Exam practice
Every question from the end of every unit, with hints and answers to open. After you check an answer, mark how you did, and come back to the ones you didn’t get.
21 questions
Unit 14 · Classification of Elements
2. Give reasons
Classification of elements is necessary.
Answer
There are 118 elements, too many to study one by one. Classification puts elements with similar properties together, so by studying one member of a group we learn the properties of the whole group. It shows how properties change in a regular way, makes the elements easier to remember and compare, shows how elements are related, and even let Mendeleev predict elements not yet discovered.
The size of atoms increases from top to bottom in a group.
Answer
Going down a group, each element has one more shell than the element above it. The valence shell gets farther and farther from the nucleus, so the atom gets bigger, even though the nucleus has more protons.
The size of atoms decreases from left to right across a period.
Answer
Across a period, the number of shells stays the same, but each element has one more proton and one more electron. The added electron goes into the same shell, while the added proton increases the positive charge of the nucleus. The nucleus pulls the electrons in more strongly, so the atom contracts.
Hydrogen is a non-metal, but it is kept with the metals in the modern periodic table.
Answer
Hydrogen’s configuration is 1s¹: one electron in its valence shell, like the alkali metals (ns¹). Like them, it has valency 1 and can lose its one electron to form a positive ion (H⁺). Since the modern table is arranged by electronic configuration, hydrogen is placed at the top of group IA, with the alkali metals.
Potassium is more reactive than sodium.
Answer
Potassium (2, 8, 8, 1) has four shells and sodium (2, 8, 1) has three, so potassium’s valence electron is farther from the nucleus and held more loosely. Potassium loses it more easily, and metals react by losing electrons, so potassium is more reactive.
Fluorine is more reactive than chlorine.
Answer
Fluorine (2, 7) has two shells and chlorine (2, 8, 7) three. Fluorine’s atom is smaller, so its nucleus pulls an incoming electron more strongly. Non-metals react by gaining electrons, so fluorine, which gains one most easily, is more reactive.
The metallic character of elements decreases and the non-metallic character increases from left to right across a period.
Answer
Metallic character is the tendency to lose electrons; non-metallic character is the tendency to gain them. Across a period, the nuclear charge increases while the number of shells stays the same, so the atoms get smaller and hold their valence electrons more tightly. They lose electrons less easily (less metallic) and gain them more easily (more non-metallic).
Inert gases are kept in group 0 of the modern periodic table.
Answer
The inert gases have a completely filled valence shell (an octet, ns² np⁶, or a duplet, 1s², for helium). They don’t need to lose, gain or share electrons, so their valency, and their combining power, is zero. That is why their group is called group 0.
A part of the periodic table is shown: Li, Be, B, C, N, O, F, Ne (groups IA to VIIIA). The atomic size decreases from Li to Ne.
Answer
All eight elements are in period 2, so all have two shells (K and L). From Li to Ne the number of protons increases from 3 to 10, while every new electron goes into the same L shell. The increasing nuclear charge pulls the L shell closer to the nucleus, so the atoms get smaller from Li to Ne.
