L19. Molecules and Lewis Models

Chemical Names and Formulas

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L19. Molecules and Lewis Models

How do dots and lines on paper show how atoms share electrons to make real molecules like water and carbon dioxide?

A quick observation to explain

Light a small candle (with an adult) and hold a cool metal spoon above the flame. You will feel heat and see steam if water is present. That steam is many H2O molecules moving around. At the particle level, forming a water molecule means hydrogen and oxygen atoms arrange their outer electrons so each atom 'feels' a full outer shell. Lewis models are a simple picture students use to show where those outer (valence) electrons are and which ones are shared as bonds. You already know how to write chemical formulas; Lewis models add a particle-level view that shows the electrons making each bond. This helps explain why some atoms form single bonds, why others form double bonds, and why molecules like CO2 and H2O look different even when both are made from oxygen and carbon or oxygen and hydrogen.

Drawing Lewis electron-dot structures (step-by-step)

A Lewis model uses element symbols and dots for valence electrons. Lines often replace shared pairs of dots to show bonds. The goal is to show how atoms share or keep electrons so most atoms achieve a full valence shell (an "octet" for many second-row elements). Follow these steps to draw a basic Lewis structure:

• 1) Count total valence electrons from all atoms (use periodic table group number for main-group elements). 2) Place the least electronegative atom in the center (often C, never H). 3) Connect atoms with single bonds (one shared pair = one line). 4) Subtract electrons used by bonds and place remaining electrons as lone pairs to complete octets on outer atoms. 5) If the central atom lacks an octet, form double or triple bonds by converting lone pairs to shared pairs. 6) Check total electrons used equals the count from step 1 and enclose charged structures in brackets with a net charge if needed.

Reading and using Lewis models

Once drawn, use a Lewis model to answer simple questions: How many bonds does each atom form? Where are lone pairs? Are multiple bonds present? For example, in O2 each oxygen shares two pairs (a double bond), while in H2O oxygen shares two single pairs and keeps two lone pairs. Lewis models also connect to balanced chemical equations. The formation of water from hydrogen gas and oxygen gas is written and balanced as 2 H2(g) + O2(g) -> 2 H2O(l). A Lewis view shows each H atom bringing one electron and each O atom bringing six valence electrons; when paired, electrons are conserved and bonds form so every atom reaches a stable arrangement. Lewis models have limits: they are two-dimensional sketches and do not fully show 3D shape or bond angles (that comes next in Structure and Properties), but they are powerful for predicting bond counts, lone pairs, and where charges would appear in ions.

Bringing it together

Lewis electron-dot models let you draw where valence electrons sit, which electrons are shared as bonds, and which remain as lone pairs. They make the invisible arrangement of electrons visible so you can predict bond counts and spot multiple bonds or formal charges.

Use the step-by-step drawing method to check that the total valence electrons are conserved and that atoms reach expected valence shells. These sketches prepare you to connect bonding patterns to molecular shape and properties in the next lesson.