The Particle Nature of Matter PDF

# **EXPERIENCE 1** ## The Particle Nature of Matter Go online to explore and explain the properties of matter. ## Atoms and Molecules Matter is anything that has mass and occupies space. Matter is made of particles that are too tiny to see without powerful microscopes. These particles, called ato...

# **EXPERIENCE 1** ## The Particle Nature of Matter Go online to explore and explain the properties of matter. ## Atoms and Molecules Matter is anything that has mass and occupies space. Matter is made of particles that are too tiny to see without powerful microscopes. These particles, called atoms, are the fundamental building blocks of matter, and they combine to form larger particles of matter. Each kind of atom is the smallest representative unit of an element. An element is the simplest form of matter that has a characteristic set of properties. Each element has a unique name and chemical symbol. A chemical symbol, such as H or Cl, is one or two letters representing an element. Hydrogen (H), nitrogen (N), and oxygen (O) are examples of elements. Because atoms are so small, scientists use models to represent atoms and their interactions with each other. Chemical symbols are the most commonly used models for elements. ### Marbles as Models You can visualize atoms as tiny spheres with different sizes, much like these marbles. ### Spheres as Models Scientists often use spheres with different colors and sizes to represent atoms. Each color represents an atom of a specific element. The sizes of the spheres show the relative sizes of the atoms. For example, a gallium (Ga) atom is larger than a carbon (C) atom. **SEP Identify Limitations of a Model** What is a limitation of using marbles as models for atoms? ## Molecules When two or more atoms combine, they form molecules. Molecules are held together by one or more chemical bonds. Some molecules, such as a water molecule, are made of only a few atoms. Others are much more complex. A single DNA molecule is made of billions of atoms. Chemical symbols are combined to make the chemical formula for a molecule. ### Models of Molecules How can models show the composition and structure of molecules? ### Molecular Models These models how bonded atoms can be represented as joined spheres. Some molecules are made of the same element, or kind of atom. Other molecules are made of two or more elements. ### Chemical Formulas In a chemical formula, symbols for the elements are combined to show how many atoms of each element make up a molecule of a substance. ### Binding Force A chemical bond is a strong force of attraction that holds two atoms together. **SEP Develop Models** Use colored pencils to draw models for a chlorine molecule, Cl₂, and for a hydrogen chloride molecule, HCl. Use the colors and relative sizes from the atom key on the previous page. ## Communities of Particles Most matter exists as groupings of atoms and molecules. The attractive forces that hold the particles together and the motions of the particles determine the state of matter—solid, liquid, or gas. ### Water Vapor (Gas) The molecules in gaseous water, found in Earth’s atmosphere, are not held together by attractive forces. They are far apart and move freely. ### Water (Liquid) The molecules in liquid water are held close together by attractive forces, but the molecules can still move around. ### Ice (Solid) In solid water, or ice, the molecules are locked in a fixed pattern by attractive forces. The water molecules still vibrate back and forth. **SEP Construct an Explanation** Density is the mass of matter in a given volume. Materials that are less dense float on denser materials. Use the models to explain, on a molecular level, why ice floats on water. ## Energy Energy is the capacity to move, do work, change matter, or produce heat. **Kinetic energy** is the energy that matter has because of its motion. **Thermal energy** of matter is the total kinetic energy of all the particles that make up the matter. **Potential energy** is the stored energy in matter due to its position relative to other matter. **Chemical potential energy** is the energy stored in the chemical bonds that hold atoms together; this energy can be released or absorbed during a chemical process. ### Kinetic Energy A moving object, such as this ball, has kinetic energy. Atoms are constantly moving. They have kinetic energy, like moving balls. Even atoms in solids have kinetic energy because they vibrate back and forth. ### Potential Energy An object can also have potential energy because of its position relative to other objects. A ball held above the ground has potential energy. If dropped, it will fall. Two oppositely charged particles have potential energy. When they are close enough together, they are attracted to each other. **CCC Cause and Effect** Draw an arrow in the figure of the player holding a soccer ball to show the direction the ball will move if the person stops holding it. Add arrows to show how the charged particles in the figure will move. Discuss your reasoning with a partner. ## Interaction of Matter and Energy A system is a portion of the universe that a scientist identifies for study. The surroundings are everything else around the system. ### Energy can flow into a system from the surroundings. It can also flow out of a system into the surroundings. A change in the temperature of a system is one indication that the energy