3. Write the differences between
Group and period
Answer
| Group | Period |
|---|---|
| a vertical column of the periodic table | a horizontal row of the periodic table |
| there are 18 groups | there are 7 periods |
| its elements have the same number of valence electrons | its elements have the same number of shells |
| its elements have similar properties and the same valency | properties and valency change from one element to the next |
| atomic size increases down a group | atomic size decreases across a period |
Chlorine and sodium
Answer
| Chlorine | Sodium |
|---|---|
| non-metal | metal |
| in group VIIA (17), period 3 | in group IA (1), period 3 |
| configuration 2, 8, 7 (1s², 2s² 2p⁶, 3s² 3p⁵) | configuration 2, 8, 1 (1s², 2s² 2p⁶, 3s¹) |
| gains one electron to form Cl⁻ (electronegative) | loses one electron to form Na⁺ (electropositive) |
| smaller atom | larger atom |
| a greenish-yellow gas | a soft, silvery solid |
| its oxide is acidic | its oxide is basic |
Elements of group IA and group VIIA
Answer
| Group IA (alkali metals) | Group VIIA (halogens) |
|---|---|
| metals (except hydrogen) | non-metals |
| one valence electron, ns¹ | seven valence electrons, ns² np⁵ |
| lose an electron to form positive ions | gain an electron to form negative ions |
| electropositive | electronegative |
| reactivity increases down the group | reactivity decreases down the group |
| e.g. Li, Na, K | e.g. F, Cl, Br, I |
Electronegativity and electropositivity
Answer
| Electronegativity | Electropositivity |
|---|---|
| tendency to gain electrons and form negative ions | tendency to lose electrons and form positive ions |
| a property of non-metals | a property of metals |
| increases across a period | decreases across a period |
| decreases down a group | increases down a group |
| highest in fluorine | highest in cesium and francium |
4. Answer these questions
State the modern periodic law.
Answer
The physical and chemical properties of elements are the periodic functions of their atomic numbers. (When the elements are arranged in increasing order of atomic number, elements with similar properties recur at regular intervals.)
How many groups and periods are there in the modern periodic table?
Answer
18 groups and 7 periods.
Write the position of sodium in the modern periodic table. Why is it known as an alkali metal?
Answer
Sodium (atomic number 11, configuration 2, 8, 1 or 1s², 2s² 2p⁶, 3s¹) is in period 3 (three shells) and group IA (1) (one valence electron). It is an alkali metal because it reacts with water to form sodium hydroxide, a strong base that dissolves in water as an alkali: 2Na + 2H₂O → 2NaOH + H₂.
Write the positions of these in the periodic table: alkali metals, alkaline earth metals, inert gases, transition metals, lanthanides and actinides.
Answer
- Alkali metals: group IA (1), the first column on the left (Li to Fr).
- Alkaline earth metals: group IIA (2), the second column (Be to Ra).
- Inert gases: group 0 (18), the last column on the right (He to Og).
- Transition metals: groups IIIB to IIB (3 to 12), the ten columns in the middle, in periods 4 to 7.
- Lanthanides: the 15 elements from La (57) to Lu (71), in period 6, written in the first separate row below the table.
- Actinides: the 15 elements from Ac (89) to Lr (103), in period 7, in the second separate row below the table.
Write the electronic configuration of sulphur and its position in the modern periodic table.
Answer
Sulphur, atomic number 16: 1s², 2s² 2p⁶, 3s² 3p⁴ (by shells, 2, 8, 6). It has three shells, so it is in period 3; it has six valence electrons, so it is in group VIA (16), below oxygen.
On what basis are groups and periods separated in the modern periodic table?
Answer
Groups are set by the number of valence electrons (the outer-shell configuration): elements with the same number of valence electrons, and so similar properties, are in the same group. Periods are set by the number of shells: elements with the same number of shells are in the same period, in order of increasing atomic number.
In which group are fluorine, chlorine and bromine? Which of them is the most reactive?
Answer
They are in group VIIA (17), the halogens. Fluorine is the most reactive: it is the smallest, so its nucleus attracts an extra electron most strongly, and non-metals react by gaining electrons.
If you were given the chance to improve the modern periodic table, what changes would you make? Give reasons.
Answer
There is no single right answer; good answers give a reason. Some ideas chemists themselves argue about:
- Give hydrogen a place of its own, floating above the middle of the table, since it is like the alkali metals in some ways and like the halogens in others, and is not a metal.
- Put helium above beryllium (group 2), since its configuration is 1s², or keep it with the noble gases, since it behaves like them. The table can’t show both.
- Fit the lanthanides and actinides into the main body (a 32-column table), so the table shows all elements in true order of atomic number without a gap; it is only printed separately to fit on a page.
- Use only the numbers 1–18 for groups, dropping the confusing A/B names that differ between countries.
- Show color bands or arrows for the trends (size, electronegativity), and the metal/metalloid/non-metal staircase clearly.