Atoms and the Periodic Table Chapter 2 Atoms and the Periodic Table

Classification of Matter

Elements A pure substance that cannot be broken down into simpler substances by a chemical reaction Identified by a one- or two-letter symbol Arranged in the periodic table Its location on the periodic table indicates a lot about its chemical properties Can be a metal, nonmetal, or a metalloid

Elements Some of the element symbols will be familiar to you already It will take time and practice to learn others

Periodic Table

Elements Metals- located to the left of the stair step line (top located near Boron) Shiny Good conductors of heat and electricity Solid at room temperature except for mercury

Elements Nonmetals – except for hydrogen (H) these are located to the right of the stair step line (top located near Boron) Do not have a shiny appearance Generally poor conductors of heat and electricity Usually a solid or gas at room temperature (exception: Br2 is a liquid at room temperature)

Elements Metalloids – 7 elements located along the stair step line (top located near Boron) Properties in between metals and nonmetals Metallic shine but brittle Boron (B) Silicon (Si) Germanium (Ge) Arsenic (As) Antimony (Sb) Tellurium (Te) astatine (At)

Classification of Matter

Compounds A pure substance formed by chemically combining two or more elements A chemical formula for a compound consists of: Element symbols to show the identity of the elements that form a compound Subscripts to show the ratio of atoms in the compound H2O H2O C3H8 C3H8 2 H atoms 1 O atom 3 C atoms 8 H atoms

Compounds Can be drawn many ways but we are always making a simplified representation of reality

4 Common ways to draw compounds H H Line Lewis Dot Diagram

Atoms All mater is composed of building blocks called atoms Atoms are composed of three subatomic particles Proton Neutron Electron

Atoms The mass of an atom is very small since each proton and neutron has a mass of 1.6726 X 10-24 Chemists use the atomic mass unit (amu) to measure the mass of an element

Subatomic Particles The nucleus is the dense core that contains the protons and neutrons Most of the mass of an atom is in the nucleus Electron cloud is composed of electrons that are in almost empty space around nucleus

Subatomic Particles Opposite charges attract each other Like charges repel each other Protons and electrons attract each other but electrons repel each other

Atomic Number The number of protons in the nucleus of an atom is its atomic number Every atom of a given element has the same number of protons in the nucleus We can symbolize the number of protons with the generic letter Z A neutral atom has no overall chare so: Z= number of protons = electrons Example: Lithium Atomic number (Z) is the number of protons in the nucleus. 3 Li

Isotopes Atoms of the same element that have a different number of neutrons The number of protons remains constant Different forms of the same element The mass number (A) represents the number of protons plus the number of neutrons A = Z + number of neutrons Example – two forms of carbon: C-12 has 6 neutrons C-14 has 8 neutrons Z = 6 for both forms of carbon

Example Cl Mass number (A) Atomic number (Z) # of protons = 17 35 Cl Atomic number (Z) 17 # of protons = 17 # of electrons = 17 # of neutrons = A – Z = 35 – 17 = 18

Atomic Weight The weighted average of the masses of the naturally occurring isotopes of a particular element reported in atomic mass units (amu) This information can be found on the periodic table 6 C 12.01 atomic number (Z) element symbol atomic weight (amu)

Atomic Weight Calculation 1 The weighted average of the ass of the naturally occurring isotopes Example What is the atomic weight of chlorine? List each isotope, it’s mass in atomic mass units, and it’s abundance in nature. Step [1] Isotope Mass (amu) Isotopic Abundance Cl-35 34.97 75.78% = 0.7578 Cl-37 36.97 24.22% = 0.2422

Atomic Weight Calculation 2 Multiply the isotopic abundance by the mass of each isotope, and add up the products. Step [2] The sum is the atomic weight of the element. 34.97 x 0.7578 = 26.5003 amu 36.97 x 0.2422 = 8.9541 amu 35.45 amu 35.4544 amu = 4 sig. figs. Answer 4 sig. figs.

