Chapter 2 - Atoms and the Periodic Table

Chapter Two: Atoms and Periodic Table Elements and the Periodic Table elements are arranged in the periodic table; the position of an element in the periodic table provides information about its chemical properties. they are divided into three groups: metals, nonmetals, and metalloids. metals • located on the left side of the periodic table • good conductors of heat and electricity • shiny solids at room temperature, except for mercury, which is liquid nonmetals • located on the right side of the periodic table • have a dull appearance • usually poor conductors of heat and electricity • can be solids (sulfur, carbon), liquids (bromine), or gases (nitrogen, oxygen) at room temperature metalloids • located on the solid line that starts at boron and angles down towards astatine • have properties intermediate between metals and nonmetals • only seven elements: boron, silicon, germanium, arsenic, antimony, tellurium, and astatine Focus on the Human Body Compounds a compound is a pure substance formed by chemically combining two or more elements together. chemical formulas are used to represent compounds. one consists of element symbols to show the identity of the elements as well as subscripts to show the ratio of the atoms in the compound. compounds can be drawn in different ways, and different elements are represented by different colors. Structure of the Atom atoms are composed of subatomic particles. mass (amu) subatomic particle charge mass (g) proton +1 1.6726 x 10 neutron 0 1.6749 x 10 electron -1 -24 -24 9.1093 x 10 -28 1 1 negligible amu (atomic mass unit) is the mass unit relative to a standard, onetwelfth of the mass of carbon (12 C), which has six protons and six -24 neutrons. this mass is equal to 1.661 x 10 g. thus, the atomic weight of (12 C) is 12.0 amu. nucleus • location of the protons and neutrons • dense core of the atom • location of most of the atom’s mass electron cloud: • location of electrons • comprises most of the atom’s volume • mostly empty space opposite charges attract: the positive charge of a proton attracts the negative charge of an electron. like charges repel. atomic number • represented by “Z” it is the number of protons in the nucleus. • every atom of a given element has the same number of protons in the nucleus. • different elements have different atomic numbers. • a neutral atom has no net overall charge, so Z is also equal to the number of electrons in this case. • the periodic table is arranged according to the atomic number of the elements. Isotopes isotopes are atoms of the same element that have the same number of protons but a different number of neutrons. thus, the isotopes of an element have different mass numbers. mass number (A) = number of protons (Z) + number of neutrons most elements in nature exist as a mixture of isotopes; the isotope symbols of Cl are: mass number (A) 35 atomic number (Z) 17 Cl 37 17 Cl mass number (A) atomic number (Z) # of protons = 17 # of protons = 17 # of electrons = 17 # of electrons = 17 # of neutrons = 35 - 17 = 18 # of neutrons = 37 - 17 = 20 isotopes of hydrogen Atomic Weight since an element can have several naturally occurring isotopes and different isotopes have different masses, the atomic weight of the elements is the weighted average of the masses of naturally occurring isotopes in amu. in the periodic table, it is as shown: 6 C 12.0 99% of the naturally occurring isotope is 12 C or 126 C. the natural abundance of other isotopes, such as 13 C and 14C are low. how to determine the atomic weight of an element example: what is the atomic weight of chlorine? two naturally occurring isotopes: 35 17 Cl 37 17 1. list each isotope, its mass in amu, and its abundance in nature. Cl - isotope mass (amu) isotopic abundance Cl-35 34.97 75.78% = 0.7578 Cl-37 36.97 24.22% = 0.2422 2. multiply the isotopic abundance by the mass of each isotope and add up the products; the sum is the atomic weight of the element. 34.97 x 0.7578 = 23.5003 amu 36.97 x 0.2422 = 8.9541 amu 4 sig figs 35.4544 amu = 35.45 amu The Periodic Table a row in the periodic table is called a period, and a column in the periodic table is called a group. main group elements • consist of the tall columns on the right and left of the periodic table • numbered 1A to 8A transition metal elements • 10 short columns in the middle • numbered 1B to 8B inner transition elements • consist of the lanthanides and actinides • no group numbers assigned Characteristics of Groups 1A and 2A elements that comprise a particular group have similar chemical properties. 