Elements
• Elements are the primary types of matter
• Matter is anything that has mass and volume
• Elements can’t be broken down into more simple parts
(that retain the properties of the element)
• An element is made up of only one type of atom
• Each element has unique physical properties (color, odor,
density, melting point, etc.): no two are exactly alike
• Living things are made up mainly of CHON
• Each element is represented by a one or two letter symbol
(Table 2.2): learn these
The Periodic Table
• Elements can be arranged into groups based on
similar physical and chemical properties
• These groups are the columns of the periodic
table
• There are 8 main groups, some of which have
special names:
- Group 1A = alkali metals
- Group 2A = alkaline earth metals
- Group 7A = halogens
- Group 8A = noble gases
Some Group 1A Elements
Some Group 7A Elements
Metals, Nonmetals and Metalloids
• Elements are classified as metals, nonmetals or
metalloids (as shown on periodic table)
• Each type has different physical properties:
• Metals: shiny, ductile/malleable, conduct heat
and electricity, high m.p. and density
• Nonmetals: dull, brittle, poor conductors, low
m.p. and density
• Metalloids: have some properties of each,
used as semiconductors
The Atom
• Atoms are the building blocks of all matter
• An atom is the smallest particle that retains the
properties of the element
• Atoms of different elements can combine to
form compounds (discussed in Ch. 4)
• Chemical reactions involve changes in the
atomic make-up of reacting substances
(discussed in Ch. 6)
Structure of the Atom
Nucleus (contains protons and neutrons)
Electron cloud (where electrons are most likely found)
•
•
•
•
•
Atoms are mostly empty space
They have 3 main sub-atomic particles:
Protons: positively charged, in nucleus (mass = 1 amu)
Neutrons: neutral, in nucleus (mass = 1 amu)
Electrons: negatively charged, outside of nucleus
(mass is very small)
• 1 amu is defined as 1/12 the mass of a carbon-12 atom
Atoms and Charge
• Atoms are always neutral
- same number of protons as electrons
• Like charges repel and opposite charges attract
• Negative electrons are attracted to positive nucleus
(keeps them around it)
• Positive protons in two nuclei repel each other, but
negative electrons are attracted to positive nucleus of
neighboring nucleus, so atoms in a substance are kept
at an optimum distance from each other (bond length)
Atomic Number and Mass Number
• All atoms of an element have the same number of
protons (and electrons)
• Atomic number (Z) = number of protons
• Periodic table is arranged in order of increasing Z
• Mass number (A) = number of protons plus
number of neutrons
Isotopes and Atomic Mass
• Not all atoms of an element have the same mass
number (why?)
• Because they can have a different number of neutrons
• Atoms of the same element with different mass
numbers are called isotopes
• Each isotope has a specific isotopic mass (not the
same as atomic mass or mass number)
• The atomic mass of an element, as reported on the
periodic table, is a weighted average of all the isotopic
atomic masses
• Atomic symbols are used to show an elements atomic
number (subscript) and mass number (superscript)
12
C
6
Calculating the Atomic Mass of an Element
Isotope
% of Sample
x
Mass Number
=
Mass Contribution
(75.8% = ) 75.8/100
x
35
=
26.5 amu
(24.2% = ) 24.2/100
x
37
=
8.95 amu
=
35.5 amu
35
Cl
17
37
Cl
17
Atomic Mass of Ch lorine
Calculating the Atomic Mass of an Element
• Example:
If a sample of chlorine is found to contain 75.8% chlorine-35 and
24.2% chlorine-37, what is the approximate atomic mass of
chlorine?
% Isotope in Sample
x
Mass Number
=
(75.8% = ) 75.8/100
x
35
=
26.5 amu
(24.2% = ) 24.2/100
x
37
=
8.95 amu
=
35.5 amu
Atomic Mass of Chlorine
Mass Contribution