Basic Chemistry

Mini LectureAP Biology

AtomElectron – negative charge

Neutron; no charge

Most atoms are neutral because the number of electrons equals the number of protons.

Element—pure substance that contains only one kind of atom

• Living things are mostly composed of 6 elements:

• Carbon (C) Hydrogen (H) Nitrogen (N)

• Oxygen (O) Phosphorus (P) Sulfur (S)

Basic Chemistry Concepts

• The number of protons identifies an element.• Number of protons = atomic number

• See periodic table handout!• For electrical neutrality, # protons = #

electrons.• Mass number—total number of protons and

neutrons

Figure 2.1 Electron Shells

• Atoms with unfilled outer shells tend to undergo chemical reactions to fill their outer shells.

• They can attain stability by sharing electrons with other atoms (covalent bond) or by losing or gaining electrons (ionic bond)

• The atoms are then bonded together into molecules.

• Octet rule—atoms with at least two electron shells form stable molecules so they have eight electrons in their outermost shells.

Ionic bonds • Ions are charged particle that form when

an atom gains or loses one or more electrons.

• Cations—positively charged ions• Anions—negatively charged ions

• Ionic bonds result from the electrical attraction between ions with opposite charges.

• The resulting molecules are called salts.

Covalent bonds• Covalent bonds form when two atoms

share pairs of electrons.• The atoms attain stability by having full

outer shells.• Each atom contributes one member of the

electron pair.

• Carbon atoms have four electrons in the outer shell—they can form covalent bonds with four other atoms.

Strength and stability—covalent bonds are very strong; it takes a lot of energy to break them.

• Multiple bonds• Single—sharing 1 pair of electrons

• Double—sharing 2 pairs of electrons

• Triple—sharing 3 pairs of electrons

C H

C C

N N

Degree of sharing electrons is not always equal.

• Electronegativity—the attractive force that an atomic nucleus exerts on electrons

• It depends on the number of protons and the distance between the nucleus and electrons.

• If two atoms have similar electronegativities, they share electrons equally, in what is called a nonpolar covalent bond.

• If atoms have different electronegativities, electrons tend to be near the most attractive atom, in what is called a polar covalent bond