Reactivity, Melting & Boiling Points | ICSE Class 10 Chemistry

01Rreact

What makes an element reactive?

An element is chemically reactive when its atoms can easily change their outer electronic arrangement.

Chemical reactivity

The chemical reactivity of an element depends on its tendency to lose or gain electrons in order to complete its outermost shell.

METALS NON-METALS lose electrons gain electrons ↓ ↓ form positive ions form negative ions ↓ ↓ more easily lost = more reactive more easily gained = more reactive

Think of completing a set of eight cards. An atom with one outer electron may find it easier to give it away. An atom with seven outer electrons may find it easier to gain one. An atom with four has neither easy route.

Check · 1 of 18

A highly reactive metal has a strong tendency to:

Correct: metals react by losing electrons; easier loss means greater metallic reactivity.

02→period

Reactivity across a period

The chapter’s key pattern is not one-directional: chemical reactivity first decreases and then increases from left to right across a period.

Nahighly reactive metal

Mgless reactive

Alless reactive

Sileast reactive

Prising

Smore reactive

Clreactive non-metal

From sodium to silicon, the tendency to lose electrons decreases. From silicon to chlorine, the tendency to gain electrons increases. Thus silicon is the least reactive element in this Period 3 sequence, while chlorine is the most reactive non-metal in it.

Trend explorer · tap a property

Check · 2 of 18

Which statement best describes reactivity across Period 3?

Correct: reactivity is lowest near the middle of the sequence and rises on the non-metallic side.

Write it · 3 of 18

Name the least reactive element in the Period 3 sequence discussed.

Answer: silicon. It is the least reactive element in the sequence shown.

03↓group

Reactivity down a group

Metals: reactivity increases

Down a group, new shells are added. The valence electron is farther from the nucleus and is held less strongly, so it is lost more easily.

Down Group 1 ↓ atomic size increases → outer electron is farther away ↓ electron loss becomes easier → metal reactivity increases Li < Na < K < Rb < Cs

Therefore potassium is more reactive than sodium, and sodium is more reactive than lithium. Francium is radioactive and is not treated in the same way here.

Non-metals: reactivity decreases

Non-metals react by gaining electrons. Down Group 17, the larger atom attracts an incoming electron less strongly, so electron gain becomes harder.

Down Group 17 ↓ atomic size increases → attraction for incoming electron decreases ↓ electron gain becomes harder → non-metal reactivity decreases F > Cl > Br > I

Opposite down-group reactivity trends
Type	How it reacts	Trend down the group
Metal	Loses electrons	Reactivity increases
Non-metal	Gains electrons	Reactivity decreases

Check · 4 of 18

Why is potassium more reactive than sodium?

Correct: added shells make the outer electron easier to remove.

Check · 5 of 18

Which halogen is more reactive?

Correct: fluorine is smaller and attracts an incoming electron more strongly.

Tap sorter · 6–9 of 18

For each pair, tap the more reactive element.

or

K — Group 1 metal reactivity increases down the group.

or

Cl — Group 17 non-metal reactivity decreases down the group.

or

Na — it loses one valence electron more easily than Mg loses two.

or

Cl — tendency to gain electrons increases towards chlorine.

Keep the terms clear

Metallic character is the tendency to lose electrons and form positive ions. Reactivity of a metal is how readily it undergoes chemical change by losing electrons. They are closely related, but not identical labels.

04°m.p./b.p.

Melting and boiling points

Two useful meanings

Melting point: the temperature at which a solid changes into a liquid. Boiling point: the temperature at which a liquid changes into a gas.

The chapter gives the supported patterns and values; learn the broad trends without adding explanations not supplied by the chapter.

Down groups: opposite families, opposite directions

Group 1 metals — points decrease down the group
Metal	Melting point	Boiling point
Lithium	180.5°C	1347°C
Sodium	94.5°C	883°C
Potassium	63.5°C	774°C

Group 17 halogens — points increase down the group
Halogen	Melting point	Boiling point	State
Fluorine	−219.6°C	−187°C	Gas
Chlorine	−101°C	−34.6°C	Gas
Bromine	−7.2°C	58.8°C	Liquid
Iodine	113.6°C	184°C	Solid

As halogen melting and boiling points rise, their stated physical states change: fluorine and chlorine are gases, bromine is a liquid, and iodine is a solid.

Across a period

Melting and boiling points usually increase up to Group 14 and then decrease. In Period 3, silicon (Group 14) has the highest listed melting point: Na 98°C, Mg 650°C, Al 660°C, Si 1410°C, P 44.2°C, S 115.2°C. The sequence after Group 14 is not perfectly smooth — sulphur is higher than phosphorus — so use the word usually.

Check · 10 of 18

Among Li, Na and K, which has the lowest melting point?

Correct: Group 1 melting points decrease from Li to Na to K.

Check · 11 of 18

Which statement is supported by the halogen table?

Correct: the values increase from fluorine to iodine.

Write it · 12 of 18

Which has the higher boiling point: chlorine or iodine?

Answer: iodine. Halogen boiling points increase down Group 17.

05ρdensity

Density: broad patterns

Density

Density tells us how much matter is packed into a given volume. Here, recognise the periodic trend rather than calculating it.

Across a period, density increases gradually to a maximum and then a slight decrease may be noticed. Period 3 values are 1.0 (Na), 1.7 (Mg), 2.7 (Al), 2.3 (Si), 1.8 (P), 2.1 g/cm³ (S). They broadly rise to aluminium and then fall, but do not change perfectly smoothly.

