Chemistry By Example

By Tim Prichard

Chemistry by Example is the second book in a GCSE revision guide series. Along with Physics by Example it is a proven successful way of consolidating and reaffirming scientific facts and concepts for students who are studying GCSE Chemistry. My books are aimed at the AQA courses in particular, but the areas covered cover all other exam boards as well.

My guides are unique in that they are based upon using example questions of which each book has over 200 questions, and they are broken down into specific topic areas. Students will find around 10 example questions for every topic, with the questions being varied in style. Importantly the answers are included and broken down, step by step so they can be easily followed and understood. At the end of each chapter there is a self-testing section with space provided for workings and then followed by the answers.

• Language: English
• Publisher: Brown Dog Books
• Release date: Mar 1, 2021
• ISBN: 9781839522628

Book preview

ATOMIC STRUCTURE

The Atom

The ancient Greeks were the first people to put forward the theory of atoms as particles which everything is made from. This was around 2500 BC by the philosopher Leucippus and his understudy called Democritus. Atoms were believed to be small spherical balls which could not be broken down any further. The word ‘atom’ comes from the ancient Greek ‘Atomos’, which means ‘uncuttable’, suggesting atoms can’t be broken down any further. This idea did not progress any further until about 200 years ago by an English scientist called John Dalton. In the early 1800s he published work which suggests that all matter was made up of tiny particles which he also called atoms. He did, however, suggest there were many different types of atom which could join up with other types of atoms to make new materials called compounds. These joined-up atoms were also called molecules.

The model of the atom was then refined by J.J. Thomson in the late 1800s when he described the atom as a sphere of positive charge with negative charges dotted inside this cloud of charge randomly like the dried fruit in a plum pudding. This gave rise to the ‘Plum Pudding’ model. (Note a plum pudding is a Christmas pudding!)

Later in 1909 the nuclear structure of the model of the atoms was described by Geiger, Müller, Rutherford and Marsden who showed the atom had a tiny centre called a nucleus which contained positive charge and was nearly where all the atoms’ mass was concentrated. This is referred to as the ‘Nuclear Model’ of the atom.

The last piece of the structure was the discovery of the neutron in the nucleus by Chadwick in 1932. The structure is now complete, having changed little since the discovery of the neutron.

Question 1

Using a diagram, describe J.J. Thomson’s Plum Pudding model of the atom.

Answer

Sphere of positive charge with negative charges randomly distributed inside.

J.J. Thomson discovered the electron and found that they exist in all matter. He thought the negative charges, which he called electrons, were randomly distributed throughout the atom just like dried fruit in a plum pudding. He also recognised that atoms had no overall charge so the number of positive and negative charges must be equal as they cancel each other out.

Question 2

Why was the famous Rutherford and Marsden gold leaf experiment so significant?

Answer

Put simply, this experiment proved the existence of the ‘nucleus’ of an atom.

Rutherford and Marsden fired alpha particles at gold leaf. Gold leaf can be made very thin to almost just a few atoms thick. Because the Plum Pudding model was the current model at that time, Rutherford and Marsden expected the alpha particles to pass straight through the atoms of gold leaf. They used a detector similar to Geiger and Müller to detect alpha particles at all angles emerging from the gold leaf.

To their astonishment, the alpha particles did not just pass straight through the gold leaf – some were diverted off at large angles and some were ‘bounced’ straight back towards the source! This led to the theory that at the centre of an atom is a densely packed area called a nucleus containing lots of positive charges which we call ‘protons’.

The positive alpha particles were either repelled by the positive charges at angles or repelled straight back since like charges repel and unlike charges attract. This tiny space called a nucleus was thought to contain most of the mass in an atom.

Question 3

Draw a timeline for the evolution of the modern-day nuclear model of the atom.

Answer

Question 4

Draw and label the currently accepted nuclear model of the atoms.

Answer

The nuclear model atom structure

The current nuclear model has a nucleus at the centre of the atom containing protons (positively charged particles) and neutrons (particles with no charge). Surrounding this nucleus are electrons in orbits. These orbits are not random; they are arranged in layers similar to the layers in an onion.

The number of protons determines the type of atom. The number of protons is called the atomic number.

Scientists have found that the mass of a neutron is the same as the mass of a proton. Put this another way: The relative mass of a neutron compared to a proton is 1.

Because electrons have almost no mass we consider the mass of an atom to be concentrated all in the nucleus. The number of protons added to the number of neutrons is called the mass number.

Because the charge of an atom is always zero it suggests the number of positive charges (protons) is equal to the number of negative charges (electrons). This means the numbers of protons and electrons in an atom are equal. This is true, we have an atom not an ion, which we will consider later.

Students must be familiar with the following table for questions on atomic structure:

Note: An electron actually has a mass of 1/2000 of a proton. This is so small we can say it is negligible or zero. (It does, however, have a tiny mass.)

Question 5

Consider the atom below. If the black dots represent protons, white dots neutrons and ×’s are electrons, answer the following questions and then use a periodic table to identify the atom.

i. How many protons are there?

ii. How many electrons were there?

iii. How many neutrons are there?

iv. What is the atomic number?

v. What is the atomic mass?

Answer

i. Number of protons = 6

ii. Number of electrons = 6

iii. Number of neutrons = 6

iv. Atomic number = 6

v. Atomic mass = 12

This atom is carbon because it has 6 protons (see periodic table).

Question 6

Describe in as much detail as you can the atom below. The black dots are protons. Then use a periodic table to identify this atom.

Answer

This atom has 3 protons and therefore 3 electrons.

The electrons are orbiting in 2 shells with 2 electrons in the first shell and 1 in the second shell. The number of neutrons is 4. The atomic number is 3 so this atom is lithium. The atomic mass is 7, as this is the number of protons and neutrons in the nucleus.

How are the electrons arranged?

Electrons are not randomly floating around the nucleus. They are arranged in levels, orbits or shells. Each orbit corresponds to an ‘energy level’. The energy levels increase in energy the further they are from the nucleus. Electrons are added to the levels starting at the lowest energy level (closest to the nucleus) and working outwards.

The number of electrons in each orbit is specific as well. The first level closest to the nucleus can contain a maximum of 2 electrons. The next shell/orbit can contain up to 8 electrons, and the next also another 8 electrons maximum. This is written as 2, 8, 8 and we refer to it as its ‘electronic structure’.

Further electrons can be added to another shell but at GCSE it is usually only another 2 or 3 more electrons.

Question 7

Sodium has 11 electrons and 11 protons. Draw its electronic structure and state its electronic structure.

Answer

Electronic structure = 2, 8, 1

Note: The number of electrons in the outer shell indicates the group the element is in. This example of sodium shows it is in Group 1 as it has 1 electron in its outer shell.

Hint: The outer electron and group idea only works in the main groups 1–8, not transition elements

Question 8

Nitrogen has 7 electrons: draw its electronic structure.

Answer

Note: we put 2 electrons in the first inner shell then the other 5 fit in the outer shell which is not full.

Question 9

Calcium has 20 electrons. Draw its electronic structure and write its electronic arrangement.

Answer

Electronic arrangement = 2, 8, 8, 2

Note: the first shell has 2