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Transcript
Cellular Chemistry (1)
Unit 2
THE PERIODIC TABLE
THE PERIODIC TABLE
• A table of chemical elements.
• Invented by Russian chemist: Dmitri
Mendeleev in 1869
– To show the recurring periodic trends in the
properties of the elements.
THE PERIODIC TABLE
• Widely used in chemistry, biology, physics,
engineering
– to classify, systemize, and compare chemical
behavior
• There have been many models.
• The current standard form has 118 elements.
Start With Atoms
• All substances consist of atoms
• An element is a substance that contains only
one type of atom
Start With Atoms
• Atoms
– Are the fundamental building-block particle of
matter
• Life’s unique characteristics start with the
properties of different atoms
Subatomic Particles and Their
Charge
• Atoms differ in numbers of subatomic
particles
– Atoms consist of electrons moving around a
nucleus of protons and neutrons
Subatomic Particles and Their Charge
• Charge
– Electrical property of some subatomic
particles
– Opposite charges attract; like charges repel
• Electron (e-)
– Negatively charged subatomic particle that
occupies orbitals around the atomic nucleus
Subatomic Particles in the Nucleus
• Nucleus
– Core of an atom, occupied by protons and
neutrons
• Proton (p+)
– Positively charged subatomic particle found in
the nucleus of all atoms
• Neutron
– Uncharged subatomic particle found in the
atomic nucleus
Different Elements: Different
Types of Atoms
• Element
– A pure substance that consists only of atoms with
the same number of protons
– The element oxygen contains only oxygen atoms etc.
• Atomic number
– Number of protons in the atomic nucleus
– Determines the element
Elements in Living Things
• The proportions of different elements
differ between living and nonliving things
• Some atoms, such as carbon, are found in
greater proportions in molecules made
only by living things – the molecules of
life
Why Electrons Matter
• Electrons travel around the nucleus in
different orbitals (shells) – atoms with
vacancies in their outer shells tend to
interact with other atoms
– Atoms get rid of vacancies by gaining or
losing electrons, or sharing electrons with
other atoms
• Shell model
– Model of electron distribution in an atom
Shell Models
Fig. 2-3 (top), p. 22
A) The first shell corresponds to
the first energy level, and it can
hold up to 2 electrons. Hydrogen
has one proton, so it has one
vacancy. A helium atom has 2
protons, and no vacancies. The
number of protons in each shell
model is shown.
B) The second shell corresponds
to the second energy level, and it
can hold up to 8 electrons. Carbon
has 6 protons, so its first shell is
full. Its second shell has 4
electrons, and four vacancies.
Oxygen has 8 protons and two
vacancies. Neon has 10 protons
and no vacancies.
Shell Models
first shell
6
second shell
carbon (C)
11
C) The third shell, which
corresponds to the third energy
level, can hold up to 8 electrons,
for a total of 18. A sodium atom
has 11 protons, so its first two
shells are full; the third shell has
one electron. Thus, sodium has
seven vacancies. Chlorine has 17
protons and one vacancy. Argon
has 18 protons and no vacancies.
third shell
2
1
1 proton
1 electron
hydrogen (H)
sodium (Na)
helium (He)
8
oxygen (O)
17
chlorine (Cl)
10
neon (Ne)
18
argon (Ar)
Fig. 2-3 (a-c), p. 22
Ions
• The negative charge of an electron balances
the positive charge of a proton in the
nucleus
• Changing the number of electrons may fill its
outer shell, but changes the charge of the
atom
• Ion
– Atom that carries a charge because it has an
unequal number of protons and electrons
Ion Formation
electron
gain
17
Chlorine
atom
17p+
17e–
charge: 0
Chloride
ion
17
electron
loss
11
11
17p+
18e–
charge: –1
Sodium
atom
11p+
11e–
charge: 0
Sodium
ion
11p+
10e–
charge: +1
Fig. 2-4, p. 23
From Atoms to Molecules
• Atoms can also fill their vacancies by
sharing electrons with other atoms
• A chemical bond forms when the
electrons of two atoms interact
• Chemical bond
– An attractive force that arises between two
atoms when their electrons interact
From Atoms to Molecules
• Molecule
– Group of two or more atoms joined by chemical
bonds
• Compound
– Type of molecule that has atoms of more than one
element
• All compounds are molecules; but not all
molecules are compounds!
Referring to a Molecule
Ionic Bonds and Covalent Bonds
• Depending on the atoms, a chemical bond
may be ionic or covalent
• Ionic bond
– A strong mutual attraction formed between ions
of opposite charge
• Covalent bond
– Two atoms sharing a pair of electrons
An Ionic Bond: Sodium Chloride
Covalent Bonds
• Molecular hydrogen (H—H) and
molecular oxygen (O=O)
Hydrogen Bonds
• Hydrogen bond
– Attraction that forms between a covalently
bonded hydrogen atom and another atom
taking part in a separate covalent bond
Importance of Hydrogen Bonds
• Hydrogen bonds form and break more
easily than covalent or ionic bonds – they
do not form molecules
• Hydrogen bonds impart unique
properties to substances such as water,
and hold molecules such as DNA in their
characteristic shapes