BIO-1801 Lecture Notes - Lecture 2: Atomic Mass, Atomic Number, Chemical Element

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9 Sep 2015
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Biology
Based on the principles of chemistry and physics
All living organisms are a collection of atoms and molecules
All life forms composed of matter
Anything that contains mass and occupies space
Atoms
Smallest functional units of matter that form all chemical substances
Cannot be further broken down into other substances by ordinary chemical or
physical means
Each specific type of atom is a chemical element
Three subatomic particles
Protons – positive, found in nucleus, same number as electrons
Neutrons – neutral, found in nucleus, number can vary
Electrons – negative, found in orbitals, same number as protons
Entire atom has no electric charge
Electrons Occupy Orbitals
Scientists initially visualized an atom as a mini solar system
This is an oversimplified but convenient image
Electrons travel within regions surrounding the nucleus (orbitals) in which the
probability is high of finding that electron
Can be depicted as a cloud
Orbitals
S orbitals are spherical
P orbitals are propeller or dumbbell shaped
Each orbital can hold only two electrons
An atom with more than two electrons has more than one orbital
Atoms with progressively more electrons have orbitals within electron shells that
are at greater and greater distances from the center of the nucleus
First shell – one spherical orbital (1s) – holds two electrons
Second shell – one spherical orbital (2s) and three dumbbell-shaped
orbitals (2p) – can hold four pairs of electrons
Nitrogen Example
A nitrogen atom has seven protons and seven electrons
Two electrons fill first shell
Five electrons in second shell
Two fills 2s orbital
One each in the 3p orbitals
Outer second shell is not full
Electrons in the outer shell that are available to combine with other atoms
are called the valence electrons
Protons
Number of protons distinguishes one element from another
Atomic number
1
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Also equal to the number of electrons in the atom so that the net charge is
zero
Periodic Table
Organized by atomic number
Rows correspond to number of electron shells
Columns, from left to right, indicate the numbers of electrons in the outer shell
Similarities of elements within a column occur because they have the same
number of electrons in their outer shells, and therefore they have similar
chemical bonding properties
Atomic Mass
Protons and neutrons are nearly equal in mass, and both are more than 1,800
times the mass of an electron
Atomic mass scale indicates an atom’s mass relative to the mass of other atoms
Most common form of carbon has six protons and six neutrons, is assigned an
atomic mass of exactly twelve
A hydrogen atom has an atomic mass of one, indicating that it has 1/12
the mass of a carbon atom
A magnesium atom, with an atomic mass of twenty-four, has twice the
mass of a carbon atom
Mass or Weight?
Weight is derived from the gravitational pull on a given mass
A man weighs 154 pounds on Earth
On the moon he weighs about twenty-five pounds
On a neutron stars surface, he would weigh twenty-one trillion pounds
His mass is the same in all locations
Units
Dalton
Unit of measurement for atomic mass
Also known as atomic mass unit (amu)
One Dalton (Da) equals 1/12 the mass of a carbon atom
Carbon has an atomic mass of twelve Daltons
Mole
One mole of any element contains the same number of atoms –
6.022×1023
Avogadro’s number
Isotopes
Multiple forms of an element that differ in the number of neutrons
12C contains 6 protons and 6 neutrons
14C contains 6 protons and 8 neutrons
Atomic masses are averages of the weights of different isotopes of an element
Hydrogen, Oxygen, Carbon, and Nitrogen
Typically make up about 95% of the atoms in living organisms
Hydrogen and oxygen occur primarily in water
Nitrogen is found in proteins
2
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