Chapter Two Atoms & the Periodic Table General, Organic & Biological Chemistry  Janice Gorzynski Smith   2010 McGraw-Hill

CHE120 Chapter Two 2 Classification of Matter

CHE120 Chapter Two 3 Atomic Theory Chemistry is founded on four fundamental assumptions about atoms and matter, which together make up modern Atomic Theory: 1.All Matter is composed of atoms. 2.The atoms of a given element differ from the atoms of all other elements. 3.Chemical compounds consist of atoms combined in specific ratios. 4.Chemical reactions change only the way the atoms are combined in compounds; the atoms themselves are unchanged.

CHE120 Chapter Two 4  Atoms are composed of tiny subatomic particles called protons, neutrons, and electrons.  The masses of atoms and their subatomic particles are extremely small when measured in grams. These masses are expressed on a relative mass scale. That is, one atom is assigned a mass, and all others are measured relative to it.  The basis for the relative atomic mass scale is an atom of carbon that contains 6 protons and 6 neutrons. This carbon atom is assigned a mass of exactly 12 atomic mass units (amu).

CHE120 Chapter Two 5  Protons and a neutrons have a mass of 1 amu. Hydrogen atoms have mass of 1 amu, oxygen atoms have mass of 16 amu, etc.  Subatomic particles are not distributed randomly throughout an atom.  The protons and neutrons are packed closely together in a dense core called the nucleus.  Surrounding the nucleus, the electrons move about rapidly through a large, mostly empty volume of space.

CHE120 Chapter Two 6  Diameter of a nucleus is only about m.  Diameter of an atom is about m. Fig 3.1 The structure of an atom

CHE120 Chapter Two 7 The structure of the atom is determined by an interplay of different attractive and repulsive forces.  Unlike charges attract - the negatively charged electrons are held close to the nucleus due to this attraction.

CHE120 Chapter Two 8  Like charges repel each other - electrons try to get as far away from each other as possible thereby occupying a large volume in space. The positively charged protons in the nucleus also repel each other, however, they are held together by a unique attraction called the nuclear strong force.

CHE120 Chapter Two 9 Element and Atomic Number Atomic Number (Z): Number of protons in the nucleus of an atom. The number Z is unique to each element. We can identify the element if we know the number of protons in the nucleus. Mass Number (A): The total number of protons and neutrons in an atom. The electrical charge on an atom is neutral because the number of positively charged protons and the number of negatively charged electrons are the same.

CHE120 Chapter Two 10 Isotopes and Atomic Weight Isotopes: Atoms with identical atomic numbers (Z) but different mass numbers (A) are called isotopes. Hydrogen, deuterium, and tritium are three isotopes of hydrogen. - Most abundant hydrogen isotope has one proton and no neutron (A=1); - deuterium isotope has one proton and one neutron (A=2), and - tritium isotope has one proton and two neutrons (A=3).

CHE120 Chapter Two 11 Isotopes of hydrogen

CHE120 Chapter Two 12 Atomic Weight: The weighted average mass of an element’s atoms in a large sample that includes all the naturally occurring isotopes of that atom.

CHE120 Chapter Two 13 The Periodic Table  Beginning at the upper left corner of the periodic table, elements are arranged by increasing atomic number into seven horizontal rows, called periods, and 18 vertical columns, called groups.  The elements in a given group have similar chemical properties. Lithium, sodium, potassium and other elements in group 1A have similar properties. Likewise, chlorine, bromine, iodine, and other elements in group 7A behave similarly.

CHE120 Chapter Two 14 Fig 3.2 The periodic table

CHE120 Chapter Two 15 The Periodic Table

CHE120 Chapter Two 16 The table has 114 boxes, each of which has the symbol, atomic number and atomic weight of an element.

CHE120 Chapter Two 17 Some Characteristics of Different Groups Various elements in a given group of the periodic table show remarkable similarities in their properties. For example, The Alkali Metals (1A)  Lithium(Li), Sodium(Na), Potassium(K), Rubidium(Rb), Cesium(Cs) and Francium(Fr) are shiny and soft and have low melting points. All react rapidly with water to form products that are highly alkaline - hence the name alkali metals.

CHE120 Chapter Two 18  Group 2A – Alkaline Earth metals: Beryllium(Be), Magnesium(Mg), Calcium(Ca), Strontium(Sr), Barium(Ba), and Radium(Ra) are lustrous,, silvery metals. They are less reactive than the Alkali group.  Group 7A - Halogens: Fluorine(F), Chlorine(Cl), Bromine(Br), Iodine(I), and Astatine(At) are colorful and corrosive nonmetals. All are found in nature in combination with other elements, such as sodium in sodium chloride (NaCl)

CHE120 Chapter Two 19  Group 8A-Noble gases: Helium (He), Neon (Ne), Argon (Ar), Krypton (Kr), Xenon (Xe), and Radon (Rn) are colorless gases of very low chemical reactivity.

