Edexcel GCSE Chemistry: Atomic Structure and the Periodic Table

In 1909, Geiger and Marsden fired a beam of positively charged alpha particles at a very thin sheet of gold foil. They observed that most alpha particles passed straight through, some were deflected through small angles, and a very few bounced almost straight back.

Before this experiment, the accepted model was Thomson's "plum pudding" model, in which the atom was a ball of positive charge with electrons embedded in it.

Describe and explain how the results of this experiment led scientists to reject the plum pudding model and replace it with the nuclear model of the atom. (6 marks)

Chlorine exists as two isotopes: chlorine-35 and chlorine-37. A sample of chlorine contains 75 % chlorine-35 and 25 % chlorine-37.

(a) Explain what is meant by the term isotopes. (2 marks)

(b) Calculate the relative atomic mass (Ar) of chlorine in this sample. Give your answer to 1 decimal place. (2 marks)

An atom of an element has the nuclear notation shown below:

mass number = 39, atomic number = 19, symbol K

(a) State the number of protons, neutrons and electrons in one neutral atom of this element. (2 marks)

(b) Give the electronic configuration of this atom and use it to state which group of the periodic table the element is in. (1 mark)

Dmitri Mendeleev published an early version of the periodic table in 1869. He arranged the known elements in order of atomic weight (relative atomic mass), but he also left gaps in his table and in a few places changed the order of elements.

(a) Explain why Mendeleev left gaps in his table. (1 mark)

(b) Modern periodic tables arrange elements in order of atomic (proton) number, not atomic weight. Explain why this order correctly places elements that Mendeleev had to swap. (2 marks)

A magnesium atom can lose electrons to form a magnesium ion, $Mg^{2+}$Mg2+.

A magnesium atom has the electronic configuration 2, 8, 2.

Explain, in terms of electrons, how a magnesium atom forms a $Mg^{2+}$Mg2+ ion, and give the electronic configuration of the ion. (2 marks)

The neutron was not discovered until 1932.

Name the scientist who discovered the neutron. (1 mark)