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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 Further Reading To learn more about covalent radius. Follow the links below: BBC Bitesize: https://www.bbc.co.uk/bitesize/guides/zxc99j6/revision/1 Read page 6 Evans2 chem web: https://www.evans2chemweb.co.uk/login/index.php# Username: snhs password: giffnock Select any teacher revision material CfE Higher Periodicity Questions Complete Exercise 1.2 and check your answers CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 6 Higher Chemistry St. Ninian’s High School Exercise 1.2 – Periodicity: Covalent Radius 1. The modern periodic table was first proposed by a Russian Chemist, Dmitri Ivanovitch Mendeleev. He listed the elements in order of atomic mass. Where he felt it necessary he left gaps. For example, he left a gap between the elements gallium and arsenic where he felt an element similar to silicon would be discovered. He called this element Eka-Silicon (a) Identify the modern element which was Mendeleev's Eka-Silicon. (b) With reference to the modern periodic table explain the following terms (i) Group (ii) Period 2. The data book lists the covalent radius of elements in units called picometres. One picometre is equal to 10-12 metres. (a) Compile a table showing the atomic number, electron arrangement, and covalent radius for hydrogen, lithium, sodium, and potassium. (b) Explain the trend shown in your table. 3. As we cross a period in the periodic table the covalent radius decreases. This trend is very evident if we examine data relating to the second period. (a) Construct a table of atomic number and covalent radii for carbon, nitrogen, oxygen and fluorine. (b) Explain the trend shown in your answer to part (a). 4. The table below shows the covalent radius of some elements in group 2 Element beryllium magnesium calcium Covalent radius (pm) 129 145 174 Explain the trend in covalent radii for these elements 5. The table below shows the atomic radii of some period three elements. Element sodium magnesium aluminium Covalent radius (pm) 154 145 130 Explain why the atomic radius decreases as we cross a period. CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 7 Higher Chemistry St. Ninian’s High School Exercise 1.2 – ANSWERS 1. (a) Germanium (b) (i) A group is a (vertical) column in the periodic table. Elements in a group display similar chemical properties because they have the same number of electrons in their outer shell. (ii) A period is a (horizontal) row in the periodic table. Elements in the same period have the same number of occupied electron shells. 2. (a) Element hydrogen lithium sodium potassium Atomic number 1 3 11 19 Electron arrangement 1 2,1 2,8,1 2,8,8,1 Atomic radius (pm) 37 134 154 196 (b) Covalent radius increases down a group because the number of occupied electrons shells increases. 3. (a) Element carbon nitrogen oxygen fluorine Atomic number 6 7 8 9 Atomic radius (pm) 77 75 73 71 (b) Covalent radius decreases along a period because nuclear charge increases, pulling the electrons closer to the nucleus. 4. Beryllium, magnesium and calcium are all in group 2. Covalent radius increases down a group because the number of occupied electrons shells increases. 5. Covalent radius decreases along a period because nuclear charge increases, pulling the electrons closer to the nucleus. CCS: Part A Lesson 2 – Periodicity: Covalent Radius Page 8 .
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