy level can hold a maximum of 98 electrons. Which energy level is it? How many orbitals does this energy level have?

Review

1.    What are energy levels?

2.    Relate energy levels to the amount of energy their electrons have.

3.    What must happen for an electron to jump to a different energy level?

4.    How many electrons can the fourth energy level have? How many orbitals are there at this energy level?

5.    An atom of sodium has 11 electrons. Make a sketch of a sodium atom, showing how many electrons it has at each energy level. Infer how reactive sodium atoms are.