Radioactivity

Radioactivity

Stable and Unstable Isotopes 

Elements can be made up of different isotopes. Isotopes are atoms with the same number of protons and electrons, but a different number of neutrons. Sometimes isotopes are stable and happy. These are the elements that we see around us and find in nature. However, some isotopes are unstable. These isotopes are called radioactive isotopes. You can go here to learn more about isotopes. 

What is radioactive decay? 

When isotopes are unstable they emit energy in the form of radiation. There are three main types of radiation or radioactive decay depending on the isotope. 

Different Types of Radioactivity

• Alpha decay - Alpha decay is caused when there are too many protons in a nucleus. In this case the element will emit radiation in the form of positively charged particles called alpha particles.
• Beta decay - Beta decay is caused when there are too many neutrons in a nucleus. In this case the element will emit radiation in the form of negatively charged particles called beta particles.
• Gamma decay - Gamma decay occurs when there is too much energy in the nucleus. In this case gamma particles with no overall charge are emitted from the element.

How is it measured? 

Radioactivity is measured using a unit called the "curie". It is abbreviated as "Ci". The curie measures how many atoms spontaneously decay each second. The curie was named after Marie and Pierre Curie who discovered the element radium. 

What is the half-life of an isotope? 

The half-life of an isotope is the time on average that it takes for half of the atoms in a sample to decay. 

For example, the half-life of carbon-14 is 5730 years. This means that if you have a sample of carbon-14 with 1,000 atoms, 500 of these atoms are expected to decay over the course of 5730 years. Some of the atoms may decay right away, while others will not decay for many thousands more years. 

The thing to remember about half-life is that it is a probability. In the example above, 500 atoms are "expected" to decay. This is not a guarantee for one specific sample. It is just what will happen on average over the course of billions and billions of atoms. 

Radioactive Decay to other Elements 

When isotopes decay they can lose some of their atomic particles (i.e. electrons and protons) and turn from one element into another. Sometimes isotopes decay from one unstable isotope into another unstable isotope. This can happen continuously in a long radioactive chain. 

An example of a radioactive chain is uranium-238. As it decays, it transforms through a number of elements including thorium, radium, francium, radon, polonium, and bismuth. It finally ends up as a stable isotope as the element lead. 

Why is radiation dangerous? 

Radiation can alter the structure of cells in our bodies causing mutations which can produce cancer. The more radiation a person is exposed to, the more dangerous it is. 

Is some radiation good? 

Despite the risks, there are a number of good ways that science has used radiation. These include X-rays, medicine, carbon dating, energy generation, and to kill germs. 

Interesting Facts about Radioactivity

• Uranium in the ground can decay into radon gas which can be very dangerous to humans. It is thought to be the second leading cause of lung cancer.
• The half-life of carbon-14 is used in carbon dating to determine the age of fossils.
• Bismuth is the heaviest element with at least one stable isotope. All elements heavier than bismuth are radioactive.
• Radioactivity was discovered by the scientist A. H. Becquerel in 1896.

Chemical Reactions

Chemical Reactions

A chemical reaction is a process where a set of substances undergo a chemical change to form a different substance. 

Where do chemical reactions occur? You may think that chemical reactions only happen in science labs, but they are actually happening all the time in the everyday world. Every time you eat, your body uses chemical reactions to break down your food into energy. Other examples include metal rusting, wood burning, batteries producing electricity, andphotosynthesis in plants.

What are reagents, reactants, and products? 

Reactants and reagents are the substances that are used to bring about the chemical reaction. A reactant is any substance that is consumed or used up during the reaction. 

The substance that is produced by a chemical reaction is called the product. 

Reaction Rate 

Not all chemical reactions occur at the same rate. Some happen very quickly like explosions, while others can take a long time, like metal rusting. The speed that the reactants turn into products is called the reaction rate. 

The reaction rate can be changed by adding energy such as heat, sunlight, or electricity. Adding energy to a reaction can increase the reaction rate significantly. Also, increasing the concentration or pressure of the reactants can speed up the reaction rate. 

Types of Reactions 

There are many types of chemical reactions. Here are a few examples:

• Synthesis reaction - A synthesis reaction is one where two substances combine to make a new substance. It can be shown in an equation such that A + B --> AB.







• Decomposition reaction - A decomposition reaction is where a complex substance breaks down to form two separate substances. It can be shown in an equation such that AB --> A+ B.







• Combustion - A combustion reaction occurs when oxygen combines with another compound to form water and carbon dioxide. Combustion reactions produce energy in the form of heat.
• Single displacement - A single displacement reaction is also called a substitute reaction. You can think of it as a reaction where one compound takes a substance from another compound. Its equation is A + BC --> AC + B.
• Double displacement - A double displacement reaction is also called a metathesis reaction. You can think of it as two compounds trading substances. Its equation is AB + CD --> AD + CB.
• Photochemical reaction - A photochemical reaction is one involving photons from light. Photosynthesis is an example of this kind of chemical reaction.

Catalyst and Inhibitors 

Sometimes a third substance is used in a chemical reaction to speed up or slow down the reaction. A catalyst helps to speed up the rate of reaction. Unlike other reagents in the reaction, a catalyst is not consumed by the reaction. An inhibitor is used to slow down the reaction. 

