What are methods for calculating the valency of elements for grade 8 students in Vietnam? What must grade 8 students in Vietnam do to take extra classes of Natural Sciences?
What are methods for calculating the valency of elements for grade 8 students in Vietnam?
Valence is the capacity of an atom to bond with other atoms. The valence is determined by the number of electrons an atom can donate, accept, or share to achieve the most stable electron configuration.
Students in grade 8 can refer to the method of calculating the valence of elements as follows:
| Step 1: Determine the chemical formula of the compound by writing down the chemical formula of the compound under consideration. Step 2: Identify the known valence of the elements in the compound - Use the valence table or information about the valence of known elements in the compound. - Example: Oxygen usually has a valence of 2, Hydrogen usually has a valence of 1. Step 3: Use the valence rule - The total valence of the atoms in a compound always equals 0 if the compound is neutral or equals the charge of the compound if it is an ion. - Example: In H₂O, oxygen has a valence of 2, and each hydrogen atom has a valence of 1. The total valence is 0 because H₂O is a neutral compound. Step 4: Calculate the valence of the unknown element - Based on the total valence and the known valence of other elements, calculate the valence of the unknown element. - Example: In the compound NH₃, nitrogen usually has a valence of 3 and hydrogen has a valence of 1. Since there are three hydrogen atoms, the total valence of hydrogen is 3. The total valence of the compound is 0, so nitrogen has a valence of 3. |
Examples of calculating element valence
| 1. H₂O (Water): - Formula: H₂O - Known valence: H (1), O (2) - Calculate valence: (2 x 1) + (1 x 2) = 2 + 2 = 4, total valence is 0 (neutral) 2. CO₂ (Carbon dioxide): - Formula: CO₂ - Known valence: O (2) - Calculate valence: The valence of carbon in CO₂: 2 * (-2) + C = 0 (neutral) C = 4 3. Na₂SO₄ (Sodium sulfate): - Formula: Na₂SO₄ - Known valence: Na (1), O (2) - Calculate valence: The valence of sulfur (S) in Na₂SO₄: 2 * 1 + S + 4 * (-2) = 0 (neutral) 2 + S - 8 = 0 S = 6 |
Practice exercises on calculating element valence:
| Question 1: Calculate the valence of C in compound CO₂, knowing the oxygen valence is II. A. I. B. II. C. III. D. IV. Answer: Choose D Let a be the valence of C in compound CO₂, we have: a.1 = 2.2 which implies a = 4. Thus, the valence of C in the compound is IV. Question 2: Which of the following sequences consists of elements with valence I in the compound A. H, Na, K. B. Mg, O, H. C. O, Cu, Na. D. O, K, Na. Answer: Choose A Question 3: Choose the correct answer: In the chemical formula, (1) of the subscript and (2) of this element equals the (3) of the subscript and the valence of the other element. A. (1) total, (2) valence, (3) product. B. (1) product, (2) valence, (3) total. C. (1) total, (2) valence, (3) total. D. (1) product, (2) valence, (3) product. Answer: Choose D Question 4: Atom Fe has a valence of II in which formula? A. FeO. B. Fe₂O₃. C. Fe. D. FeCl₃. Answer: Choose A FeO has valence II because O has valence II. Question 5: Choose the incorrect statement: A. Valence is a number indicating the bonding capacity of one element's atom with another element's atom. B. The valence of an element is determined according to the valence of H chosen as the unit and the valence of oxygen is 2 units. C. Valence rule: x.a = y.b. D. Phosphorus only has valence IV. Answer: Choose D |
Note: The information is for reference only!
What are methods for calculating the valency of elements for grade 8 students in Vietnam? What must grade 8 students in Vietnam do to take extra classes of Natural Sciences? (Image from Internet)
What must grade 8 students in Vietnam do to take extra classes of Natural Sciences?
According to Article 5 Regulations on extra classes and tutoring issued together with Circular 17/2012/TT-BGDDT:
Organization of extra classes and tutoring in schools
- Students wishing to take extra classes must write an application for extra classes submitted to the school; parents or guardians (hereinafter referred to as parents) of students applying for extra classes must directly sign and commit to the school regarding extra classes and tutoring in the application and are responsible for implementing the commitment.
- The school principal receives the application for extra classes of the students, organizes the grouping of students according to academic ability, assigns teachers responsible for the subject, and organizes extra classes according to the academic ability groups of students.
- Teachers wishing to conduct extra classes must have an application for extra teaching; in the application, there is a commitment with the school to fully complete all tasks of the teacher according to general regulations and other tasks assigned by the school, while strictly complying with the regulations on extra classes and tutoring in schools.
- The school principal approves the list of extra class teachers, assigns extra class teachers, and schedules extra classes suitable to the students' academic ability.
Thus, students wishing to study more Natural Sciences grade 8 must write an application for extra classes submitted to the school.
What are the principles of extra classes and tutoring in schools in Vietnam?
Based on Article 3 of the Regulations on extra classes and tutoring issued together with Circular 17/2012/TT-BGDDT, the principles of extra classes and tutoring in schools include:
- The activities of extra classes and tutoring must contribute to consolidating and enhancing knowledge, skills, and the personality education of students; suitable to psychological characteristics and not causing an overwhelming burden on the learner.
- Do not reduce the content in the official curriculum of general education to deliver during extra classes; do not teach extra classes before introducing the content in the official general education curriculum.
- Students taking extra classes are those who need, voluntarily want to participate, and have family consent; no form of coercion should be used to force students and their families to participate in extra classes.
- Do not organize extra classes and tutoring according to the official class levels; students in the same extra class should have equivalent academic abilities; organizing students into extra classes should be based on their academic abilities.
- Organizations and individuals organizing extra classes and tutoring must be responsible for the contents registered and authorized for extra classes and tutoring activities.
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