(a) Describe Thomson's model of the atom. Which subatomic particle was not present in Thomson's model of the atom?
​(b) The mass number of an element is 18. It contains 7 electrons. What is the number of protons and neutrons in it? What is the atomic number of the element?

(a) Thomson's model of an atom is similar to the model of plum pudding or watermelon. He said that the positive charge in an atom is spread all over like the red edible part of the watermelon. Also, electrons are embedded in the positively charged sphere like the seeds in the watermelon.

Explanation of Thomson' model:
(i) An atom consists of a positively charged sphere with electrons embedded in it.
(ii) The negative and positive charges present inside an atom are equal in magnitude. Therefore, an atom as a whole is electrically neutral.

Thomson only explained about electrons and protons, he did not take neutrons into consideration.

(b) Mass number of element = 18
Number of electrons = 7

For an atom, number of electrons = number of protons

Number of protons = number electrons = 7

Number of neutrons = (Mass number)$-$(number of protons)
Number of neutrons = 18$-$7
Number of neutrons = 11

The atomic number of an element is defined as the number of protons present. Therefore, the atomic number of the given element is 7.