Chemistry Chapter 2: Atoms, Molecules, and Ions

• Chapter 2 covers atoms, molecules, and ions

Learning Objectives

• Summarize atomic milestones in the development of modern atomic theory
• Describe the three subatomic particles
• Define isotopes
• Write and interpret symbols that depict the atomic number, mass number, and charge of an atom or ion
• Calculate average atomic mass and isotopic abundance
• Name and determine formulas for ionic and molecular compounds

Dalton's Atomic Theory (1808)

• All matter consists of atoms; tiny indivisible particles that cannot be created or destroyed
• Atoms of one element cannot be subdivided, created, or destroyed during chemical reactions
• Atoms of an element are identical in mass and other properties
• Atoms of different elements will vary in mass and properties
• Compounds result from the chemical combination of a specific ratio of atoms of different elements

Atomic Structure: The Electron (1897)

• J.J. Thomson utilized cathode ray tubes
• Cathode rays are invisible, but the fluorescence of a zinc sulfide coating on the glass causes it to appear green
• Ray bends in a magnetic field which shows ray consists of charged particles
• Ray bends toward positive plate, showing rays consist of negative particles
• The Ray is identical for any cathode material and contains electrons

J.J. Thomson's Model of the Atom (1904)

• Plum-pudding model
• Atom = a positively charged sphere, with an equal amount of negative charge (uniformly distributed e¯) to be electrically neutral overall

Atomic Structure: The Nucleus (1911)

• Ernest Rutherford: Bombarded a thin gold foil with positively-charged a (alpha) particles
• Findings showed that a few alpha particle are deflected at large angles
• Proposed that most of the atom is empty space, as the majority of a particles passed through the gold foil
• The atom's positive charges are concentrated in an extremely dense central core: the nucleus

Modern Atomic Structure

• Neutrons were discovered in 1932 by James Chadwick
• Experimental evidence revealed that atoms are made up of smaller, subatomic particles:
• Positively charged particles = protons
• Neutral particles of roughly same mass = neutrons
• Negatively charged electrons with a mass of only about 1/2000th that of other particles

Elements and Atomic Number

• Each of the 100+ elements has a characteristic number of protons in its nucleus
• The number of protons is the defining trait of an element.
• The number of protons defines the atomic number, symbolized by Z
• Atomic number is the integer number located at the top of the box for each element
• Chemical symbol is a one, two or three letter symbol representing each element

The Periodic Table of Elements

• Organized by atomic number (number of protons, Z)
• Elements in a column (called groups or families) tend to have similar chemical and physical properties
• The rows are numbered and called periods

Mass Number

• Mass number (A) of an atom is the number of "massive" particles (protons + neutrons) in the nucleus
• Always an integer, NOT on the periodic table

Isotopes

• These are atoms of an element containing the same number of protons, but different numbers of neutrons
• Same atomic number but different mass numbers
• Isotopes of the same element have different masses due to different number of neutrons
• Isotopes occur in different amounts depending on the element
• Natural abundance: proportion of a particular isotope, relative to all isotopes for that element
• Some isotopes are not stable and decay radioactively

Subatomic Particles

• The number of protons is determined by the element (atomic number, Z)
• Atoms are electrically neutral, so the number of electrons must equal the number of protons
• The number of neutrons may vary in different atoms of the same element
• Only the number of protons determines the identity of an element
• To calculate the number of neutrons, subtract Z from A
• Mass number (A) = number of protons + neutrons
• Atomic number (Z) = number of protons
• A – Z = number of neutrons

Atomic Mass and the amu

• The true mass of an atom is extremely small and most of science uses a relative mass
• To set up a relative scale of masses, one atom must be chosen as the standard
• The current standard is carbon-12, whose mass is set at 12 amu exactly
• The unit "amu" stands for atomic mass unit
  • 1 amu = 1/12 the mass of carbon-12
  • 1 amu = 1 g/mol = 1 Dalton (Da)
• Mass on this relative scale:
  • Proton = 1.01 amu
  • Neutron = 1.01 amu
  • Electron = 0.000549 amu
• Average atomic mass is also reported in units of amu (= g/mol)

