Atoms and molecules by iV0ZpZ


Elements are homogeneous substances made up of identical
  atoms. (Homoatomic)
There are 118 elements known to exist. Each element has
  unique set of physical and chemical properties. Each
  assigned unique name and symbol.

Elemental Symbols- A symbol is assigned to each element.
  The symbol is based on the name of the element and
  consists of one capital letter or a capital letter followed by a
  lower case letter
Used to represent the element in general way or an atom of
  the element.

Elemental Song
Compounds are pure substances made up of two or more
  different kinds of atoms (Heteroatomic)
A molecular compound is symbolized by a compound

Compound formula - consists of the symbols of the elements
  found in the compound. Each elemental symbol represents
  one atom of the element. If more than one atom is
  represented, a subscript following the elemental symbol is

Examples ?
Learning Check 2.1
       Reading Assignment
Revisit 2.1-2.2
Read 2.4-2.7
Homework due on Mon, Jan 22nd
             What is an atom?
An atom is the smallest unit of an element that can
  exist as a stable, independent entity.
Greek word "a-tomos" and signifies "indivisible”

Modern science (19th century) has shown atom
 consists of sub-atomic particles. Fundamental
 particles in all theories are: Protons, Neutrons and

Masses of single atoms ranges between 1.67 ×10-24
 and 5.0 ×10-22
           Subatomic particles
Characteristics of the fundamental subatomic
           Subatomic particles
The protons and the neutrons are tightly bound in the
  central portion of an atom called the nucleus

Protons carry a +1 electrical charge and have a
  mass of 1 atomic mass unit (u).
Neutrons carry no electrical charge and have a
  mass of 1 atomic mass unit (u).

Electrons are located outside the nucleus of an
  atom. They carry a -1 electrical charge and have a
  mass of 1/1836 atomic mass unit (u). They move
  rapidly around the heavy nucleus.
              Atomic Number
The atomic number of an atom is equal to the
  number of protons in the nucleus of the atom.
Atomic numbers are represented by the symbol Z.

Atomic number stands for the number of electrons as

Atomic Mass number is the sum of the number of
  protons and the number of neutrons in a nucleus.
Atomic mass numbers are represented by the
  symbol A.

Elements are arranged in the periodic table based on
  their atomic number.
Periodic Table
Many atoms of elements have same atomic
  number but different atomic mass numbers
  i.e. atoms of the same element that contain
  different numbers of neutrons in their nuclei.
  They are called isotopes.
Most elements are made up of mixtures of two
  or more isotopes.
Hydrogen has three isotopes:
Protium (A=1), Deuterium (A=2), Tritium (A=3)
           1          2           3
           1   H      1   H       1   H

Learning Check: 2.2
Relative Masses and Atomic Mass unit
Earliest chemists had no means of determining the
  actual masses of atoms. The extremely small size of
  atoms and molecules makes it inconvenient to use
  their actual masses for measurements or calculations.
For instance, with modern instruments (mass
  spectrometers) actual mass of Hydrogen atom = 1.674
  × 10-24 g
System using relative masses is used.

Atomic Mass unit is a unit used to express the relative
  masses of atoms. One atomic mass unit is equal to
  1/12 the mass of a carbon-12 atom.
  – A carbon-12 atom has a relative mass of 12 u
  – An atom with a mass equal to 1/12 the mass of a carbon-12
    atom would have a relative mass of 1u.
   Atomic and Molecular weights
The atomic weight of an element is the relative
  mass of an average atom of the element
  expressed in atomic mass units.
Learning Check 2.3

The relative mass of a molecule in atomic mass units
  is called the molecular weight of the molecule.
  Because molecules are made up of atoms, the
  molecular weight of a molecule is obtained by
  adding together the atomic weights of all the atoms
  in the molecule.
Learning Check 2.4
    Isotopes and atomic weights
The atomic weight of an element is the relative
  mass of an average atom of the element
  expressed in atomic mass units.
Many elements occur naturally as a mixture of
  several isotopes.
The atomic weight of elements that occur as mixtures
  of isotopes is the average mass of the atoms in the
  isotope mixture.
The average mass of a group of atoms is obtained by
  dividing the total mass of the group by the number
  of atoms in the group.
     Atomic weight calculation
A practical way of determining the average mass
  of a group of isotopes is to assume the group
  consists of 100 atoms and use the percentage of
  each isotope to represent the number of atoms
  of each isotope present in the group.
The use of percentages and the mass of
 each isotope leads to the following
 equation for calculating atomic weights of
 elements that occur naturally as a mixture
 of isotopes.

Learning Check 2.5
           Avogadro’s Number
Mass of an average carbon atom 1.994 × 10-23 g
Atomic weight of carbon 12.01 u
Number of C atoms required to give 12.01 g C?

1.994  10-23 g  1 C atom
12.01g  (1 C atom)
                     1.994  10-23 g
Example 2.6
Learning check 2.6

The number 6.022 ×1023 is called as Avogadro’s
  number in honor of Amadeo Avogadro
           Mole and 6.022 ×                    1023
The number of particles (atoms or molecules)
  contained in a sample of element of compound with
  a mass in grams equal to the atomic or molecular
  weight, respectively is called a mole (mol).

   – 1 mole Na = 22.99 g Na = 6.022x1023 Na atoms
   – 1 mole Ca = 40.08 g Ca = 6.022x1023 Ca atoms
   – 1 mole S = 32.06 g S = 6.022x1023S atoms

Learning check 2.7
   – 1 mole H2O = 18.02 g H2O
                      = 6.022x1023 H2O molecules
   – 1 mole CO2 = 44.01 g CO2
                     = 6.022x1023 CO2 molecules
   – 1 mole NH3 = 17.03 g NH3
                      = 6.022x1023 NH3 molecules
Calculations involving moles (Compounds)
 1 mole CO2 molecules = 1 mole C atoms = 2 moles
   O atoms
 44.01 g CO2 = 12.01 g C = 32.00 g O
 6.022x1023 CO2 molecules = 6.022x1023 C atoms
 = (2) 6.022x1023 O atoms

 How many moles of O atoms are contained in 31.2
  g of CO2

 Example 2.10
Calculations involving moles (Compounds)
 What is the mass percentage of oxygen in CO2?

 1 mole CO2 = 44.01g CO2 = 32.00g O
        Reading Assignment
Revise Chapter 2
Go over Study Skills 2.1
Chapter 3 reading- up to section 3.3

Answer prelab questions

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