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1ATOMS, MOLECULES ANDSTOICHIOMETRYMatter is anything that has mass and occupies space. Everything aroundus consists of matter.Matter is classied into two main groups, elements and compounds.Elements are substances that cannot be broken down into simplersubstances. They contain only one type of atom. The particles in elementscan be atoms or molecules. For example, sodium and magnesium containatoms while oxygen and nitrogen contain molecules.Compounds are substances that contain more than one type of atomscombined chemically. The particles in compounds can be molecules orions. For example, water and carbon dioxide contain molecules whilesodium chloride and magnesium oxide contain ions.Subatomic particles were discovered during the 1800s. We will concentrateonly on the fundamental subatomic particles, which are the protons,neutrons and electrons.We will also study how atoms combine in simple ratios, that is, thestoichiometry of reactions.

Concept Map

Atoms

Subatomic particles Proton number andnucleon number

Stable andunstable isotopes

Relative chargeand relative mass

Behaviour inelectric field and

magnetic field

Relative atomicmass based on 12C

Mass spectrometry

Mass spectrum

Mole and Avogadros constant Mole concept of gases and solutions

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1 Atoms, Molecules and Stoichiometry 3

Solution

1 mol of 12C has a mass of 12 g.1 mol of 12C contains 6.02 10 23 atoms.

Mass of 1 atom of 12C =12

6.02 10 23 = 1.99 10 23 g

Effect of an Electric Field on Subatomic Particles1 When a beam of protons, neutrons and electrons is passed through

an electric field, the observations are shown in Figure 1.1.

+

protons

neutrons

electrons

beam of protons,electrons andneutrons

2 The positive protons are deflected towards the negative plate. Thenegative electrons are deflected towards the positive plate whilethe neutrons, being electrically neutral, are undeflected.

3 The lighter electrons are deflected more than the heavier protons,that is, < .

Effect of a Magnetic Field on Subatomic Particles1 When a beam of protons, neutrons and electrons is passed through

an electric field, the observations are shown in Figure 1.2.

Southpole ofmagnet

electrons

neutrons

protons

beam of protons,electrons andneutrons

Northpole ofmagnet

Left Hand Rule

directionof force

magneticfield

directionof current

Figure 1.1

Figure 1.2

2003/P2/Q1

LINK TO STPM

The deflections of the protonsand electrons in a magneticfield can be determined byusing Flemings left handrule.

TAKE NOTE!

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ACE AHEAD Chemistry First Term8

14 The stability of radioisotopes is measured by its half-life . Theshorter the half-life, the less stable the radioisotope.

15 The half-lives of some radioisotopes are given in Table 1.6.

Radioisotope Half-life

Iodine-131 8.14 days

Carbon-14 5760 years

Uranium-238 4.51 10 9 years

Radium-226 1602 years

Table 1.6

EXAMPLE 1.4

Uranium-235 decays by emitting an alpha particle.Determine the proton number and the nucleon number of the isotopeformed.

Solution

Proton number = 92 2 = 90 Nucleon number = 235 4 = 231235

92U 23190Th + 42He

1 The fundamental particles in an atom are protons, neutrons andelectrons.

2 Protons are positively charged, electrons are negatively chargedand neutrons are neutral.3 Protons and neutrons are found in the nucleus while electrons

move around the nucleus in specific energy levels.4 A neutral atom has an equal number of protons and electrons.5 A positive ion is formed when an atom or molecule loses electrons.

A negative ion is formed when an atom or molecule receiveselectrons.

6 Isotopes are atoms of an element that have the same number ofprotons but different number of neutrons.

7 Unstable isotopes decay spontaneously.

REFRESHER

1 Determine the number of subatomic particles of the following species. (a) 21D2168O (b) 188O2 (c) [ 147 N168O3]

2 A beam of 1H+ and 16O ions is passed into an electric field as shown in the diagram below.

+

path X

x y

path Y

beam of1H+ and16 O ions

Checkpoint 1.1

Radium was discovered in1898 by Marie S. Curieand her husband Pierre.Radium has 33 isotopes, with

nucleon numbers 202 to234 and all are unstable.226 Ra has the longest half-lifeof 1602 years.

CHEM FILE

When a tree is alive, C-14is continually disintegrating,and it is continually beingreplaced by photosynthesis.However, when the tree dies,photosynthesis does not takeplace. Only disintegrationis occurring and the C-14concentration in the treebegins to decrease. Bymeasuring the C-14 level inthe tree, it is possible totell how long the tree hasbeen dead. This is calledcarbon dating .

CHEM FILE

Radioactive disintegrationis a first order reaction . Thehalf-life of a radioisotope isindependent of the amountof the radioactive substancepresent and is not influenced

by catalyst or changes intemperature and pressure.

TAKE NOTE!

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ACE AHEAD Chemistry First Term24

OBJECTIVE QUESTIONS

Focus on Exam 1

1 Which combination shows the correctnumber of electrons and neutrons of therespective species?

Species Number ofelectrons

Number ofneutrons

A 146C 6 8

B 188O2 10 10

C 2411 Na + 10 11

D 3717Cl2+ 19 20

2 An atom P has a nucleon number of 48. P forms P 2+ ion which is isoelectronic with

4020Ar atom. Which of the following showsthe correct number of protons, electronsand neutrons in an atom of P ?

