Inside the atom

We are told the atomic number = 9 and the mass number = 19. The atomic number tells us the number of protons, and the mass number is the total of protons and neutrons.

Use the formula: $\text{Number of neutrons}=\text{Mass number}-\text{Atomic number}$ Substituting in: $\text{Neutrons}=19-9=10$

A fluorine atom contains 10 neutrons. Remember, the mass number counts both protons and neutrons, so subtracting the protons leaves us with the neutrons.

We need to find the number of neutrons in fluorine, given atomic number = 9 and mass number = 19.

9 is the number of protons (the atomic number), not the number of neutrons. Confusing protons with neutrons is a common mistake.

19 is the mass number — the total of protons and neutrons together. It cannot be the number of neutrons alone.

28 comes from adding the atomic number and mass number ($9+19=28$), which has no chemical meaning. This is not a valid operation.

The correct answer is 10, calculated as $19-9=10$. This uses the formula: neutrons = mass number − atomic number.

J.J. Thomson discovered the electron in 1897. He knew atoms were electrically neutral overall, so if they contained negative electrons, there must also be positive charge to balance them.

In the plum pudding analogy, the sponge (pudding) represents the spread-out positive charge, and the plums represent the electrons dotted throughout it.

The 'plums' represented electrons scattered throughout a positive charge. Thomson had not yet discovered protons or neutrons — those came later.

The question asks what the 'plums' in Thomson's plum pudding model represent.

This is backwards — Thomson's model had a positive background, not a negative one. Also, Thomson had not discovered protons at this stage.

Neutrons were not discovered until 1932, long after Thomson's model. They play no part in the plum pudding model.

This describes Rutherford's later nuclear model, not Thomson's plum pudding. In Thomson's model, there was no concentrated nucleus.

The 'plums' represented electrons scattered throughout a positive charge — negative particles dotted inside a ball of positive charge, just like plums inside a sponge pudding.