Chapter 2 - Chemical Basis of Life

Matter occupies space and has mass.
Elements are substances that cannot be broken down into simpler substances by ordinary chemical reactions.
Total number of elements is $109$ , consisting of $92$ natural and $17$ "artificial" elements.
Atoms are the building blocks of elements.

Subatomic particles:
- Neutron: no electrical charge, mass of $1\,\text{amu}$ , located within the nucleus.
- Proton: $+1$ electrical charge, mass of $1\,\text{amu}$ , located within the nucleus.
- Electron: $-1$ electrical charge, mass of $0\,\text{amu}$ .
Atoms are electrically neutral, where the number of protons equals the number of electrons.
Atomic number ( $Z$ ) equals the number of protons (which also equals the number of electrons).
Mass number equals the number of protons plus the number of neutrons.
Isotopes are atoms with the same number of protons and electrons but different numbers of neutrons (e.g., ${}^1\text{H}$ , ${}^2\text{H}$ , ${}^3\text{H}$ ).
Radioactive isotopes feature unstable nuclei that undergo decay; they are utilized in clinical imaging techniques and ${}^{14}\text{C}$ dating of biological materials.

Molecules are formed by $2$ or more atoms joined together by chemical bonds.
Chemical bonds occur when atoms transfer or share electrons in their outermost shell.
Ionic bonds: attraction between oppositely charged ions (e.g., $\text{NaCl}$ ).
- Ions: atoms that have gained or lost an electron.
- Cation: loses electron, becomes positively charged, formed by metals.
- Anion: gains electron, becomes negatively charged, formed by nonmetals.
Covalent bonds: sharing of electron pairs between atoms in a molecule.
- Nonpolar covalent: equal sharing of electrons (e.g., $\text{H}_2$ , $\text{O}_2$ , $\text{N}_2$ ).
- Polar covalent: unequal sharing of electrons, resulting in positive and negative regions on the molecule (e.g., $\text{H}_2\text{O}$ ).