of the system has changed. As the energy of matter changes, its macroscopic properties can change. A macroscopic property is a characteristic that is large enough to see, handle, or measure without magnification. An example is melting point. A solid can melt into a liquid when it gains enough energy and reaches its melting point. ### System and Surroundings A piece of gallium can be defined as a system. The hand is part of the surroundings. The hand is warmer than the piece of gallium. The hand transfers thermal energy to the gallium, causing it to melt. **CCC Systems and System Models** Liquid gallium changes to a gas at 2229°C. Describe how a model of gaseous gallium would compare to the model of liquid gallium shown in the picture. ## Modeling Atoms Go online to explore and explain the make-up and properties of atoms and isotopes. ### Visualizing the Atom Matter is made of particles, called atoms, that are too tiny to see without powerful microscopes. An atom is the smallest particle of an element that retains its identity in a chemical reaction. Atoms are extremely small. A single row of 100,000,000 copper atoms would produce a line only 1 cm long. Because of the creative experiments of many scientists, we know that atoms are made of even smaller particles called *subatomic particles*. * **Protons** are positively charged subatomic particles. * **Neutrons** are subatomic particles with no charge. * **Electrons** are negatively charged subatomic particles. An electron cloud surrounds the *nucleus*, which is the dense central core made of protons and neutrons. ### Electron Cloud The electron cloud, where the electrons are found, takes up most of an atom's volume. It is mostly empty space. ### Nucleus The nucleus is made of protons and neutrons. It accounts for most of an atom's mass. An *atomic mass unit *(amu)* is the unit used to measure the mass of subatomic particles. **CCC Scale, Proportion, and Quantity** Which subatomic particles account for most of an atom’s mass and volume? ## Types of Atoms An *element* is the simplest form of matter that has a unique set of properties. The number of protons in an atom is what makes one element different from another. The number of protons in the nucleus of an atom is called an element’s *atomic number*. For example, carbon has 6 protons and has an atomic number of 6. Atoms are electrically neutral particles because they have no net charge. Therefore, for an atom to be neutral, the number of protons (positively charged particles) must equal the number of electrons (negatively charged particles). ### Helium The element helium is less dense than air, so it is used in balloons. * Atomic number: 2 * Number of protons: 2 * Number of electrons: 2 ### Silver The element silver is reflective and resists corrosion, so it is often used in jewelry. * Atomic number: 47 * Number of protons: 47 * Number of electrons: 47 **CCC Scale, Proportion, and Quantity** Identify the numbers of protons and electrons in fluorine and iodine. * The element fluorine helps prevent tooth decay and is found in many toothpastes. * Atomic number: 9 * Number of protons: * Number of electrons: * The element iodine is often used to clean skin before surgery. * Atomic number: 53 * Number of protons: * Number of electrons: ## The Periodic Table There are 118 elements, all with different atomic numbers. They are organized into a table called a *periodic table*. A periodic table is an arrangement of elements in which the elements are separated into groups based on a set of repeating properties. The elements are listed in order from left to right and top to bottom by atomic number. The organization of the periodic table allows you to easily compare the properties of one element (or a group of elements) to another element (or group of elements). Elements above and below each other tend to have similar properties. ### Chemical Symbols All elements have a name, but an element’s name is often not convenient to use. Each element can be represented by a one- or two-letter chemical symbol. It is these chemical symbols that are used to represent the elements in the periodic table, as well as in chemical formulas for compounds. When writing the formula for a compound, you combine the symbols of the elements that make up the compound. For example, the chemical formula for water is H₂O **Infer** Based on the organization and colors in the periodic table, which two elements do you think are most similar in terms of their properties: magnesium (Mg), barium (Ba), and gold (Au)? Explain. ## Mass Number Most of the mass of an atom comes from protons and neutrons. The total number of protons plus neutrons in an atom is called the *mass number*. If you know the atomic number and mass number of an atom, then you can determine the number of neutrons in the atom. ### Representing Atoms The composition of any atom can be represented in shorthand notation using the element’s chemical symbol, atomic number, and mass number. ### Mass Number and Neutrons You can calculate the number of neutrons from the atomic number and the mass number of an atom. For example, let’s calculate the number of neutrons in neon. Number of neutrons = mass number – atomic number Number of neutrons = 22 – 10 = 12 **SEP Use Mathematics** Complete the table by identifying the atomic number and mass number for each atom and then using those data to determine the numbers of protons, neutrons, and