Physical Science · Unit 3: How Atoms Hold Hands

Remember from Unit 1: every atom wants a full outer shell of 8 electrons (or 2 for the smallest ones). That's the whole drive. The whole motivation. Atoms are like people at a party — they want to feel settled. Bonds are how they settle.

Two ways to settle:

Ionic bonds — "Just take it. I don't want it." When a metal meets a nonmetal, the metal usually has 1 or 2 extra electrons it doesn't want. The nonmetal needs 1 or 2 more. So the metal just hands them over. Now the metal has a positive charge (it lost negative electrons) and the nonmetal has a negative charge (it gained them). Opposite charges attract → they stick together. That's an ionic bond.

Salt-shaker friends: Sodium (Na) is a soft, silvery metal that explodes if you drop it in water. Chlorine (Cl) is a poisonous green gas. Sodium says "take this electron, I hate it." Chlorine says "thank you, I needed exactly that." Now you have NaCl — table salt. The thing on your fries. Built from the social anxiety of two atoms that individually would kill you.

Covalent bonds — "Let's share, neither of us has enough." When two nonmetals meet, neither of them wants to give up electrons. They're both hungry. So they share. They overlap their outer shells and both get to count the shared electrons as theirs. That's a covalent bond.

Example: Water (H₂O). Oxygen needs 2 more electrons to fill its outer shell. Each hydrogen needs 1. So two hydrogens each share their one electron with oxygen, and oxygen shares back. Everyone counts a full shell. Water exists.

Now — reactions. A chemical reaction is just atoms breaking old bonds and making new ones. Nothing gets created. Nothing gets destroyed. The same atoms just rearrange themselves into new combinations. This is the Law of Conservation of Mass: whatever atoms you start with, you end with. That's why when you balance an equation, both sides have to match.

The simplest way to draw bonds is with Lewis dot structures — you draw the element symbol and put dots around it for the outer electrons.

Na • → loses 1 electron
• Cl • → gains 1 electron
• • • • •

Na⁺ [ Cl⁻ ] ← ionic bond (NaCl)

Sodium gives, Chlorine takes — held together by opposite charges

Now look at covalent bonding (sharing):

H • • •
O • • H
• •

H — O — H (H₂O — water)

each hydrogen shares 1 electron with oxygen · oxygen shares 1 back · everyone is happy

And here's CO₂ (carbon dioxide), which has DOUBLE bonds — each oxygen shares TWO pairs with the carbon:

O = C = O (CO₂)

each "=" is a double bond · 2 shared pairs of electrons

For reactions, you write what you start with (reactants) on the left, and what you end with (products) on the right. Then you balance so atoms match:

2H₂ + O₂ → 2H₂O

2 hydrogen molecules + 1 oxygen molecule → 2 water molecules · atoms balanced

Count it: left side = 4 H + 2 O. Right side = 4 H + 2 O. Match. Conservation of mass — done.

Balancing a chemical equation always uses the same method. Here's the recipe:

Step 1. Write the equation as-is. Example: H₂ + O₂ → H₂O

Step 2. Count atoms on each side.

LEFT: 2 H, 2 O
RIGHT: 2 H, 1 O

oxygens don't match — that's our problem

Step 3. Pick the atom that's out of balance. You can ONLY add big numbers in front of whole molecules (coefficients). You can NEVER change a subscript — that would change the compound itself.

Step 4. Put a 2 in front of H₂O so we have 2 oxygens on the right:

H₂ + O₂ → 2H₂O
LEFT: 2 H, 2 O
RIGHT: 4 H, 2 O

oxygens balanced ✓ but now hydrogens don't match

Step 5. Put a 2 in front of H₂ on the left to fix hydrogens:

2H₂ + O₂ → 2H₂O
LEFT: 4 H, 2 O
RIGHT: 4 H, 2 O ✓

balanced! done.

The whole game is whack-a-mole. Adjust a number, recount, adjust the next thing. You'll get faster. Promise.

How Atoms Hold Hands: Bonds & Reactions

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tutor bonds, reactions, balancing

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