CHAPTER 6 – CHEMISTRY
6.1 Atoms and Their
Interactions
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Earth supports an enormous variety of organisms.
The laws of chemistry govern the structure &
function of all living things.
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Section Objectives:
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Define element, atom, compound, and molecule.
Draw a model of the structure of an atom.
Explain what determines an atom’s stability.
Contrast ionic and covalent bonds.
MATTER
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Matter is
– Anything that occupies space and has mass.
Mass is
– The amount of matter an object has.
Weight is
– The force produced by gravity acting on mass.
The same mass would have less weight on the moon because
the moon exerts less force (gravity) on the object than the Earth
does.
ELEMENTS
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Elements are
– Pure substances that can’t be broken down into more
simple kinds of matter.
Information about the elements is summarized on a chart
(back of the room) known as the periodic table of elements.
– Each element is represented by a chemical symbol, made
of 1, 2 or 3 letters.
– In most cases, the symbol comes from the 1st letter or
other letters in the name of the element.
– Others come from the Latin names of elements, i.e., Na
from the Latin word natrium.
ATOMS
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Atoms are
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Parts of an ATOM
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The simplest particle of an element that retains all
of the element’s properties.
Nucleus
 Proton
 Neutron
Electron
NUCLEUS
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Is found in the central region of the atom
Makes up the majority of the bulk of the atom
Is made of two subatomic particles
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Proton (+)
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Atomic number of the element
Neutron (no charge)
Mass number is the total of protons and
neutrons of the atom.
ELECTRONS
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Small, negatively charged particles in an
atom
The number of electrons must = the number
of protons in an atom of the same element.
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The net electrical charge of an atom = zero.
They are high-energy particles with very little
mass.
ELECTRONS, CONT’D…
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Electrons move about the nucleus at very high
speeds.
They are located in orbitals.
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Orbital rules for biology…
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First energy level = 2 eSecond energy level = 8 eThird energy level = 8 e-
The technical rules for orbitals will be
covered in chemistry next year.
ISOTOPES
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Isotopes are atoms of the same element that have a
different number of neutrons.
Additional neutrons change the mass of the element.
Most elements are made up of a mixture of isotopes.
Average atomic mass of an element takes into
account the relative amounts of each isotope in the
element.
The average is the mass found in the periodic table.
COMPOUNDS
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Under natural conditions, most elements do
not exist alone.
Instead, they combine to form compounds.
Chemical formula: shows the kinds and
proportions of atoms of each element that
forms a particular compound.
H2O = 2-H; 1-O
COMPOUNDS, CONT’D…
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How elements combine to form compounds
depends on the number and arrangement of
electrons in their orbitals.
An atom is chemically stable when its highest
orbital (the one farthest from the nucleus) is
filled with the maximum number of electrons.
COMPOUNDS, CONT’D…
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Most atoms aren’t stable in their natural state.
They tend to react with other atoms in different ways to
become more stable.
Carbon, nitrogen, and oxygen atoms have unfilled
orbitals at their highest energy levels (orbitals).
Chemical bonds are the attractive forces that hold
atoms together.
– Most elements interact with other atoms to form
chemical bonds.
COVALENT BONDS
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Forms when two atoms share one or more pairs of
electrons.
H2O = held together by covalent bonds
Molecule = the simplest part of a
substance that retains all of the
properties of that substance, and can
exist in a free state.
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i.e., one molecule of the compound water is
H2O.
IONIC BONDS
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Figure 2.4, p. 34
1. Sodium and chlorine atoms have unfilled outer
orbitals.
Because of this, they are reactive.
In order to achieve stability, the outer e- of the Na atom
is transferred to the Cl atom.
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This makes sodium slightly positive. (Na+)
This makes both atoms more stable than they were
before.
IONIC BONDS, CONT’D…
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2. The chlorine that has gained an electron from
sodium now has 8 e- in it’s outer orbital. It is
stable.
With this additional e- chlorine is slightly negative.
– ClBoth Na+ and Cl- have a charge.
This makes them ions.
An ion is an atom or molecule with an electrical
charge.
Ionic Bonds, cont’d…
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3. Positive and negative electrical
charges attract.
Na+ and Cl- are attracted to each other and
form the ionic compound NaCl.
Table salt