Protons Electrons NeutronsA

20 20 28B 22 20 26C 22 22 26D 22 22 22

Clone STPM 2004/P1/Q8

3 Which ion has the same number of protonsand neutrons but has more electrons than

protons?A 2H+ C [16O2H] B [2H316O]+ D [32S18O4]2

4 Silver has two isotopes, 10747Ag and 10947Ag. Ifthe relative atomic mass of silver is 107.87,what is the percentage abundance of theheavier isotope?A 35.7% C 56.5%B 43.5% D 59.2%

5 The symbol of a sulphur atom is

3216S.

Whichstatement is true regarding sulphur?

A The relative atomic mass of sulphur is 32 g.B A sulphur atom has 32 electrons.C 16 g of sulphur contain 6.02 10 23 atoms.D There are 16 protons and 16 neutrons in

the nucleus of a sulphur atom.

6 The mass ratio of one atom of an isotopeof Q to one atom of carbon-12 is 1.50.What is the relative isotopic mass of theisotope of Q?A 8.0 C 15.0B 10.0 D 18.0

7 The function of the magnetic field in amass spectrometer is toA vaporise a solid sampleB produce positive ionsC separate ions with different massesD separate ions from neutral molecules

8 Which species will be deflected the most ina mass spectrometer?A 21H+ C 126 C+

B 73Li+ D 147 N2+

9 The nitrogen dioxide molecule, NO 2,consists of 14 N, 16O and 18O isotopes. Howmany molecular peaks would there be in themass spectrum of a sample of NO 2?A 1 C 3B 2 D 4

10 The mass spectrum of a compound showingsome major peaks is shown below.

m / e 2814

r e

l a t i

v e

i n

t e n s

i t y

12

Which compound will give the above massspectrum?

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1 Atoms, Molecules and Stoichiometry 25

[Relative atomic mass: H, 1; C, 12; N, 14;O, 16]A N2 C COB C2H4 D NO

11 An element Y occurs naturally as 65Y and67Y

in the ratio of 2 : 1. Based on thisinformation, which graph could representthe mass spectrum of Y 2?

65

% a

b u n

d a n c e

67m / e

A

m / e 130

% a

b u n

d a n c e

132 134130

% a

b u n

d a n c e

132 134m / e

B D

m / e 130

% a

b u n

d a n c e

132 134

C

12 The relative atomic mass of 90Sr based onthe 16O standard is 89.936. Based on the 12Cstandard, the relative atomic mass of 16O is15.995. What is the relative atomic mass of90

Sr based on the12

C standard?A 89.908 C 89.964B 89.931 D 90.000

13 The compound NCl 3 is formed from 14 N,35Cl and 37Cl isotopes. The relative abundanceof 35Cl to 37Cl is 3:1. Which statement aboutthe mass spectrum of NCl 3 is true?A The base peak corresponds to N + ion.B The m/e value for the last peak is 123.C The number of peaks for NCl 3+ ion is 4.D

The relative abundance of N35

Cl3+

ion to N37Cl3+ ion is 3:1.STPM 2010/P1/Q1

14 0.1 dm 3 of argon contains x atoms. Howmany atoms are there in 0.2 dm 3 ofhydrogen gas under the same conditions?A

x

2 C 2 x

B x D 4 x

15 The relative molecular mass of carbon

dioxide is 44. What is the mass, in g, of acarbon dioxide molecule?

[Avogadros constant = 6.02 10 23 mol 1]

A 44 6.02 10 23 C 446.02 10 23

B 6.02 1023

44 D 1

44 6.02 10 23

16 The formula of a type of polystyreneis Br 3C6H3(C8H8)n . One sample of the

polystyrene contains 10.46% bromine.What is the value of n ?

[Relative atomic mass: H, 1; C, 12; Br, 80]A 3 C 19B 12 D 22

17 0.85 g of magnesium powder is reactedwith 2.0 mol dm 3 hydrochloric acid. Whatis the volume of hydrochloric acid requiredfor complete reaction?

[Relative atomic mass: Mg, 24.3]A 17.5 cm 3 C 42.5 cm 3B 35.0 cm 3 D 70.0 cm 3

A B C D

1 only 1 and 2 only 2 and 3 only 1, 2 and 3

18 Which statement(s) is/are true about theisotopes 168 X and 188Y ?1 They are isotopes of the same element.2 The 168 X isotope has more neutrons per

atom than the 188Y isotope.3 They have the same density.

19 Which statement(s) is/are true of thenuclides of silicon, 2814Si and 3014Si?1 The mass spectrum shows two peaks.2 2814SiCl 4 and

3014SiCl 4 molecules have different

shapes.3 Isotope 2814Si is more reactive than isotope

3014Si.

STPM 2010/P1/Q41

20 Which statement(s) is/are true about theisotope AZ X ?1 Its relative isotopic mass is A g.2 Its relative isotopic mass is based on the

12C standard.3 The isotope AZ X and its ion AZ X 2+ have the

same relative mass.

21A sample of bromine gas contains

79

Br and81Br isotopes. Which mass/charge ratio(s)