The periodic table
2 marks · EasyExplain why the number of electrons in an atom must always equal the number of protons.
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Atoms are electrically neutral; protons have a positive charge and electrons have a negative charge, so their numbers must be equal to balance the charges.
Also accept: To balance the positive charge of protons with the negative charge of electrons, making the atom neutral.
Common mistakes: Because that's how atoms are made; Not mentioning charge or balance
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Marking points
M1 · 1 mark · Mentions that atoms are electrically neutral.
A1 · 1 mark · States that protons and electrons have equal but opposite charges, so their numbers must be equal to balance the total charge.
Worked solution
Atoms are electrically neutral, which means their total charge is zero.
Protons are positively charged and electrons are negatively charged.
Their numbers must be equal in an atom so that their charges cancel each other out.