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Structure of the Atom

Every atom has a dense central nucleus containing protons and neutrons, surrounded by electrons arranged in fixed energy shells. Protons carry a relative charge of +1 and neutrons are neutral, so the nucleus is positively charged. Almost all of the atom’s mass sits in the nucleus.

Misconception

Electrons are not inside the nucleus. They occupy shells around it, and it is the shell arrangement that determines how an element reacts.

Exam-style questions

State where almost all of an atom’s mass is found.

Answer: In the nucleus, because protons and neutrons each have a relative mass of 1.
Explain why an atom has no overall charge.

Answer: The number of protons equals the number of electrons, so the charges cancel.

Atomic Number and Mass Number

The atomic number is the number of protons in the nucleus and it defines the element — change it and you have a different element. The mass number is the total number of protons and neutrons. Subtracting one from the other gives the neutron count.

Examiner Insight

Marks are lost for answering “mass number” when the question asks for atomic number. Always tie atomic number back to the proton count.

Question

What determines the atomic number of an element?

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Answer

The number of protons in the nucleus.

Card 3 of 12

Question 3 of 10

Which set of elements is common to all four classes of biological macromolecules?

A Hydrogen, oxygen, sulfur
B Carbon, oxygen, phosphorus
C Carbon, hydrogen, nitrogen
D Carbon, hydrogen, oxygen

Why A is wrong Sulfur is in proteins only, not in all four classes.

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Your answer

“The process of transcription occurs in the nucleus. RNA polymerase binds to the promoter region of the DNA and unwinds the double helix. It then reads the template strand and adds complementary RNA nucleotides to form mRNA, but it also translates them into proteins immediately.”

Mark scheme evaluation

Occurs in the nucleus [1]
RNA polymerase binds to promoter and unwinds DNA [1]
Adds complementary RNA nucleotides to form mRNA [1]
Translation into proteins does not occur immediately during transcription; translation happens later at the ribosome. [0]

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