Periodicity Covalent Radius

Higher Chemistry St. Ninian’s High School

Higher Chemistry: Unit 1 – Chemical Changes and Structure Part A – Periodicity and Bonding Lesson 2 – Periodicity: Covalent Radius

Learning Outcomes

By the end of this lesson you should know: 1. What is meant by the term covalent radius

2. That there is a pattern in the periodic table in covalent radius as you go along a period and down a group.

3. How to explain this pattern across the periods terms of the positive force of the nucleus and its pull on electrons

4. How to explain this pattern in groups in terms of number of occupied electron shells.

Success Criteria

You will have been successful in this lesson if you: 1. Read and learn the notes given

2. Watch the links provided

3. Complete Exercise 1.2 and check your answers.

There is also a further reading section to help you gain more depth of understanding for this section. MS Teams will be monitored throughout the week by a chemistry teacher. If you need help or clarification with either the task or the content of the lesson, just ask.

Links to Prior Knowledge

You may wish to revise the following to help you understand this lesson: - National 5 chemistry – atomic structure, protons, electrons and electron arrangement

CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 1

Higher Chemistry St. Ninian’s High School

For reference, the periodic table is given in the data booklet. Download or print a copy of the Higher Chemistry Data Booklet from MS Teams or from the SQA website - https://www.sqa.org.uk/sqa/files_ccc/ChemistryDataBooklet_NewH_AH-Sep2016.pdf

Notes

Structure of the Atom

The structure of an atom consists of a dense nucleus, containing protons which carry a positive charge and neutrons which carry no charge. Negatively charged electrons orbit the nucleus in what are referred to as “energy levels” or “electron shells”.

The electrons remain “attached” to the atom because of the attraction between the positive nucleus and the negative electrons.

WATCH - TWIG: Atomic Structure: http://bit.ly/2ubTE8P

Covalent Radius

The covalent radius is a measure of the size of the atom. The atomic size is very difficult to measure because electrons are practically invisible. However an X-ray technique can measure the distance between the nuclei of covalently bonded atoms – this distance is called the bond length.

The covalent radius of an element is half the distance between the nuclei of two of its covalently bonded atoms. Covalent Radii are listed on page 7 of the Higher data booklet (download a copy from the SQA website).

(Atoms are extremely small, the unit of measurement for atomic size is pm, picometre = 10-12 metres)

CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 2

Higher Chemistry St. Ninian’s High School

Look at the table above. Can you see a general pattern in the covalent radius: - Going along a period? (left to right) - Going down a group? (top to bottom) Take a moment to think about why these patterns occur…

Patterns in Covalent Radius in Periods If you go along a period, you may be surprised to notice that covalent radius decreases, meaning atoms actually get smaller. To explain this, we need to understand a concept called “nuclear charge”. Nuclear charge is the positive pull of the nucleus and is proportional to the number of protons in an atom. In other words, an atom of beryllium, with 4 protons, has a greater nuclear charge than an atom of lithium, with 3 protons. A greater nuclear charge results in a stronger pull on the electrons, bringing them closer to the nucleus. This results in the size of the atom decreasing.

Patterns in Covalent Radius in Groups If you look down any group in the periodic table, you will notice that the covalent radius is increasing, meaning atoms are getting bigger. This can be explained by looking at the electron arrangement of an atom. Take group 1 for example, the electron arrangement for lithium is 2,1 – meaning two occupied electron shells, whereas sodium has an electron arrangement of 2,8,1 – meaning 3 occupied electron shells. Unsurprisingly, sodium is a larger atoms.

CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 3

Higher Chemistry St. Ninian’s High School

Electron arrangements can be found on page 8 of the Higher data booklet.

The graph below shows the covalent radius for the first 25 elements. Look at the pattern as you increase atomic number (number of protons) and as you move to a new period (more occupied electron shells).

CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 4

Higher Chemistry St. Ninian’s High School

SUMMARY If you are asked a question about covalent radius, you should use the following phrases to explain the patterns: (LEARN THESE)

Covalent radius decreases along a period because: Nuclear charge increases, pulling the electrons closer to the nucleus.

Covalent radius increases down a group because: The number of occupied electrons shells increases

Learning Outcomes

You should now know: 1. The covalent radius is a measure of the size of an atom.

2. The covalent radius decreases going across a period of elements because of an increasing positive nuclear charge.

3. The covalent radius increases going down a group of elements. - This is due to increasing number of occupied electron shells.

CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 5

Higher Chemistry St. Ninian’s High School