Atoms - Structure and Properties PDF

# ATOMS - **Structure** - Everything is made up of atoms. All solids, liquids and gasses are made up of atoms. - The diameter of an atom is about $0.1 \times 10^{-9}m$ ($0.0000000001m$). - Atoms have a mass of about $4.52 \times 10^{-25}kg$ ($0.000000000000000000000000452kg$). - Ato...

# ATOMS - **Structure** - Everything is made up of atoms. All solids, liquids and gasses are made up of atoms. - The diameter of an atom is about $0.1 \times 10^{-9}m$ ($0.0000000001m$). - Atoms have a mass of about $4.52 \times 10^{-25}kg$ ($0.000000000000000000000000452kg$). - Atoms are *so tiny* that we cannot see them. Therefore, their existence is a *theory*. - The picture on the right shows a computer-generated image of atoms. - They are made up of a **positively** charged nucleus and **negatively** charged electrons spinning around the nucleus. - The nucleus is made up of **protons** and **neutrons**. - The following are properties about protons, neutrons and electrons: | | **Protons** | **Neutrons** | **Electrons** | |--------------|-------------|--------------|---------------| | **charge** | Positive | Neutral | Negative | | **mass** | aprox. 2000 times mass of electron| aprox. 2000 times mass of electron | $9.11 \times 10^{-31} kg$ | | **position in the atom** | nucleus | nucleus | outside nucleus | - So the **nucleus** of the atom is made up of neutrons and protons. - Since neutrons have no charge and protons have a positive charge, the nucleus will have a positive charge. - Negatively charged electrons spin around the nucleus since opposite charges attract. # Particle Arrangement ## Q: What is the difference between an atom of oxygen and an atom of gold? ## A: The difference between the atoms of all the different elements which exist is the number of protons in each atom! - For example, *Helium* is a colourless, odourless, tasteless, and non-toxic gas. If a balloon is filled up with helium, it rises up into the air because it has a lower density than air. - Its atoms are made up of 2 protons, 2 neutrons, and 2 electrons. - Since protons have positive charge, both protons would repel each other. - Neutrons hold these protons together, that is, they make the nucleus **stable** (meaning that the nucleus will stay as it is). - If the atom has a neutral charge, the number of protons is equal to the number of electrons. - **Representation of atomic nuclei** - How can we represent nuclei of atoms without drawing them? - Each element has its own **symbol** (symbol of Helium: *He*). - An element symbol is written in the form: - **A** = nucleon (or mass) number - **Z** = proton (or atomic) number - **X** = element symbol - So the symbol for Helium is $ ^4_2He$. - These are some other elements: | | Hydrogen | Helium | Lithium | Oxygen | Copper | |------|----------|--------|---------|--------|--------| | **Protons** | 1 | 2 | 3 | 8 | 29 | | **Neutrons** | 0 | 2 | 4 | 8 | 34 | | **Electrons** | 1 | 2 | 3 | 8 | 29 | | **Symbol**| $^1_1H$ | $^4_2He$ | $^7_3Li$ | $^16_8O$| $^63_{29}Cu$| # Isotopes - **What are isotopes?** (important) - Isotopes are atoms having the same number of protons but a different number of neutrons. - **What is an element?** - Element is a material with all atoms having the same number of protons! - Iron, for example, is an element with 26 protons in each of its atoms. Water is not an element because it is made up of oxygen and hydrogen atoms. - **Look at these examples:** - Chlorine has an atomic number of 17. Two isotopes of chlorine are chlorine-36 and chlorine-50: - $^{36}_{17}Cl$ and $^{50}_{17}Cl$ - Protons = 17 - Neutrons = 19 - Protons = 17 - Neutrons = 33 - **Example:** - i) Which atoms on the left is NOT an isotope of Hydrogen ($^1_1H$)? - Ans: [$D$]. Why? It has two protons. - ii) What is the symbol of each atom? - [$A$] $^1_1H$ - [$B$] $^2_1H$ - [$C$] $^3_1H$ - [$D$] $^4_2He$ # Radioactivity - Some atoms have a **stable** nucleus. Other atoms have an **unstable** nucleus. An unstable nucleus will emit **radiation**! - Either: **Too much energy** in the nucleus. - Or: **Wrong number of particles** in the nucleus. - **It is important to learn the meaning of the following terms:** - **Radioactive atoms** - atoms with unstable nuclei - **Radioactive decay** - when an atom throws out particles or energy from its nucleus. Since the atom is losing some of its particles, it is said to **disintegrate**. - **Radiation** or **radioactive emission** - the particles or enegry given out from an atom. - **Daughter** - the atom left behind (the 'daughter') is different from the original atom (the 'parent'). It is an atom of a new element. For example, uranium breaks down to radon which then breaks