Electric Charge

Structure of an atom

Atoms are basic building blocks of matter. Every body consists of atoms or molecules.

The structure of an atom is roughly divided into the nucleus, which consists of neutrally-charged neutrons und positively-charged protons and the electron shell, which contains the negatively-charged electrons. According to the Bohr model these electrons move on circular orbits around the nucleus.

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The neutrons are not important for the electric charge of a body and as such not shown in the sketches below.

Electric charge of an atom

The electric charge of an atom is the sum of the positive charges minus the sum of negative charges which is equivalent to the number of protons minus the number of electrons. An atom with more protons than electrons is positively-charged. When an atom has more electrons than protons, it is negatively-charged. An atom that has as many protons as electrons is neutrally-charged.

The following sketch shows a sodium and a chlorine atom. Both are usually neutrally-charged. But during specific chemical processes atoms can receive or give away electrons.

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Charged atoms are ions, positively-charged are called cations and negatively-charged anions.

Cations have less electrons than protons and anions have more electrons than protons.

Electric charge as a physical

The electric charge is a physical property of matter.

The electric charge is always a multiple of the elementary charge \( e \), the smallest freely existing charge. It is defined as:

$$ e = 1,602 \cdot 10^{-19} C $$

$$ Q = N \cdot e \qquad \qquad \mathrm{Einheit:} \qquad \left[ 1   C \right] $$ \( N \) = Number of charges, \( e \) = elementary charge

The electric charge of an electron is \( Q_e = -e \) and the electric charge of an proton is \( Q_p = e \).

The sodium ion from the animation above has the electric charge \( Q_{Na^+} = e = 1,602 \cdot 10^{-19} C \) and the chlorine ion has the charge \( Q_{Cl^-} = -e = -1,602 \cdot 10^{-19} C \).