Final answer:
Lewis structures depict the arrangement of atoms within molecules, and the molecular geometry describes the shape around each carbon atom. Methane has a tetrahedral geometry, ethene has a planar trigonal geometry, propane's carbons are tetrahedral, and butyne's carbons vary between linear and tetrahedral.
Step-by-step explanation:
Writing Lewis structures and describing the molecular geometry at each carbon atom for the given compounds involves visualizing how the atoms are bonded and their three-dimensional arrangement. Here's a brief description for each compound:
Methane (CH4) - The Lewis structure consists of a central carbon atom with four single bonds to hydrogen atoms. The molecular geometry is tetrahedral, with a bond angle of approximately 109.5 degrees. Carbon is sp3-hybridized.
Ethene (C2H4) - The Lewis structure shows a double bond between the two carbon atoms and single bonds to the hydrogen atoms. The geometry around each carbon is planar trigonal, with bond angles of approximately 120 degrees. Carbon is sp2-hybridized.
Propane (C3H8) - The Lewis structure has a chain of three carbon atoms with single bonds and hydrogen atoms filling the valencies. Each carbon atom has a tetrahedral geometry similar to methane, and again, carbon is sp3-hybridized.
Butyne (C4H6) - Depending on whether it's 1-butyne or 2-butyne, you'll have a triple bond starting at the first or the second carbon respectively. The geometry around the carbons with a triple bond is linear, with a bond angle of approximately 180 degrees (indicating sp hybridization), while the rest of the carbons are tetrahedral (sp3 hybridized).
Describing these molecules based on their Lewis structures allows us to predict their physical properties and reactivity.