electrons. | Atom | Atomic number | Mass number | Protons | Electrons | Neutrons | |---|---|---|---|---|---| | ²³Na | | | | | | | ³²S | | | | | | | ¹⁰⁸silver | 47 | | | | | | ¹⁹⁷gold | 79 | | | | | ## Isotopes All atoms of an element have the same number of protons. However, atoms of the same element may have different numbers of neutrons. *Isotopes* are atoms that have the same number of protons but different numbers of neutrons. Since isotopes of an element have different numbers of neutrons, they also have different mass numbers. ### Sizes of Dogs Dogs come in a variety of sizes. The mass of each can be large or small, but they are all still dogs. Their DNA determines they are dogs. ### Isotopes of Oxygen Similarly, atoms may have different numbers of neutrons and different masses but still be the same type of atom. All three of these atoms are isotopes of oxygen. They have 8 protons, but different numbers of neutrons. **SEP Communicate Information** How does the overall charge of oxygen-16 compare to the overall charge of oxygen-17 and oxygen-18? Use the models of isotopes to help explain your answer. ## Atomic Mass Comparing the Masses of Atoms It’s not convenient to measure the mass of a single atom because the mass is so small. Instead, it’s more useful to compare the relative masses of atoms using a reference isotope. A reference isotope is an isotope whose mass can be measured accurately and is used as the basis for a scale to compare the masses of other atoms. ### Understanding Atomic Mass How is the mass of an atom determined? * **Carbon-12** The isotope carbon-12 is used as a standard to compare the relative masses of atoms. Carbon-12 has 6 protons, 6 neutrons, and 6 electrons. Scientists have assigned carbon-12 a mass of exactly 12 atomic mass units. ### Piano Key to Piano If you measure the mass of a piano with and without a piano key, you likely wouldn’t see a difference between the two masses. The mass of a piano key is negligible compared to the mass of the whole piano. Even though the mass is insignificant, each key is essential to the function of the piano. ### Electron to Atom Similarly, the mass of an electron is negligible compared to the mass of a proton or neutron. Each electron is essential to the properties of an atom. * Mass of proton 1.67 x 10^-24 g * Mass of neutron 1.67 x 10^-24 g * Mass of electron 9.11 x 10^-28 g ### Atomic Mass Unit An atomic mass unit *(amu)* is one twelfth of the mass of a carbon-12 atom. 1 amu = 1/12 x 12 amu = about the mass of a proton or neutron **SEP Develop Models** Each helium atom has two protons. Sketch models of helium-3 and helium-4, which have approximate masses of 3 amu and 4 amu, respectively. Label and differentiate protons, neutrons, and electrons in your models. ## Isotope Abundance and Atomic Mass In nature, most elements occur as a mixture of two or more isotopes. Each isotope of an element has a fixed mass and a natural percent abundance. ### Comparing Abundances of Bromine, Chlorine, and Silicon Isotopes **SEP Analyze Data** Look at the graphs. How does an element’s atomic mass compare to the mass number of its most abundant isotope? The *average atomic mass*, or *atomic mass* for short, of an element is a weighted average of the masses of its isotopes. A weighted average mass reflects the masses and the relative abundances of the isotopes as they occur in nature. **SEP Use Computational Thinking** Copper has two isotopes, copper-63 and copper-65. It has an atomic mass of 63.546 amu. Sketch a pie graph that shows the approximate abundance of each copper isotope based on the atomic mass. ## Calculating Atomic Mass Isotopes that are more common in nature have greater importance in atomic mass calculations—just like tests may be assigned greater importance in calculating your final grade. To calculate the atomic mass of an element, multiply the mass of each isotope by its natural abundance, expressed as a decimal, and then add the products. The resulting sum is the weighted average mass of the atoms of the element as they occur in nature. ### Class Grade Teachers sometimes assign different weights to different types of assignments when calculating your class grade. First, the teacher determines the types of assignments and assigns each type a weight based on its relative importance. | Assignment Type | Weight | Score | Weighted Score | |---|---|---|---| | Homework | 20% | 85 | 17.0 | | Tests | 50% | 75 | 37.5 | | Labs | 30% | 90 | 27.0 | | **Total:** | **100%** | | **81.5** | As assignments are completed, the weights are used to calculate a student's class grade. ### Atomic Mass Atomic mass is calculated similarly. You can even set it up in the same table format, if you wish. Here’s how to calculate the atomic mass of magnesium. The percent abundance is like the weight in the class grade table. It’s the multiplier factor. The sum of the percent abundances should always be 100%. | Isotope | Percent Abundance | Mass (amu) | Mass Contribution | |---|---|---|---| | magnesium-24 | 79% | 24 | 19.0 | | magnesium-25 | 10% | 25 | 2.5 | | magnesium-26 | 11% | 26 | 2.9 | | **Total:** | **100%** | | **24.4** | The mass contribution of each isotope is like the weighted score. Add all the contributions to find the atomic mass. It should always be closer to the mass of the most abundant isotope—in this case, 24 amu. **Compare** In what ways are final grade calculations and atomic mass calculations similar?