Concept 2.1 Atomic Structure is the Basis for Life’s Chemistry
● Living and nonliving matter is composed of atoms.
● Like charges repel; diﬀerent charges attract.
● Most atoms are neutral because the number of electrons equals the number of protons.
● Dalton—mass of one proton or neutron
● (1.7 × 10–24 grams)
● Mass of electrons is so tiny, it is usually ignored.
● Element—pure substance that contains only one kind of
● Living things are mostly composed of 6 elements:
Carbon (C) Hydrogen (H) Nitrogen (N)
Oxygen (O) Phosphorus (P) Sulfur (S)
● The number of protons identifies an element.
● Number of protons = atomic number
● For electrical neutrality, # protons = # electrons.
● Mass number—total number of protons + neutrons
● Behavior of electrons determines whether a chemical bond will form and what shape the
bond will have.
● 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 or by losing or gaining
● 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 shells.
Concept 2.2 Atoms interact and Form Molecules
● A Chemical bond is an attractive force that links atoms together to form molecules.
● There are several kinds of chemical 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
● Ionic attractions are weak, so salts dissolve easily in water,
● 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
● Properties of molecules are inﬂuenced by characteristics of the covalent bonds:
These characteristics include:
● Strength and stability
● Multiple bonds
● Degree of sharing electrons