BioGT Ch2: Elements, Atoms, and Compounds

2.1

Organisms are composed of elements, in combinations called compounds

• Matter

  • Anything that occupies space and has mass

  • Composed of chemical elements

• Element: a substance that cannot be broken down into other substances by ordinary chemical means

  • Organized on the periodic table

  • Ex. Gold, copper, oxygen, hydrogen, carbon, helium

• Compound: Substance of two or more elements in a fixed ratio

  • Ex. Water is a compound made of two hydrogen atoms and one oxygen atoms

• Compounds found in living organisms contain 3-4 elements

• Humans need 25 different elements. Plants only need 17.

• Carbon, hydrogen, oxygen, and nitrogen make up 96% of all living things

• Trace elements

  • Very scarce in the human body, but still very necessary for survival.

2.2

Trace elements are often hard for humans to get, so they are common additives to food and water

• Elements such as iodine, florine, and iron are commonly added.

2.3

Atoms consist of protons, neutrons, and electrons

• Atom

  • From the Greek for “indivisible”. Smallest unit of matter that retains the properties of an element

  • Made up of subatomic particles

• Protons

  • Single positive electric charge.

  • Found in nucleus

• Neutrons

  • No charge

  • Found in nucleus

• Electrons

  • Single negative charge

  • Moves around nucleus

• Nucleus

  • Made of tightly packed neutrons and protons

  • Electrons are attracted to protons, and move around outside the nucleus, creating an atom

• Atomic number + mass number

  • Atomic number: Unique number of protons for each element

  • Mass number: sum of total neutrons and protons.

  • Atomic mass is based only on protons and neutrons

    • Protons and neutrons each have the mass of one dalton

    • Electrons have a two thousandth of that mass

• Isotopes

  • Different isotopes have the same number of protons and therefore behave the same in reactions, but have different numbers of neutrons

  • Some isotopes are radioactive as opposed to stable, have spontaneously decaying nuclei, and can be a risk to living organisms.

2.4

Radioactive Isotopes

• living cells cannot tell isotopes apart

• Radioactive isotopes are used as “trackers” in living organisms

• Some have been used to study photosynthesis.

• Radioactive isotopes have been used for medical diagnosis and treatment, although radiation is known to harm living things

2.5

Chemical bonds

Electrons can be located in different electron shells with differing distances from the nucleus

• An orbital can only hold a pair of electrons.

  • One orbital on the 1st shell, four on the 2nd and 3rd.

• Valence shell

  • The outermost electron shell

  • Atoms without full valence shells tend to interact with other atoms to fill it.

  • Elements with full valence shells are unreactive with each other.

  • Elements can share, donate, or receive electrons. These are known as chemical bonds.

2.6

Covalent bonds join atoms into molecules through electron sharing

• Covalent bond: when two atoms share a pair of electrons

• Bonding capacity depends on the needed amount of electrons to fill valence shell

• Attraction for shared elements is electromagnetivity

• Types of covalent bonds

  • Nonpolar: bond between atoms with equally shared electrons

  • Polar: Bond between atoms with unequally shared electrons

2.7

Ionic bonds are attractions between ions of opposite charge.

• Ion: an atom or molecule with a positive or negative electrical charge resulting from a gain or loss of electrons

• Atoms can a steal electrons from each other if the electromagnetivity is unbalanced enough.

2.8

Hydrogen bonds are weak and therefore important in the chemistry of life

• Weak bonds create polar molecules

• The partially positive side of a water molecule can interact with nearby atoms that have a negative charge.

• Chemical reactions re-arrange matter.

  • Atoms re-arranging by making hydrogen bonds and breaking them is essential to the life of all organisms.