Organizing the elements Periodic table was put together over time as the elements were discovered, isolated, and studied. Many versions exist but the most common is based on the one developed by Dmitri Mendeleev in 1869 A row in the periodic table is called a period Elements in the same row are similar in size A column in the periodic table is called a group Elements in the same group have similar electronic and chemical properties

Periodic Table

Periodic Table - Groups Alkali Metals Alkaline Earth Metals Transition Metals Lanthanide & Actinide Halogens Nobel Gases Very reactive Metals except for H +1 ions React with Oxygen to form compounds that dissolve into alkaline solutions in water Reactive +2 ions Oxygen compounds are strongly alkaline Many are not water soluble Metals Form ions with several different charges (oxidation states) Tend to form +2 and +3 ions Lanthanides 58 – 71 Actinides 90 – 103 Actinides are radioactive Form diatomic molecules in elemental state -1 ions Salts with alkali metals Inert Heavier elements have limited reactivity Do not form ions Monoatomic gases

Carbon is Special Carbon’s ability to join with itself and other elements gives it a versatility not seen with any other element in the periodic table Elemental forms of carbon include the following carbon-only structures: diamond graphite buckminsterfullerene

Carbon “the party animal of the atomic world, latching onto many other atoms (including itself) and holding tight, forming molecular conga lines of hearty robustness-the very trick of nature necessary to build proteins and DNA” Bill Bryson from A Short History of Nearly Everything

Electronic Structure The chemical properties of an element are determined by the number of electrons in an atom Electrons do not move freely in space – restricted to a region with a particular energy Electrons occupy discrete energy levels that are restricted to specific values – the energy is “quantized” Electrons are confined to regions called the principal energy levels or shells

Electron Shells The shells are numbers, n=1, 2, 3, 4… Moving out from the nucleus Electrons closer to the nucleus are held more tightly are lower in energy Electrons farther from the nucleus are held less tightly and are higher in energy The farther a shell is from the nucleus, the larger its volume, and the more electrons it can hold

Electron Shells Nucleus

Orbitals Each shell is divided into subshells made up of orbitals These are identified as s, p, d, f Each orbital can hold two electrons

Orbitals

Orbitals The maximum number of electrons that can occupy a shell is determined by the number of orbitals in a shell

Electron Configuration The electron configuration shows how the electrons are arranged in an atom’s orbitals The ground state is the lowest energy arrangement The outtermost shell is the valance shell The electrons in the valence shell are called valence electrons The chemical properties of an element depend on the number of electrons in the valence shell

Electron Configuration Rule 1: Electrons are placed in the lowest energy orbital beginning with the 1s orbital. Orbitals are then filled in order of increasing energy. Rule 2: Each orbital holds a maximum of 2 electrons Rule 3: 1 electron is added to each orbital until all of orbitals are half-filled Then, the orbitals can be completely filled

Electron Configuration

Example What would the electron configuration for fluorine (F) be? Atomic Number = 9 so it has nine electrons Shells fill: 1s22s22p5 How many valence electrons does it have? There are 7 electrons in the 2nd shell so there are 7 valence electrons

Examples Be Cl 1s22s2 1s22s22p63s23p5 valence shell: n = 2 Atomic number: 4 Atomic number: 17 1s22s2 1s22s22p63s23p5 valence shell: n = 2 valence shell: n = 3 # of valence electrons = 2 # of valence electrons = 7

Valence Electrons 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–8A, equals the number of valence electrons for the main group elements. The exception is He, which has only 2 valence electrons. The chemical properties of a group are therefore very similar.

Valence Electrons

Electron Dot Symbols 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 Element: # of Valence electrons: Electron-dot symbol: H 1 C 4 O 6 Cl 7

Periodic Trends The size of atoms increases down a column, as the valence e− are farther from the nucleus. Increases Decreases The size of atoms decreases across a row, as the number of protons in the nucleus increases, pulling the valence electrons in closer.

Periodic Trends The ionization energy is the energy needed to remove Decreases The ionization energy is the energy needed to remove an electron from a neutral atom. Na + energy Na+ + e– Ionization energies decrease down a column as the valence e− get farther away from the positively charged nucleus. Increases Ionization energies increase across a row as the number of protons in the nucleus increases.