1. How is an atom built and what parts does it contain, the properties of the parts?
2. What is an isotope; give examples?
3. Mass number, atomic number, and how they are written together with the chemical symbol?
4. How electrons are distributed in atom shells?
5. Radiation, radiation properties, radioactivity
6. Half-life
7. Carbon 14 method
8. Fission and fusion
9. Questions

How is an atom built and what parts does it contain, the properties of the parts?


The picture shows a helium atom. It has two protons, two neutrons and two electrons. The atom consists of a nucleus and electrons orbiting around.

The atom nucleus consists of protons and neutrons. Protons are positively charged and neutrons are neutrally charged. The atomic weight is made up essentially of the atomic nucleus. The task of the neutrons is to hold together the protons.

Electrons are negatively charged. In an atom, the number of electrons is exactly the same as the number of protons in the nucleus.

If the nucleus is big as a “putter and ball” and located in the middle of Ullevi Stadium (Sweden), the electrons orbit outside the stands and are the size of pinheads. Most of the atom is thus void or vacuum.

An atom is so small that it can only be seen through an electron microscopy. An electron microscope can magnify 500,000 times.

What is an isotope; give examples?


Above is the hydrogen atom. The hydrogen atom has one proton in its nucleus. The element will be hydrogen, although the number of neutrons varies in its nucleus. Usually the hydrogen atom has no neutron at all. When an element can have different numbers of neutrons in its nucleus it is said that the different variants are isotopes of the element. Above are the isotopes of hydrogen, which are hydrogen, deuterium and tritium. All three atoms above are still the element hydrogen. Heavy water contains the isotope deuterium instead of normal hydrogen.

Mass number, atomic number, and how they are written together with the chemical symbol?


Above is the chemical symbol of chlorine (Cl). Around is written a number of letters.

A = Atomic number. The atomic number indicates the number of protons in the nucleus: ₁H (hydrogen) and ₂He (helium).

Z = Mass number. The mass number shows the number of protons plus the number of neutrons in the nucleus, eg. ¹H (hydrogen) and ⁴He (helium).

l = Charge. If the element is in ionic form the charge is shown here. E.g. Na⁺ and SO₄^2-.

n = Number of atoms connected. E.g. H₂ (hydrogen molecule), F₂ (fluorine molecule).

How electrons are distributed in atom shells?


The atom to the left depicts Argon - an inert gas located on the far right in the table of elements.

Electrons circle in orbits around the nucleus.

The shells are called from the nucleus outwards the K, L, M and N shells. The names of the shells are shown in the picture.

The K-shell can contain a maximum of two electrons.

The L-shell can contain a maximum of eight electrons.

The M-shell can initially contain a maximum of eight electrons.

Electrons are added the different shells from inside and outward. If the atom has six electrons, then two of them are placed in the innermost K-shell and four are placed in the L-shell.

Radiation, radiation properties, radioactivity

Radiation is produced when an atom decays. Only atoms with an built-in instability decay spontaneously. Examples of unstable atoms are ²³⁵U, ¹³⁷Ce and ¹⁴C. These are converted into other elements or isotopes when decaying.

When atoms decay radiation is created. It can be alpha radiation, beta radiation or gamma radiation.

Alpha radiation consists of a helium nucleus. 4He²⁺. This radiation is charged and is easily stopped by a plain paper.

Beta radiation consists of an electron e^-. This radiation is charged and goes through a plain paper. Beta radiation is stopped by a piece of wood.

Gamma radiation consists of electromagnetic waves. The radiation is high energy and is stopped by a 20 cm thick lead plate. The radiation is dangerous because it can penetrate the body and cause cell changes.

Alpha radiation, beta radiation and gamma radiation are all called ionizing radiation. Ionizing radiation has enough energy to knock out electrons from elements thus creating ions.

Half-life

Radioactive elements decay. The time it takes for half of a particular radioactive substance to decay is called half-life. The time is different depending on the radioactive substance.

The half-life of ²³⁵U is 710 million years, for ¹³⁷Ce it is 30 years and for ¹⁴C it is 5700 years. The half-life of different radioactive atoms can vary from a fraction of a second to billions of years.

After a half-life half of the substance remains. After yet another half-life a quarter of the substance remains, and after yet another half-life an eight of the substance remains.

Carbon 14 method

Radioactive substances can be used to determine the age of different materials. To determine the age of a mountain, the ²³⁵U can be used, which decays into ²⁰⁷Pb. For age determination of organic materials the 14C isotope can be used. Organic materials all contain carbon.

14C is created in the atmosphere. It is created when the atmospheric nitrogen is hit by the solar wind giving it more neutrons. This makes the nitrogen mass number increase and the result is ¹⁴C. When a tree grows, the level of ¹⁴C is constant in the tree material. Also ¹²C is incorporated into the tree material. When the tree dies the ¹⁴C starts decaying and the relationship between ¹⁴C and ¹²C changes. The smaller the amount of ¹⁴C in the tree material, the older the piece of wood.

Fission and fusion

Fission: When splitting atoms we talk about fission. When atoms are split large amounts of energy is produced. In a nuclear power plant uranium isotopes are split. Shooting neutrons against the fuel rods containing at least 3% ²³⁵U starts the entire process. ²³⁵U decays into other radioactive substances. Neutrons also release from the split uranium isotopes. The released neutrons impact other uranium isotopes, which are also split. The process continues by itself and one controls the decay rate by using control rods. A control rod contains a substance that stops neutrons. The substance can be cadmium or boron. The entire reactor core is bathed in water. The water in this case is also a moderator. The task of the moderator is to slow the speed of the released neutrons. The reactor core heats up and the water warms up. The warm water that is often kept under pressure can have a temperature of about 300 degrees Celsius. This water warms an external water system where the water starts boiling. The steam produced makes a turbine rotate, which means that a generator is spinning. The generator generates electricity.


The image shows a nuclear reactor

Fusion: Fusion means that two atoms merge into a heavier atom. In the sun two hydrogen isotopes merge together. Deuterium merges with tritium thus forming helium. Meanwhile, a large amount of heat energy is formed that makes the sun’s interior maintain a temperature of several million degrees Celsius. From the center of the sun to the solar surface the temperature decreases. The sun's surface has a temperature of 6000 degrees Celsius. The heat from the sun’s interior makes the sun expand but gravity keeps the sun together. Those two forces are in balance.

Questions

1. Give the atomic constituents and give their properties?
2. If there are 10 protons in the atomic nucleus, how many electrons are there in the shells around?
3. Which element has 10 protons in its nucleus?
4. How are electrons distributed on the different shells?
5. What is an isotope?
6. Sometimes there is talk about tritium and deuterium - which element do they talk about?
7. Sometimes Uranium-235 is mentioned. 235 is the mass number. How is the number 235 written along with U (the chemical symbol of uranium)?
8. Why is Cesium-137 dangerous?
9. What is alpha radiation? How can it be stopped?
10. What is beta radiation? How can it be stopped?
11. What are gamma rays? How can they be stopped?
12. How much remains of Cesium-137 after 30 years, 60 years and after 90 years?
13. How do you determine the age of organic materials?
14. How many electrons can there be at the maximum in the K-shell and the L-shell?
15. What is nuclear fission and how does it work?
16. How does a nuclear power plant work – explain?
17. What is a moderator?
18. What is fusion? Where does it happen?