AP Biology 1.2 Elements of Life Study Notes - New Syllabus Effective 2025
AP Biology 1.2 Elements of Life Study Notes- New syllabus
AP Biology 1.2 Elements of Life Study Notes – AP Biology – per latest AP Biology Syllabus.
LEARNING OBJECTIVE
Describe the composition of macromolecules required by living organisms.
Key Concepts:
- Elements of Life
1.2.A.1 : Atoms, Molecules, Ions, and Bonds
An atom consists of a nucleus of positively charged protons and neutrally charged neutrons. Negatively charged elec trons are arranged outside the nucleus. Molecules are groups of two or more atoms held together by chemical bonds. Chemical bonds between atoms form because of the interaction of their electrons. The electronegativity of an atom, or the ability of an atom to attract electrons, plays a large part in determining the kind of bond that forms. There are three kinds of bonds, as follows:
1. Ionic bonds form between two atoms when one or more electrons are transferred from one atom to the other. This bond occurs when the electronegativities of the atoms are very different and one atom has a much stronger pull on the electrons (high electronegativity) than the other atom in the bond. The atom that gains electrons has an overall negative charge, and the atom that loses electrons has an overall positive charge. Because of their positive or negative charges, these atoms are ions. The attraction of the positive ion to the negative ion constitutes the ionic bond. Sodium and chlorine form ions (Na+ and Cl–), and the bond formed in a molecule of sodium chloride (NaCl) is an ionic bond.
2. Covalent bonds form when electrons between atoms are shared, which means that neither atom completely retains possession of the electrons (as happens with atoms that form strong ionic bonds). Covalent bonds occur when the electronegativities of the atoms are similar. Nonpolar covalent bonds form when electrons are shared equally. When the two atoms sharing electrons are identical, such as in oxygen gas (O2 ), the electronegativities are identical, and both atoms pull equally on the electrons. Polar covalent bonds form when electrons are shared unequally. Atoms in this kind of bond have electronegativities that are different, and an unequal distribution of the electrons results. The electrons forming the bond are closer to the atom with the greater electronegativity and produce a negative charge, or pole, near that atom. The area around the atom with the weaker pull on the electrons produces a positive pole. In a molecule of water (H2 O), for example, electrons are shared between the oxygen atom and each hydrogen atom. Oxygen, with a greater electronegativity, exerts a stronger pull on the shared electrons than does each hydrogen atom. This unequal distribution of electrons creates a negative pole near the oxygen atom and positive poles near each hydrogen atom. Single covalent, double covalent, and triple covalent bonds form when two, four, and six electrons are shared, respectively.
3. Hydrogen bonds are weak bonds between molecules. They form when a positively charged hydrogen atom in one covalently bonded molecule is attracted to a negatively charged area of another covalently bonded molecule. In water, the positive pole around a hydrogen atom forms a hydrogen bond to the negative pole around the oxygen atom of another water molecule.
1.2.A.2 : Subatomic Particles
- Protons
■ Packed with neutrons in nucleus
■ Positively charged
■ Most atoms have same amount of protons as electrons, making them electrically neutral - Neutrons
■ Packed with protons in nucleus
■ No charge
■ Isotopes
● Same element with different amount of neutrons in nucleus
● Vary in mass
● Radioactive isotopes decay spontaneously, giving off particles and energy - Electrons
■ Negatively charged
■ Spin around nucleus
■ Very small; effectively massless
■ Electrons on an atom differ in their amounts of potential energy
■ Electron’s state of potential energy is called its energy level, or electron shell
■ Valence electrons are those in the outermost shell, or valence shell
■ Chemical behavior of an atom is mostly determined by the distribution of electrons in electron shells
● Valence shell most important
● Elements with full valence shells are chemically inert
● Atoms with incomplete valence shells can share or transfer valence electrons with certain other atoms - Atoms of different various elements differ in number of subatomic particles
- $\text{Atomic number=protons in nucleus}$
- $\text{Mass Number= protons+neutrons}$
■ Average of all isotopes - Atomic mass+atom’s weighted average total mass
1.2.A.3 : Differences of electronegativities
The left end represents bonds that form when no differences exist in the electronegativities of the atoms. Electrons are shared equally, and nonpolar bonds form. The right end represents bonds that form when very large differences in electronegativities exist. Electrons are transferred from one atom to another, and ionic bonds form. When the electronegativities of the atoms are different, but not strongly so, the electrons are shared unequally, and polar covalent bonds form. The kind of bond that forms between two atoms and the strength of that bond de pend upon the difference of electronegativities of the atoms and might occur any place along the line.
