Chap 2 The Chemical Context of Life

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In a helium atom, the nucleus consists of 2 neutrons which have no electrical charge, and 2 protons which are positively charged. Two electrons, which are negatively charged, move around the nucleus.
The atomic number of an element is the number of protons in the nucleus.
The mass number (atomic mass) is the sum of protons plus neutrons.

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Single and double covalent bonds.
Hydrogen (H2). Two hydrogen atoms can form a single bond.
Oxygen (O2). Two oxygen atoms share two pairs of electrons to form a double bond.
Water (H2O). Two hydrogen atoms and one oxygen atom are joined by single covalent bonds to produce a molecule of water.
Methane (CH4). Four hydrogen atoms can satisfy the valence of one carbon atom, forming methane.

  • Q: What kind of bonds form between these hydrogen atoms and carbon?
  • A: Single covalent bond.

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    Covalent Bond.

    In each hydrogen atom, the single electron is held in its orbital by its attraction to the proton in the nucleus.
    When two hydrogen atoms approach each other, the electron of each atom is also attracted to the proton. in the other nucleus.
    The two electrons become shared in a covalent bond, forming an H2 molecule.

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    A sodium chloride crystal (salt). The sodium ions (Na+) and chloride ions (Cl-) are held together by ionic bonds. The formula NaCl indicates that the ratio of Na+ to Cl- is 1:1.

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    The rainforest ant Myrmelachista schumanni nests in stands of Duroia trees called "devil's gardens".
    The ants kill all trees except its host plant by injecting formic acid into competing trees such as Cedrela.
    Continue

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    The ants inject formic acid into trees other than its host Duroia. Competing trees such as Cedrela soon die from the acid unless they were protected by an experimental sticky insect barrier, or were outside the ants' territory.

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    Electron orbitals. Each orbital (volume of space where the electrons of an atom are most likely to be found) holds a maximum of 2 electrons. An electron shell represents the average distance of an electron from the nucleus.

    1. The first electron shell has one spherical (1s) orbital.
    2. The second shell has one larger s orbital (2s) plus three dumbbell–shaped orbitals (2p).
    3. Neon has 10 electrons.

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    A water molecule is polar because oxygen is more electronegative than hydrogen, so there is a partial negative charge on the oxygen and a partial positive charge on the hydrogens. The unequal sharing of electrons results in a polar covalent bonds.

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    Electron–shell diagrams of the first 18 elements in the periodic table. An element is a substance that cannot be broken down to other substances by chemical means. The first electron shell can contain up to 2 electrons; the 2nd and 3rd shells can contain up to 8 electrons.

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    Energy levels of an atom's electrons. Electrons exist only at fixed levels of potential energy, which are also called electron shells. The outermost shell is called the valence shell, and contains valence electrons, and most chemical reactions involve those electrons, since atoms are most stable with complete valence shells. .

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    A hydrogen bond results from the attraction between the partial positive charge on the hydrogen atom of water and the partial negative charge on the nitrogen atom of ammonia.

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