1A elements are known as alkali metals 2A elements are known as alkaline earth elements properties of both groups: • soft and shiny metals • low melting points • good conductors of heat and electricity • react with water to form basic solutions and hydrogen gas • none exist in nature as pure elements; they are always combined with other elements to form compounds. Characteristics of Groups 7A and 8A 7A elements are known as halogens; properties include: • exist as two atoms joined together: F2, Cl2, Br2, etc. • very reactive 8A elements are known as noble gases; properties include: • very stable • rarely combine with any other elements (radon, though, is radioactive) Electronic Structure of the Atom the chemical properties of an element are determined by the electronic structure of the atom. an electron is confined to a specific region around the nucleus, giving it a particular energy. the regions occupied by electrons are called principal energy levels or shells (n). each shell can have a different number of electrons, and they are numbered n = 1, 2, 3, 4, etc. electrons in lower numbered shells are closer to the nucleus and are lower in energy. electrons in higher numbered shells are further from the nucleus and are higher in energy. as the shell number increases, they are farther from the nucleus with increased volume and can hold more electrons. the maximum number of electrons in each shell is given by the formula 2n2 , where n = shell number. shell (n) number of electrons in the shell increasing 4 32 energy and 3 18 increasing 2 8 number of 1 2 electrons shells are divided into subshells, identified by the letters s, p, d, and f. the subshells consist of orbitals. an orbital is a region of space were the probability of finding an electron is high. each orbital can hold two electrons. subshell number of orbitals s 1 increasing p 3 energy d 5 f 7 when the shell number increases, the numbers of subshells and orbitals increase. electronic structure of atoms: shell > subshell > orbital the s orbital has a spherical shape. the orbital gets larger in size as the shell number increases. that is, the 3s orbital is larger than the 2s orbital, which is also larger than the 1s orbital. the p orbital has a dumbbell shape. in one shell, the p orbitals are higher in energy than the s orbital, so 2p has a higher energy than 2s. the electron configuration shows how the electrons are arranged in an atom’s orbitals. the ground state is the lowest energy arrangement. three rules to determine the ground state electronic configuration of an atom: 1. electrons are placed in the lowest energy orbital, beginning with the 1s orbital. orbitals are then filled in order of increasing energy. 2. each orbital holds a maximum of 2 electrons. 3. when orbitals are equal in energy, such as three p orbitals: • 1 electron is added to each orbital until all of the orbitals are half-filled • then, the orbitals can be filled by adding an electron to each half-filled orbital until they are completely filled an orbital diagram uses a box to represent each orbital and arrows to represent electrons. two electrons must have paired spins (opposite directions) to fit into the same orbital. the electron configuration can be shortened by using noble gas notation. write the symbol of the previous noble gas, then add the electronic configuration of the additional electrons. Electron Configurations and the Periodic Table the periodic table can be divided into four regions: s, p, d, and f blocks. the blocks are labeled according to the subshells that are filled with electrons last. Valence Electrons the chemical properties of an element depend on the number of electrons in the outermost shell (the highest value of n), called the valence shell, the most loosely held electrons. the electrons in the valence shell are called valence electrons. relating valence electrons to group number • elements in the same group have similar electron configurations • elements in the same group have the same number of valence electrons • the group number, 1A to 8A, equals the number of valence electrons for the main group elements • • the exception is He, which only has 2 valence electrons the chemical properties of a group are therefore similar electron-dot symbol • dots representing valence electrons are placed on the four sides of an element symbol. • each dot represents one valence electron. • for 1 to 4 valence electrons, single dots are used. with more than 4 valence electrons, the dots are paired. periodic trends many properties of atoms change in a regular way across a row or down a column of the periodic table, called periodic trends, as shown by two properties of atoms: atomic size and ionization energy. atomic size • the size of an atom is measured by its atomic radius, which is the distance from the nucleus to the outer edge of the valence shell. • the size of atoms decreases down a column, as the valence electrons are farther from the nucleus. • the size of atoms decreases across a row, as the number of protons in the nucleus increases, pulling the valence electrons closer. ionization energy • ionization energy is the energy needed to remove an electron from a neutral atom. • the more tightly the electron is held, the greater the energy required to remove it; removing an electron from a neutral atom forms a cation. • ionization energies decrease down a column, as the valence electrons get farther away from the positively charged nucleus. • ionization energies increase across a row, as the number of protons in the nucleus increases.