Chapter 2 Elements and Compounds Chapter 2 Elements and Compounds In This Chapter As we learned in Chapter 1 each of the 116 known elements is composed of a unique type of atom In this chapter we discover that there are actually a number of different variations or isotopes of the atom associated with each element We explore the structure of the atom in further detail and learn how different isotopes are formed We also discuss the different ways that molecules and compounds are represented and named Chapter Outline 2 1 The Structure of the Atom 2 2 Elements and the Periodic Table 2 3 Covalent Compounds 2 4 Ions and Ionic Compounds Chapter Review Chapter Summary Assignment 2 1 The Structure of the Atom Section Outline 2 1a Components of an Atom 2 1b Atomic Number Mass Number and Atomic Symbols 2 1c Isotopes and Atomic Mass Section Summary Assignment Elements are characterized by the number and type of particles of which they are composed In this section we describe the components of an atom explore the properties of subatomic particles and describe the relationship between the number of subatomic particles and the properties of an atom 1 Chapter 2 Elements and Compounds Opening Exploration 2 1 Mass Spectrometry and Subatomic Particles 2 1a Components of an Atom Atoms the smallest unit of matter consist of three subatomic particles protons neutrons and electrons A proton carries a relative charge of 1 and has a mass of 1 672622 10 24 g A neutron carries no electrical charge and has a mass of 1 674927 10 24 g An electron carries a relative charge of 1 and has a mass of 9 109383 10 27 g Two of the subatomic particles protons and neutrons are found in the atomic nucleus a very small region of high density at the center of the atom Electrons are found in the region around the nucleus As you will see when we study atomic structure in more detail in an upcoming chapter the precise location of electrons is not determined Flashforward 2 1 anchor Instead we visualize an electron cloud surrounding the nucleus that represents the most probable location of electrons Interactive Figure 2 1 1 Flashforward 2 1 As discussed in Section 6 4 the quantum mechanical view of the atom describes the allowed energy of electrons but not the exact position of electrons This is due in part to the wavelike motion of electrons which results in mathematical models that describe the probability of finding an electron at any location at a given time 2 Chapter 2 Elements and Compounds Interactive Figure 2 1 1 Explore the components of an atom The arrangement of subatomic particles in an atom not drawn to scale The atom represented in Interactive Figure 2 1 1 is not drawn to scale In reality electrons account for most of the volume of an atom and the nucleus of an atom is about 1 10 000 the diameter of a typical atom For example if an atom had a diameter the same size as a football field 100 yards or about 90 meters the nucleus of the atom would have a diameter of only about 1 cm Mass and Charge of an Atom The mass and charge of an atom affect the physical and chemical properties of the element and the compounds it forms As shown in Table 2 1 1 the three subatomic particles are easily differentiated by both charge and mass Table 2 1 1 Properties of Subatomic Particles Proton p Actual Mass kg 1 672622 10 27 Relative Mass 1836 Neutron n 1 674927 10 27 Electron e 9 109383 10 31 Mass u 1 007276 Actual Charge C 1 602 10 19 C Relative Charge 1 1839 1 008665 0 0 1 5 485799 10 4 0 1 602 10 19 C 1 The mass of an atom is almost entirely accounted for by its dense nucleus of protons and neutrons The actual mass of protons and neutrons is very small so it is more convenient to define the mass of these particles using a different unit The atomic mass unit u is defined as 1 12 the mass of a carbon atom that contains six protons and six neutrons Because neutrons and protons have very 3 Chapter 2 Elements and Compounds similar masses both have a mass of approximately 1 u The mass of an electron is about 2000 times less that of protons and neutrons and it has a mass of approximately zero on the atomic mass unit scale Atoms are neutral because protons and electrons have equal opposite charges and atoms have equal numbers of positively charge protons and negatively charged electrons An ion is an atom with an unequal number of protons and electrons because there number of protons and electrons is not equal the atom carries an overall positive or negative charge When an atom carries more protons than electrons it carries an overall positive charge and is called a cation An atom with more electrons than protons has an overall negative charge and is called an anion As discussed later in this chapter ions have very different properties than the elements they are derived from 2 1b Atomic Number Mass Number and Atomic Symbols Atoms of each element can be distinguished by the number of protons in the nucleus The atomic number Z of an element is equal to the number of protons in the nucleus For example a carbon atom has six protons in its nucleus and therefore carbon has an atomic number of six Z 6 Each element has a unique atomic number and all atoms of that element have the same number of protons in the nucleus All atoms of hydrogen have 1 proton in the nucleus Z 1 and all atoms of gold have 79 protons in the nucleus Z 79 Because protons carry a positive charge 1 in a neutral atom the atomic number also equals the number of electrons 1 in that atom An atom can also be characterized by its mass Because the mass of electrons is negligible the mass of an atom in atomic mass units u is essentially equal to the number of protons and neutrons in the nucleus of an atom called the mass number A For example a carbon atom with 6 protons and 6 neutrons in its nucleus has a mass number of 12 A 12 and a gold atom with 79 protons and 119 neutrons in its nucleus has a mass number of 198 A 198 The atomic symbol for an element consists of the one or two letter symbol that represents the element along with the atomic number written as a subscript number and the mass written as a superscript number For example the atomic symbol for a carbon C atom with 6 protons and 6 Au Note neutrons is 126 C and the symbol for a gold Au atom with 79 protons and 119 neutrons is 198 79 that the number of neutrons in an atom is equal to the difference between the mass number and the atomic number EXAMPLE PROBLEM rite atomic …