Something with a charge
Run a comb through dry hair. It lifts small bits of paper.
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Neutral means no overall charge.
- An atom is neutral. Where is the positive charge?
- Thomson: positive charge is spread through a sphere.
- The electrons sit inside it.
- Equal plus and minus make the atom neutral.
The idea
Rubbing moves electrons from one thing to the other. The charged comb then pulls the paper.
Electrons carry negative charge. One electron has a charge of −1.602 × 10⁻¹⁹ C. Here C stands for coulomb, the unit of charge. We call this charge −1.
But an atom is neutral overall. So where is the positive charge? Thomson pictured a sphere of positive charge. A sphere is a ball. Electrons were spread through it. This is the plum pudding model. Think of a bun with raisins. The bun is the positive charge. The raisins are the electrons.
Later work replaced this model. The model was an early try at making the charges balance.
Later, the positive particle in an atom was named the proton. A proton has a charge of +1. It is much heavier than an electron.
A neutral atom has as many protons as electrons. Carbon has 6 protons and 6 electrons. Neon has 10 and 10. The charges cancel. For carbon, 6 × (+1) + 6 × (−1) = 0.