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Chapter 3. Matter and Atomic Structure. Section 3.1: What are elements? All matter – that is, everything on Earth and beyond – is made of substances c alled elements. An element is a substance that cannot be broken down into simpler substances by physical or chemical means.
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Chapter 3 Matter and Atomic Structure
Section 3.1: What are elements? • All matter – that is, everything on Earth and beyond – is made of substances called elements. • An element is a substance that cannot be broken down into simpler substances by physical or chemical means. • For example, gold is still gold whether it has been melted, pulled into wire, hammered into a thin sheet, or divided into small particles.
Section 3.1: What are elements? • Ninety-two elements occur naturally on Earth and in the stars. • Other elements have been produced in laboratory experiments. • Each element is identified by a one-, two-, or three-letter abbreviation known as a chemical symbol.
Section 3.1: What are elements? • Each element has distinct characteristics, and all elements are made up of atoms. • An atom is the smallest particle of an element that has all of the characteristics of that element.
Section 3.1: What are elements? • All atoms consist of even smaller particles: protons, neutrons, and electrons.
Section 3.1: What are elements? • The number of protons and neutrons in different atoms varies widely. The lightest of all atoms is the hydrogen atom, which has only one proton in its nucleus. • The heaviest naturally occurring atoms are those of uranium. Uranium-238 has 92 protons and 146 neutrons in its nucleus.
Section 3.1: What are elements? • The number of protons in an atom’s nucleus is its atomic number. The combined number of protons and in neutrons is its mass number. • For example, the atomic number of uranium is 92 and its mass number is 238 (92 + 146). • You can find the atomic numbers and mass numbers in the periodic table of elements.
Section 3.1: What are elements? • Surrounding the nucleus of an atom are smaller particles called electrons. An electron has little mass, but it has a negative electrical charge that is exactly the same magnitude as the positive charge of a proton. • An atom has an equal number of protons and electrons; thus, the electrical charge of an electron cancels the positive charge of a proton to produce an atom that has no overall charge.
Section 3.1: What are elements? • An energy level represents the area in an atom where an electron is most likely to be found. • The volume of an atom is mostly empty space. Because electrons have little mass, the mass of an atom depends mostly upon the number of protons and neutrons in its nucleus. • However, the size of an atom depends upon the number and arrangement of its electrons.
Section 3.1: What are elements? • Note that electrons are distributed over one or more energy levels in a predictable pattern. Each energy level can hold only a limited number of electrons.
Section 3.1: What are elements? • The electrons in the outermost energy level determine the chemical behavior of the different elements. These outermost electrons are called valence electrons. • Elements with the same number of valence electrons have similar chemical properties. For example, a sodium (Na) atoms, with the atomic number 11, and a potassium (K) atom, with the atomic number 19, both have just one valence electron. • Thus, both sodium and potassium are highly reactive metals, which means that they combine easily with other elements.
Section 3.1: What are elements? • Elements such as helium (He), neon (Ne), and argon (Ar) are inert, which means that they do not easily combine with other elements. This is because they have full outermost energy levels. • The innermost energy level is filled with two electrons, and the second energy level, which is the outermost energy level, also is filled, with eight electrons. • With a filled outermost energy level, neon is unlikely to combine chemically with other elements.
Section 3.1: What are elements? • You have learned that all atoms of an element have the same number of protons. However, the number of neutrons in the nuclei of an element’s atoms can vary. • For example, all chlorine atoms have 17 protons in their nuclei, but they may have either 18 or 20 neutrons. This means that there are two types of chlorine atoms; one with a mass number of 35 (17 protons + 18 neutrons) and one with a mass number of 37 (17 protons + 20 neutrons). Chlorine-35 and Chlorine-37. • When atoms of the same element have different mass numbers, they are known as isotopes of that element.
Section 3.1: What are elements? • Many elements are mixtures of isotopes. Because the number of electrons in an atom equals the number of protons, isotopes of an element have the same chemical properties. • If many elements are mixtures of isotopes, how do scientists known how many neutrons are found in an element’s atoms? • Scientists have measured the mass of atoms of elements and found an average atomic mass for each element. The atomic mass of an element is the average of the mass numbers of the isotopes of an element. • For example, the atomic mass of chlorine is 35.453. This number is the average of the mass numbers of the naturally occurring isotopes of chlorine-35 and chlorine-37.
Section 3.1: What are elements? • The nuclei of some isotopes are unstable and release radiation. Radioactivity is the spontaneous process through which unstable nuclei emit radiation. • During radioactive decay, a nucleus can lose protons and neutrons, change a proton to a neutron, or change a neutron to a proton. • Because the number of protons in a nucleus identifies an element, decay changes the identity of an element. For example the isotope uranium-238 decays over time into lead-206, so uranium originally present in a rock gradually and predictably is replaced by lead.
Section 3.2: How Atoms Combine? • Compounds: • Table salt is a common substance. However, table salt is not an element, but a compound. A compound is a substance that is composed of atoms of two or more different elements that are chemically combined. • Water is another example of a compound, because it is composed of two elements, hydrogen and oxygen. • Most compounds have totally different properties from the elements of which they are composed. For example, both oxygen and hydrogen are gases at room temperature, but in combination they form water, a liquid.
Section 3.2: How Atoms Combine? • For most elements, an atom is chemically stable when its outer most energy level is full. We know this is true because the most stable elements are the gases helium, neon, and argon. A state of stability is achieved by other elements through chemical bonds, which are forces that hold the elements together in a compound.
Section 3.2: How Atoms Combine? • What happens in a chemical reaction? • Bonds break and bonds form. • Atoms rearrange when chemical bonds between atoms break and other chemical • bonds form. • All substances, including solids are made of particles that move constantly. As • Particles move. They collide. • If the particles collide with enough energy, the bonds between atoms can break. • The atoms separate and rearrange, and new bonds can form.
Section 3.2: How Atoms Combine? • Unlike a compound, in which the constituent atoms combine and lose their • identities, a mixture is a combination of two or more compounds that retain • their identities. • When a mixture’s components are easily recognizable, it is called a • heterogenous mixture. In a homogenous mixture, the component particles cannot • be distinguished, even though they still retain their original properties. • Brewed coffee is an example of a homogenous mixture, which is also called • a solution. • A solution may be liquid, gaseous, or solid. Seawater is a liquid solution consisting • of water molecules and ions of many elements that exist on Earth.
