General Biology 1
Chapter 2 – Chemistry of life
Atoms – the smallest unit of matter that contains properties of an element
3 subatomic particles: neutrons – neutral particles, electrons – negative particles (form a cloud),
protons – positive particles
Protons = neutrons = 1 Dalton
Electron – very light
Atomic number – no. of protons in atom = no. of electrons (uncharged)
Atomic mass – sum of protons and neutrons
Atoms of the same element have same number of protons
Atoms with different number of neutrons are called isotope
Radioactive decay – nucleus decays and gives off protons and energy (E)
Noted: If element loses protons, they became another element Ex/ Na is an isotope
Electron – the only particle involved in chemical reactions between atoms. Electrons in different
shells have different amount of energy.
Shell 1 has lowest E
Shell 3 has higher E than shell 1
Electrons can only move between shells by gaining or losing heat.
Photons – excite electrons and make electrons move to higher shell.
Valence shell – the outermost shells with electrons
st nd rd
1 shell – 2 electrons max. 2 shell – 8 electrons max. 3 shell – 8 electrons max.
Reactivity depends on valence shell – full valence shell: stable and nonreactive; not full: reacts to
fill the shells
Covalent bonds – strong bond
- 2 or more element sharing a pair of electrons