Interesting Facts about Chemical Reactions

• When ice melts it undergoes a physical change from solid to liquid. However, this is not a chemical reaction as it remains the same physical substance (H₂O).
• Mixtures and solutions are different from chemical reactions as the molecules of the substances stay the same.
• Most cars get their power from an engine that uses a combustion chemical reaction.
• Rockets are propelled by the reaction that occurs when liquid hydrogen and liquid oxygen are combined.
• When one reaction causes a sequence of reactions to occur this is sometimes called a chain reaction.

Chemical Bonding

Chemical Bonding

The world around us is made up of tiny units of matter called atoms. How these atoms stick together to form substances is called chemical bonding. 

About Atoms Each element has its own unique atom made up of a specific number of protons in its nucleus called the atomic number. Each atom also has the same number of electrons as it has protons. Electron Shells The electrons orbit around the nucleus of the atom. They stay in layers called shells. Each shell can only contain a certain number of electrons: the first layer can hold two electrons, the second layer eight electrons, the third layer eighteen electrons, etc.

The Outer Shell 

All atoms would like to have a full outer shell, but the only elements to naturally have a full outer shell are the noble gases to the right of the periodic table. As a result, when atoms without full outer shells come into contact with other atoms, they tend to want to give up or gain electrons. 

Valence Electrons 

The valence electrons are the number of electrons an atom must lose or gain to have a full outer shell. 

Atoms with a relatively empty outer shell will want to give up electrons. For example, if an atom has 1 electron out of a possible 8 in its outer shell, it will want to give up that electron so its outer shell is now full. 

Atoms with a relatively full outer shell will want to gain electrons to fill up the outer shell. For example, an atom with 6 of 8 electrons in its outer shell will try to gain 2 electrons so its outer shell is full. 

Ionic Bonding 

Ionic bonding occurs when different elements trade electrons such that both elements now have a full outer shell. 

Example: 

Here is an example showing lithium (which has 3 electrons and 1 in the outer shell) and fluorine (which has 9 electrons and 7 in the outer shell) trading an electron to form LiF or lithium fluoride. This is called an ionic bond. 

Example of ionic bonding

Covalent Bonding 

In covalent bonding electrons are shared between atoms rather than traded in order for the atoms of both elements to gain full outer shells. Electrons are always shared in pairs. 

Example: 

An example of covalent bonding is the molecule of carbon dioxide. In this example carbon has 4 of 8 electrons in its outer shell and oxygen has 6 of eight electrons. By combining two oxygen atoms with one carbon atom, the atoms can share electrons such that each atom has a full outer shell. 

Example of covalent bonding

Interesting Facts about Chemical bonding

• Because noble gases have a naturally full outer shell they seldom react.
• In metallic bonding a large number of atoms lose their electrons.
• Ionic bonding is mostly formed between metals located at the left side of the periodic table.
• Atoms in molecules are held together by the attraction between the nucleus and the shared electrons.

Melting and Boiling

As we learned in solids, liquids, and gases all matter exists in certain states or phases. Water can be liquid water, solid ice, or gas vapor. It's still all water, however, and made up of molecules of 2 hydrogen atoms and 1 oxygen atom (H2O). 

Lava is melted or liquid rock

Melting and Freezing 

When a solid turns into a liquid it is called melting. There is a temperature at which this happens called the melting point. As the energy in the molecules increases from a rise in temperature, the molecules start moving faster. Soon they have enough energy to break free of their rigid structure and start moving around more easily. The matter becomes a liquid. The melting point for water is 0 degrees C (32 degrees F). 

When the opposite happens and a liquid turns into a solid, it is called freezing. 

Boiling and Condensation 

When a liquid becomes a gas it is called boiling or vaporization. Again, at a certain temperature called the boiling point, the molecules will gain enough energy to break free and become a gas. The boiling point for water is 100 degrees C (212 degrees F).

Hot gas from steam engine condensating

When the opposite occurs and a gas becomes a liquid, it is called condensation. 

Evaporation 

Evaporation is liquid becoming a gas that happens only on the surface of a liquid. Evaporation doesn't always need a high temperature to occur. Even though the overall energy and temperature of a liquid may be low, the molecules on the surface that are in contact with the air and gases around them, can be high energy. These molecules on the surface will slowly become gases through evaporation. You can see evaporation when water on your skin dries or a puddle in the street slowly goes away. 

Standard State 

Scientist use the term "standard state" to describe the state an element or substance is in at "room conditions" of 25 degrees C and one atmosphere of air pressure. Most of the elements, like gold and iron, are solids in their standard state. Only two elements are liquid in their standard states: mercury and bromine. Some of the elements that are gases in their natural state include hydrogen, oxygen, nitrogen, and the noble gases. 

Fun facts about Melting and Boiling

• When rocks get really hot they turn into a liquid called magma or lava.
• Gas can be turned into a liquid through pressure. By squeezing all the gas molecules tightly together a gas can become liquid.
• We use natural gas in our homes in its gas state, but when it's shipped in ocean tankers it's shipped in a liquid state to save on space.
• Mercury has the interesting properties of being both a metal and a liquid in its standard state.