Group 1 density values (g/cm³)
Element	Li	Na	K	Rb	Cs
Density	0.54	0.97	0.86	1.53	1.87

Down a group, density generally increases. Notice the exception in the listed data: potassium (0.86 g/cm³) is slightly less dense than sodium (0.97 g/cm³). Trends describe broad patterns, not a guarantee that every adjacent value changes smoothly.

Check · 13 of 18

Why is “generally” the safest word for density down a group?

Correct: potassium is a noted exception to a perfectly smooth rise after sodium.

Write it · 14 of 18

Complete the statement: down a group, density __________.

Answer: generally increases.

06!mistakes

Common mistakes to avoid

For non-metals, a smaller atom attracts an incoming electron more strongly. Chlorine is therefore more reactive than iodine.

In Group 1, reactivity increases down the group. Potassium loses its valence electron more easily.

It first decreases towards the middle and then increases towards the reactive non-metals.

Metals become more reactive down a group; non-metals become less reactive.

For the families studied, Group 1 metals decrease but Group 17 halogens increase.

The broad trend is an increase, but potassium is slightly less dense than sodium in the table.

Check · 15 of 18

Bromine is a liquid while chlorine is a gas because, in the chapter’s table, bromine has:

Correct: the increasing values down the halogen group align with the change of physical state.

Check · 16 of 18

Which word best qualifies the Period 3 melting/boiling trend after Group 14?

Correct: individual values need not form a perfectly smooth sequence.

07✓mastery

Worked examples and practice

Worked comparisons

1. Which is more reactive: sodium or magnesium?

Sodium. Both are Period 3 metals, and sodium loses its one valence electron more easily than magnesium loses two.

2. Which is more reactive: sodium or silicon?

Sodium. Reactivity decreases from sodium towards silicon; silicon is the least reactive element in this sequence.

3. Which is more reactive: sulphur or chlorine?

Chlorine. On the non-metallic side, tendency to gain electrons increases towards chlorine.

4. Which is more reactive: lithium or potassium?

Potassium. Metal reactivity increases down Group 1.

5. Which is more reactive: chlorine or bromine?

Chlorine. Non-metal reactivity decreases down Group 17.

6. Which Group 1 metal has the lowest melting point among Li, Na and K?

Potassium. Melting point decreases down Group 1.

7. Which has the higher boiling point: chlorine or iodine?

Iodine. Halogen boiling points increase down the group.

8. Why is bromine liquid while chlorine is a gas?

Bromine has higher melting and boiling points than chlorine; the table lists chlorine as a gas and bromine as a liquid at ordinary conditions.

Check · 17 of 18

Which order shows increasing reactivity for Group 1 metals?

Correct: metallic reactivity increases down Group 1.

Write it · 18 of 18

State the trend in chemical reactivity across a period.

Answer: it first decreases and then increases.

Practice: answer, then open the model answer

Concept check 6–10: Na or K? F or Cl? Group 1 melting points? Halogen melting points? Density down a group?

Potassium; fluorine; Group 1 melting points decrease; halogen melting points increase; density generally increases down a group.

Easy 1–4: What determines reactivity? What is the across-period trend? Metallic trend down a group? Non-metallic trend down a group?

It depends on tendency to lose/gain electrons and complete the outer shell. Across a period it first decreases then increases. Metal reactivity increases down a group; non-metal reactivity decreases.

Easy 5–8: Most reactive non-metal? Least reactive Period 3 element? State of bromine? State of iodine?

Fluorine; silicon; liquid; solid.

Understanding: explain Na vs Mg and Cl vs S.

Na loses its one outer electron more readily than Mg loses two. Cl has a greater tendency to gain an electron than S as it is approached from the right of Period 3.

Understanding: explain K vs Na and Cl vs Br.

K has more shells, so its outer electron is lost more easily. Cl is smaller and attracts an incoming electron more strongly than Br.

Understanding: why does reactivity first decrease then increase? Why use “generally” for density?

Metallic electron loss becomes harder towards the middle; then non-metallic electron gain becomes easier. “Generally” is needed because individual values such as K and Na can be exceptional.

Order of increasing reactivity: Li, Na, K; I, Br, Cl, F; Al, Mg, Na; Si, S, Cl.

Li < Na < K; I < Br < Cl < F; Al < Mg < Na; Si < S < Cl.

Order of decreasing reactivity: K, Na, Li; F, Cl, Br, I; Na, Mg, Al; Cl, S, P.

K > Na > Li; F > Cl > Br > I; Na > Mg > Al; Cl > S > P.

ICSE: Explain the variation in reactivity across Period 3.

From Na towards Si, tendency to lose electrons decreases, so reactivity decreases. From Si towards Cl, tendency to gain electrons increases, so reactivity increases.

ICSE give reasons: K > Na, F > Cl, Si least reactive, Na > Al.

K’s outer electron is farther away; F is smaller and attracts an incoming electron more strongly; Si is the middle/least-reactive point in the given sequence; Na loses its one electron more easily than Al loses three.

ICSE data: Li, Na, K melting/boiling — state trend, lowest melting point, highest boiling point.

Both decrease down Group 1; K has the lowest melting point; Li has the highest boiling point.

ICSE halogen physical states — what change is suggested down the group?

Melting and boiling points increase down the group.

One-minute revision

REACTIVITY: across a period → first decreases, then increases metals down → increases | non-metals down → decreases MELTING / BOILING: Group 1 down → decrease | Group 17 down → increase across a period → usually rise to Group 14, then fall DENSITY: across → rises to a maximum, may then fall down → generally increases

Chemical Reactivity, Melting Point, Boiling Point & Density

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