CHE120 Chapter Two 20 Transition Between Atomic Nucleus & Outside the Nucleus Nothing stronger known in the universe than nuclear forces – that holding protons and neutrons together.  But the action is outside the nucleus, where the electrons are spinning and moving.  They don’t have much mass but they make up for it with the amount of energy involved in their movement.

CHE120 Chapter Two 21 Electronic Structure of Atoms Quantum mechanical model of atomic structure:  Electrons are not perfectly free to move about in an atom.  Each electron is restricted to moving about only in a certain region of space within the atom, depending on the amount of energy the electron has.

CHE120 Chapter Two 22  Different electrons have different amounts of energy and thus occupy different regions within the atom.  The energies of electrons are quantized, or restricted to having only certain values.  The electron energy levels in an atom are grouped around the nucleus into shells.  Within the shells, electrons are further grouped into subshells of four different types, identified as s, p, d, and f in order of increasing energy.

CHE120 Chapter Two 23 - The first shell has only an s subshell - The second shell has an s and a p subshell - The third shell has an s, p, and a d subshell. - The fourth shell has an s, p, d, and an f subshell.

CHE120 Chapter Two 24  The number of subshells in a given shell is equal to the shell number. For example, shell number 3 has 3 subshells.  Within each subshell, electrons are further grouped into orbitals, regions of space within an atom where the specific electrons are more likely to be found.  There are different numbers of orbitals within the different kinds of subshells.

CHE120 Chapter Two 25 Different orbitals have different shapes. Orbitals in s subshells are spherical (a), while orbitals in p subshells are roughly dumbbell shaped (b).

CHE120 Chapter Two 26  The first shell (nearest to the nucleus) can hold only 2 electrons. They are in a single 1s orbital.  The second shell can hold 8 electrons – 2 in the 2s orbital and 6 in the 2p orbitals.  The third shell can hold 18 electrons – 2 in a 3s orbital, 6 in the 3p orbitals, and 10 in the 3d orbitals.  The fourth shell can hold 32 electrons – 2 in a 4s orbital, 6 in the 4p orbitals, 10 in the 4d orbitals, and 14 in the 4f orbitals.

CHE120 Chapter Two 27 The overall electron distribution within an atom is summarized in table 3.2 shown below.

CHE120 Chapter Two 28 Electron Configuration Electron Configuration: The exact arrangement of electrons in atom’s shells and subshells. Rules to predict electron configuration: 1. Electrons occupy the lowest-energy orbitals available, beginning with 1s and continuing in order shown in the fig Each orbital can hold only two electrons, which must have opposite spins. 3. Two or more orbitals with the same energy – each orbital gets one electron before any orbital gets two.

CHE120 Chapter Two 29 Electrons fill orbitals in ascending order, from the lowest-energy orbitals upward. Generally, this means that lower-numbered shells fill before higher-numbered shells. However, some overlap in energy levels occurs starting in the third energy level. Fig 3.5 order of orbital energy levels

CHE120 Chapter Two 30 Electronic configuration of a few elements are shown below:

CHE120 Chapter Two 31

CHE120 Chapter Two 32 Electron Configuration and the Periodic table The periodic table can be divided into four regions or blocks, of elements according to the shells and subshells as shown in Fig 3.6. Valence Shell : Outermost shell of an atom. Valence electron: Electron in the outermost shell of an atom. These electrons are loosely held and are most important in determining an element’s chemical properties.

CHE120 Chapter Two 33 Fig 3.6 electron configurations and the periodic table

CHE120 Chapter Two 34 Chapter Summary  All matter is composed of atoms.  An atom is the smallest and simplest unit into which an element can be divided while maintaining the properties of the element.  Atoms are made up of subatomic particles called protons, neutrons and electrons. Protons have a +1 charge, electrons have a −1 charge, and neutrons have no electrical charge.  Elements differ from each other according to the number of protons their atoms contain (Atomic number, Z)

CHE120 Chapter Two 35 Chapter Summary Contd.  Mass number (A):Total number of protons and neutrons in an atom.  Isotopes: Atoms with identical number of protons but different numbers of neutrons.  Periodic table: Tabular arrangement of elements according to their valence shells.  The electrons surrounding an atom are grouped into shells. Within each shell, electrons are grouped into subshells, and within subshells into orbitals (regions of space where electrons are most likely to be found).

CHE120 Chapter Two 36 Chapter Summary Contd.  s orbitals are spherical and p orbitals are dumbbell shaped.  Each orbital and each shell can hold a specific number of electrons. -The first shell can hold only two electrons. 2 electrons in an s orbital (1S 2 ). -The second shell can hold 8 electrons. 2 electrons in an s orbital and 6 electrons in 3 p orbitals. (2S 2 2p 6 ). -The third shell can hold 18 electrons. 2 electrons in an s orbital, 6 electrons in 3 p orbitals, and 10 electrons in 5 d orbitals (3S 2 3p 6 3d 10 ); and so on.

CHE120 Chapter Two 37 End of Chapter 2