Unit notes and practice questions
Everything here is for unit 4 of the Grade 9 Chemistry textbook: what the chapter covers, the short notes, practice questions with their answers, and the lesson video where one exists.
Start here
A short video lesson on this chapter, made by Temari. It loads only when you press play.
Find an element's place from its shells · 4 min · 2 past exam questions worked through
Tap a part to play the lesson from there.
The cards filed under this chapter on the Grade 9 Chemistry page. Open one at a time.
When you use it
Use when comparing the electron affinity values of halogens in Group VIIA.
Watch out
Chlorine has a higher electron affinity than fluorine, despite fluorine being higher in the group. Fluorine has a very small 2p subshell, causing strong electron-electron repulsion that reduces the energy released.
Drafted from Grade 9 Chemistry, pages 111-142, then checked twice before it went up
| Property | Across a Period (Left to Right) | Down a Group (Top to Bottom) |
|---|---|---|
| Atomic radius | Decreases | Increases |
| Ionization energy | Increases | Decreases |
| Electron affinity | Increases | Decreases |
When you use it
Use when predicting or comparing periodic properties of elements based on their position in the periodic table.
Watch out
Across a period, nuclear charge increases while shielding remains constant, shrinking atomic radius and raising ionization energy.
Drafted from Grade 9 Chemistry, pages 111-142, then checked twice before it went up
10 multiple-choice questions written against this chapter, each with its answer and the reason. Try one before you open the answer.
Question 1
Which scientist discovered the neutron in 1932 by bombarding Beryllium nuclei with alpha particles?
Answer: A James Chadwick
Why
James Chadwick discovered the neutron in 1932 after observing neutral radiations produced when Beryllium was bombarded with alpha particles.
Question 2
What is the total number of protons and neutrons in the nucleus of an atom called?
Answer: B Mass number
Why
The mass number (A) is defined as the total number of protons and neutrons (nucleons) in an atom's nucleus.
Question 3
According to Dobereiner's law of triads, what is the atomic weight of the middle element in a triad?
Answer: A The arithmetic mean of the atomic weights of the other two elements
Why
In Dobereiner's triads, the atomic weight of the middle element is approximately the arithmetic mean (average) of the first and third elements.
Question 4
How many elements are present in the first period of the modern periodic table?
Answer: A 2
Why
Period 1 contains only 2 elements: hydrogen and helium. It is called a very short period.
Question 5
Based on Moseley's work with X-rays in 1913, the periodic arrangement of elements must be based on which fundamental property?
Answer: C Atomic number
Why
Moseley showed that the frequency of X-rays is related to the atomic number (Z), proving that atomic number is the correct basis for arranging elements in the periodic table.
Question 6
In Lothar Meyer's curve of atomic volume versus atomic mass, which family of elements occupies the peaks of the curve?
Answer: A Alkali metals
Why
Lothar Meyer observed that alkali metals (Li, Na, K, Rb, Cs) occupy the highest points or peaks of the atomic volume curve.
Question 7
According to the rules of electron filling, what is the maximum number of electrons that can be placed in the penultimate shell of an atom?
Answer: B 18
Why
The valence (outermost) shell can hold a maximum of 8 electrons, while the penultimate (second to last) shell can hold up to 18 electrons.
Question 8
Which of the following correctly arranges nitrogen (N), phosphorus (P), and silicon (Si) in order of increasing atomic radius?
Answer: A N < P < Si
Why
Atomic radius increases down a group (so N is smaller than P) and decreases from left to right across a period (so P is smaller than Si). Therefore, the correct order is N < P < Si.
Question 9
Why do Group IIA elements have a higher first ionization energy than Group IIIA elements in the same period?
Answer: A Group IIA elements have completely filled s orbitals, which are more stable
Why
Group IIA elements have a completely filled ns2 configuration. It takes extra energy to remove an electron from a completely filled subshell compared to an incompletely filled subshell.
Question 10
Why is the electron affinity of fluorine (-328 kJ/mol) lower in magnitude than that of chlorine (-349 kJ/mol)?
Answer: B Stronger electron-electron repulsion in the small 2p subshell of fluorine
Why
Fluorine's 2p subshell is very small and compact, causing strong repulsion among electrons. This makes it release less energy when taking in an extra electron compared to chlorine's larger 3p subshell.
The app holds 12 questions on this unit, marked as you go. These are the first 10.
Unit 4 runs from page 111 to page 142 of the Ministry of Education edition.
Ministry of Education edition, English, 186 pages
Temari turns this chapter into timed practice and marks it as you go, with an AI tutor to explain anything that did not land.
Start free
Temari.et is also an Android app, free on Google Play.