Average Atomic Mass

• Multiple isotopes of an atom are present in a typical sample
• Isotopes differ in terms of mass (due to different #'s of neutrons) and natural abundance
• The periodic table gives an average atomic mass
• Atomic mass is the weighted average of all of an element

Method for Calculating Average Atomic Mass

• Atomic mass= ∑(fractional abundance × isotopic mass)
• The fractional abundance is found by the: % Abundance of isotope/ 100

Ions and Charge

• Atoms are electrically neutral
• An atom can gain or lose electrons to form electrically charged species called ions
  • If an atom loses electrons, it will have a positive charge aka a cation
  • If an atom gains electrons, it will have a negative charge aka an anion
  • The identity of the element is determined by the number of protons, not the number of electrons (or neutrons) it has
  • Ions are not the same as isotopes

Writing Chemical Formulas

• Charges on ions are written in the upper right-hand corner (Mg2+, Cl-) implied
• Any charge of 1 (either 1+ or 1-) is implied (no need to write out)
• Subscripts tell you how many atoms
• H₂O has 2 hydrogen atoms and one oxygen atom and N2O4 has 2 nitrogen atoms and four oxygen atoms
• The subscript of 1 is implied
• Parentheses and subscripts = how many of each group, Mg(NO3)2 has two NO3 groups

Oxidation Numbers

• This the real or almost real charge that an element has in a compound
• Atoms get a charge by gaining/losing electrons.
  • If an atom loses 1 e-, the charge on the ion is 1+
  • If an atom gains 2 e-, the charge on the ion is 2–
• Charges of main group metals (1 and 2 families) and non-metals can be predicted from group number
• Group 14, transition metals, Lanthanides, and Actinides are less predictable
• "Zero Rule"- the sum of the oxidation states for all atoms of an electrically neutral molecule must add up to zero
• "Cross Rule" helps: remember to reduce to the simplest ratio

Naming Binary lonic Compounds: Example

• The binary ionic compound consists of cations and anions
• cation (metal) is named using the name of the element (Mg = magnesium)
• The anion uses the -ide suffix to the name of the element. (CI = chlorine → chloride)
• Formulas for ionic compounds must always be neutral

Ionic Compounds with Variable Charges

• Metals that can form cations with different charges require a Roman numeral to indicate the charge of the cation
• Assume that transition metals – except for silver (+1) and zinc (+2) – can have multiple charges
• Group 14 elements lead (Pb) and tin (Sn) can also have multiple charges (+2 or +4)

Molecular (Covalent) Compounds

• These are composed of atoms held together in molecules by covalent bonds
• A covalent bond = a bond between two atoms created by sharing one or more pairs of electrons
• These are composed of two or more nonmetals
• Binary covalent compounds are made of only two elements (may be more than two atoms) Example: SO3 (sulfur trioxide)
• Subset of binary compounds: diatomic molecules = molecules composed of only two atoms
• The Earth's trophosphere is ~ 99% diatomic molecules, with 21% oxygen and 78% nitrogen
• Diatomic molecules are not made up of the same kind of elements
• Naming Binary Covalent Compounds:
• Molecules contain only two different elements (both nonmetals).
• Element Number One:The first of the two elements has no changes
• The Second Element: gets the suffix -ide
• The number of atoms present for each element is indicated by prefixes:

Rules for Using Prefixes

Do not use the prefix mono- when naming the first element, it is omitted o- or a- prefixes are replaced to avoid double vowels. one - mono- two - di- three - tri- four - tetra- five - penta- six - hexa- seven - hepta- eight - octa- nine - nona- ten - deca-

Polyatomic lons

• They are charged with two or more atoms joined together by covalent bonds
• They can be either anions or cations
• Subset of polyatomic anions: Oxoanions
• Polyatomic anions containing oxygen in combination with one or more other elements