1122 WORDS


Define element, atom, compound, molecule, and trace element.

ELEMENT: a substance that cannot be broken down to other substances by chemical reactions.

COMPOUND: a substance consisting of two or more elements combined in a fixed ratio.

MOLECULE: two or more atoms held together by covalent bonds

TRACE ELEMENTS: those required by an organism in only minute quantities.

Give the mass, charge, and location of each sub-atomic particle.


Mass= 1 Dalton or 1.009 grams or 1.7 x 10-24

Charge= neutral

Location=Center of the atom


Mass= 1 Dalton or 1.007 grams or 1.7 x 10-24

Charge= positive

Location= Center of the atom


Mass= we ignore the mass of an electron


Location=moving nearly at the speed of light, electrons form a cloud around the nucleus.

Define and use atomic number and atomic mass number.

ATOMIC NUMBER: the number of protons in an element. Is written as a subscript to the left of the symbol for the element.

ATOMIC MASS NUMBER: the sum of protons plus neutrons in the nucleus of an atom. The mass number is written as a superscript to the left of an element’s symbol.

Explain why isotopes have similar chemical and therefore biological properties.

How are isotopes used by biologists?

Radioactive isotopes in particular have many useful applications in biology. Radioactive isotopes date fossils, are used as tracers to follow atoms through metabolism, and are helpful in diagnosing kidney disorders.

Describe the relative energy of electron shells.

FIRST ENERGY LEVEL (shell): closest to the nucleus and has the lowest energy.

SECOND ENERGY LEVEL (shell): more energy....

THIRD ENERGY LEVEL (shell): more energy

And so on...

Describe the placement of electrons into the 1s, 2s, and 2p orbitals?

No more than two electrons can occupy the same orbital.

1s-The first energy shell has a single orbital and can thereby accommodate a maximum of two electrons. This orbital is spherical in shape. The lone electron of a hydrogen atom occupies the 1s orbital, as do the two electrons of a helium atom. *Electrons like all matter tend to exist in the lowest available state of potential energy.

2s-The second electron shell can hold eight electrons, two in each of the four orbitals. Electrons in the four different orbitals all have nearly the same energy, but they move in different volumes of space. The 2s orbital is spherical in shape, but of a greater diameter than the 1s orbital.

2p-The other three orbitals, are dumbbell-shaped, each oriented at right angles to the other two. At higher energy levels these orbitals are referred to as 3s, 3p, and so on. **see fig. 2.8 page 27

What is valence?
***Valence electrons-the chemical properties of an atom depend mostly on the number of electrons in its outermost shell; these outermost electrons are referred to as valence electrons, and the outermost shell as the valence shell.

Each atom that can share valence electrons has a bonding capacity corresponding to the number of covalent bonds the atom can form. When the bonds form, the give the atom a full complement of valence electrons. This bonding capacity is called the atom’s valence and equals the number of unpaired electrons in the atom’s outermost (valence) shell.
**Since electrons tend to exist in their lowest state of potential energy multiple electron shells exist. We can readily determine the valences of many elements from their electron configurations. The valence of hydrogen is 1; oxygen , 2; nitrogen, 3; and carbon, 4. Biologically important molecules generally have a valance of 5, forming three single bonds and one double bond. Of the first 18 elements, only helium, argon, and neon have atoms with full valence shells; these elements are called inert because they are unreactive. **see fig. 2.9 page 28

List the 6 main covalently bonding atoms and 5 main ions. Give each its appropriate symbol - with valence or ionic indicated.

C-Carbon, valence.

H-Hydrogen, valence.

N-Nitrogen, valence.

O-Oxygen, valence.

P-Phosphorous, valence.

S-Sulfur, valence.

Define ion, cation, and anion.
ion- an atom that has gained or lost electrons, thus acquiring a charge.
cation- an ion with a positive charge, produced by the loss of one or more electrons.
anion- a negatively charged ion.

What determines the number of covalent bonds that form?

The number of electrons required to complete required to complete an atom’s valence shell determines how many bonds that atom will form. If two unattached hydrogen atoms meet, they can form a single covalent bond. **see fig. 2.10 page 29

Redraw these molecular formulas as

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