down into other elements. - | **Description** | **Keyword** | - |-------------------|-------------------| - | This is emitted from an unstable nucleus. | radiation | - | This is an atom which has just changed because it emitted energy or particles. | daughter | - | This happens to an atom which loses energy, protons or neutrons. It now has something less. | decay | - | This is the atom before changing into another atom by radiation. | parent | - | The process where an atom loses some of its energy or nuclear particles by radiation. | disintegrate | - | This is a parent atom which will emit radiation. | radioactive atom | - **IONISATION:** The radiation emitted by radioactive substances has a huge amount of energy, which is why it is so dangerous. The energetic radiation causes **ionization**. - When radiation hits a neutral atom, some of the energy from the radiation is passed to the atom. - This energy can cause an electron from the atom to escape, leaving the atom with a positive charge. This positively charged atom is called an **ion**, so the process is called **ionisation**. # 3 Types Of Radiation - There are three main types of radiation that can be emitted by radioactive particles. They are called **ALPHA**, **BETA**, and **GAMMA**. - All three types of radiation come from the **nucleus** of the atom. - 1. **Alpha** is letter α in Greek and written as **α** - 2. **Beta** is letter β in Greek and written as **β** - 3. **Gamma** is letter γ in Greek and written as **γ** - **Practice here:** - ααα - βββ - γγγγγγγγ # α Alpha Radiation - Alpha radiation is the emission of a particle from the **nucleus** of an unstable atom. Alpha radiation is a **Helium nucleus** $^4_2He$ therefore made up of 2 **neutrons** and 2 **protons**. - Moreover, it is also **positively** charged. Due to its **mass**, it is highly energetic! - These are large **particles** with a positive charge. They can ionise atoms quickly and lose their energy by ionising nearby **atoms**. This means they can be absorbed by just a few centimetres of **air**, a sheet of **paper** or by skin. # -iβ Beta Radiation - Beta radiation is the emission of a highly energetic electron, released from inside a **nucleus**. - It has negligible **mass** (almost 0kg). - When a beta particle is produced a neutron in the nucleus divides into a proton and an **electron**. It is the electron that is rejected at high speed from the nucleus. - That is the beta particle. - These are small particles with a **negative** charge. - They can ionise fairly easily so can only **penetrate** through thin materials before they are absorbed. # γ Gamma Radiation - Gamma rays are high frequency waves at the **end** of the **electromagnetic spectrum**. - Like all other waves, gamma radiation has no **Mass**. - They are **emitted** from the **nucleus** during radioactive decay and occasionally accompanying the emission of an alpha or beta particle. - These waves are very penetrating, and it is almost impossible to absorb them completely. Lead lined clothing can reduce the amount of waves reaching the **body**. Gamma waves are the **least** ionising of the three types of radiation, but it is extremely difficult to prevent them entering the body. # Summary of the three types of radiation: | **Symbol** | **Mass** | **Charge** | **lonising power** | **Range in air** | **Stopped by** | |---|---|---|---|---|---| | α | 4 | +2 | High | < 5 cm | Sheet of paper | | β | 2000 | -1 | Medium | < 1 m | Sheet of metal, thick sheet of lead, or concrete | | γ | 0 | 0 | Low | Infinity (decreases with distance) | | # Electromagnetic spectrum - **The Electromagnetic Spectrum** - It is important to remember the order of the waves in the electromagnetic spectrum. On the diagram on the left, the waves are increasing in wavelength and decreasing in frequency towards the right. - **You can remember the order using:** - `Gamma ray, X-ray, Ultraviolet, Visible, Infrared, Microwave, Radio` # Alpha Decay - A nucleus changes into a new element by emitting alpha or beta particles. These changes are described using nuclear equations. Alpha decay (two protons and two neutrons) changes the mass number of the element by -4 and the atomic number by -2. An alpha particle is the same as a helium-4 nucleus. - For example, as shown in the picture, Radium-226 decays into Radon-222. This can be represented using the following nuclear equation: - $^{226}_{88}Ra$ → $^{222}_{86}Rn$ + $^4_2He$ - The mass number of the parent nucleus, is equal to sum of the mass numbers of the daughter atom and the emitted particle: 226= 222 + 4 - The atomic number of the parent nucleus, is equal to sum of the atomic numbers of the daughter atom and the emitted particle: 88 = 86 + 2 - **Example:** - Uranium-238, $ ^{238}_{92}U$, emits an alpha particle to become thorium-234. Thorium has an element symbol Th. Write a nuclear equation that shows this decay. # Beta Decay - Beta decay changes the atomic number by +1 (the nucleus gains a proton) but the mass number remains unchanged (it gains a proton but loses a neutron by ejecting an electron, since beta particle is an electron). - For example, as shown in the picture, Caesium-137 decays into Barium-137. This can be represented using the following nuclear equation: - $ ^{137}_{55}Cs$ → $ ^{137}_{56}Ba$ + $^0_{-1}β$ - The mass number of the parent nucleus, is equal to sum of the mass numbers of the daughter atom and the emitted particle: 137 = 137 + 0 - The atomic number of the parent nucleus, is equal to sum of the atomic numbers of the daughter atom and the emitted particle: 55 = 56 + (-1) # Detecting Radiation: Geiger-Müller tube - A special apparatus is used to detect radiation. This apparatus is the ***Geiger-Müller tube*** (also known as GM tube). The tube is connected to a rate-meter. Each time radiation enters the tube, the rate-meter usually beeps once, the counter increases and it can be read. # Background Radiation - At this very moment, radiation is penetrating through your body. - **WHAT?!?!?** :S - Yes, there is a certain amount of radiation around us (and even inside us) all the time. There always has been - since the beginning of the Earth. It is called **background radiation**. Background radiation comes from a huge number of sources; these include: - radon gas, medical uses of radiation, rock and soil, inside the body, - cosmic radiation, fallout from nuclear tests, and nuclear waste. - But relax, In most areas, background radiation is safe. It is at such a low level that it doesn't hurt you. - You need to be exposed to many times the normal background level before you notice symptoms. - However, some areas of the United Kingdom have a higher level of background radiation than others because the rocks near the surface contain more radioactive isotopes. # Half-Life (λ) - Radioactive substances will give out radiation all the time. As they decay the atoms change to daughter atoms, until eventually there won't be any of the original (parent) atoms left. - Different substances decay at different rates and so will last for different lengths of time. - We use the half-life of a substance to tell us which substances decay the quickest. - **Half-life is the time it takes for half of the radioactive particles to decay.** - The half-life of a substance can be found by measuring the count-rate of the substance with a Geiger-Muller tube over a period of time. By plotting a graph of count-rate against time the half-life can be seen on the graph. - Look carefully at the graph on the right: - **Time/min.** 0 5 10 15 20 25 30 35 40 45 50 55 - **Count rate/min.** 150 118 92 75 63 52 38 35 26 19 17 - **How long is the half-life of the radioactive element used for this graph?** - **Why?** - **For example, Thorium-232 is an element that has a half-life of 14 billion years (14, 000, 000, 000 years or 14x10º years). The following is how it decays as time passes.** - 1st half-life - $ ^{232}_{90}Th$ - 14 billion years - 100,000 unstable atoms - 2nd half-life - $ ^{232}_{90}Th$ - 14 billion years - 50,000 unstable atoms - 3rd half-life - $ ^{232}_{90}Th$ - 14 billion years - 25,000 unstable atoms - 4th half-life - $ ^{232}_{90}Th$ - 14 billion years - 12,500 unstable atoms - **Find the count rate of the following materials with different half-lives after a length of time. The initial count of each material is 1000 counts per minute? ** | **Half-Life** | **Length of Time** | **Count rate/minute (initial 1000 counts/min)** | |---|---|---| | 20 years | 60 years | | | 5 minutes | 5 minutes | | | 6 months | 1 year | | | 30 seconds | 1½ minutes | | # Uses of radioactivity - Different radioactive substances can be used for different purposes. The type of radiation they emit and the half-life are the two things that help us decide what jobs a substance will be best for. Here are some of the main uses you will be expected to know about: - **1. Uses in medicine to kill cancer - radiation** - damages or kills cells, which can cause cancer, but it can also be used to kill cancerous cells inside the body. Sources of radiation that are put in the body need to have a high count-rate and a short half-life so that they are effective, but only stay in the body for a short period of time. If the radiation source is outside of the body it must be able to penetrate to the required depth in the body. (Alpha radiation can't travel through the skin remember!) - **The radiotherapy machine**