Secondary 3 Chemistry Atomic Structure - Week 1.pdf · Atom • An atom is the smallest building...

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Secondary 3 Chemistry Topic: Atomic Structure

Notes & MCQ Guide

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Date: _____________________

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CHEMISTRY Atomic Structure

Atom

• An atom is the smallest building block of all matter.

• However, each atom is made up of two main parts – the nucleus and the orbitals.

Subatomic Particles

Relative Mass Relative Charge

Proton 1 +1

Electron !

!"#$ -1

Neutron 1 0

Learning Objectives

¨ State the relative charges and approximate relative masses of a proton, a neutron and

an electron

¨ Describe, with the aid of diagrams, the structure of an atom as containing protons

and neutrons (nucleons) in the nucleus and electrons arranged in shells (energy levels).

¨ Define proton (atomic) number and nucleon (mass) number

¨ Interpret and use symbols such asdefine the term isotopes

¨ Deduce the numbers of protons, neutrons and electrons in atoms and ions give

Proton and Nucleon numbers

orbitals


Example 1:

(a) Number of Protons: 12

(b) Number of Electrons: 12

(c) Number of Neutrons: 24 – 12 = 12

There are three types of particles that exists in an atom, known as subatomic particles.

Protons

• A proton gives the identity to the atom and it NEVER change.

• Proton provides a positive charge to the atom.

• In a neutral atom, there is a SAME number of proton and electron

• Responsible for the chemical properties of an atom. (Refer below)

AtomicNumber (Protons/Electrons)

MassNumber (Protons+Neutrons) ChemicalSymbol

(FromPeriodicTable) Mg

24

12

What is the difference between Physical Properties VS Chemical Properties?

Physical Properties is an aspect of matter that can be observed or measured without changing it. For example, colour, boiling point, melting point and

volume.

Chemical Properties may only be observed by changing the chemical identity of

a substance. For example, the reactions they undergo, flammability,

oxidation states.

Specialist’s Comment


Electrons

• Electron provides a negative charge to the atom.

• When there are more electrons in the atom, it becomes a negative ion (Anion).

• When there are less electrons in the atom, it becomes a positive ion (Cation).

Neutrons

• Neutrons provide the mass for the atom.

Isotopes

• Isotope is atoms of the same element with same number of protons, but different number of

neutrons.

• Two different atoms have similar chemical properties if they have the same number of protons-

regardless of the number of neutrons present.

Example 2: Are the elements below isotopes?

Yes. They have the same number of protons (17) and different number of neutrons.

• Mass number refers to the mass of one atom, and is always a whole number (based on the

number of protons and neutrons that atom contains)

• Relative atomic mass refers to the average mass of all the various isotopes of the same

element, according to their percentage abundance. Thus, it may have decimal places.


Example 3: Why is the relative atomic mass of Chlorine not a whole number

Chlorine exists as two isotopes, chlorine-35 (75%) and chlorine-37 (25%). Hence the relative

atomic mass of 35.5 is the average of these two isotopes.

Isotope Abundance

35Cl17 75%

37Cl17 25%

Electronic configuration

• Generally, electrons are filled in the inner shells first before filling the outer shells

• All elements want to form a Noble Gas Configuration; Duplet (2 Outer Electrons – First Shell) or

Octet (8 Outer Electrons – 2nd

/3rd

/4th).

• So, in order for atoms to achieve noble gas configuration, atoms will either lose or gain electrons.

1st

Shell 2nd

Shell 3rd

Shell 4th

Shell

Max no. of

Electrons 2 8 18 32

Stable no. of

Electrons 2 8 18 8

Relativeatomicmass:

(35× ()*++)+(37×

-)*++)=35.5


Example 3: Let’s Consider the case of Calcium Atom.

• In the case of Calcium, 20 electrons are arranged in the 2.8.8.2 Manner, leaving 2 electrons at

the outermost shell (Valence). In order to form noble gas configuration, 2 electrons have to be

give away.

• After losing 2 Electrons (20 Protons, 18 Electrons), Ca atom will become Ca2+

Ion.

202(FirstShell)

8(SecondShell)

8(ThirdShell)

2(ValenceShell)

182(FirstShell)

8(SecondShell)

8(ThirdShell)

Electronic configuration of a Calcium atom

1stshell

2ndshell

3rdshell

4thshell(ValenceShell)


MCQ Walkthrough

1. The atomic number of magnesium is 12. When magnesium reacts to form an ionic compound,

the electronic configuration of the magnesium ion formed is (1980 P1 Q10)

A 2, 8

B 2, 8, 1

C 2, 8, 2

D 2, 8, 3

E 2, 8, 4 ( )

2. How many protons, neutrons and electrons does the Na+ ion contain? (1989 P1 Q2)

protons neutrons electrons

A 10 13 10

B 11 11 12

C 11 12 10

D 11 12 11

E 12 11 12 ( )

The Electronic configuration of Magnesium ATOM is 2.8.2.

So, in order for it to achieve noble gas configuration (form ion), it will need to lose 2 electrons.

New Electronic configuration, 2.8

Ans: (A)


Proton Number never change. Sodium has an atomic number of 11.

As such, there will be 11 protons and 11 electrons. With a nucleon number of 23, there are 23 – 11 =12 neutrons. In order for Sodium to become an ion, it must lose 1 electron.

Ans: (C)



3. Which one of the following particles contains 10 electrons, 11 protons and 12 neutrons?

(1988 P1 Q2)

A 2412Mg

2+

B 2311Na

+

C 2210Ne

D 2110 Ne

E 199F

- ( )

4. Three elements X, Y and Z have consecutive, increasing proton (atomic) numbers. If

element X is a noble gas, what will be the symbol for the ion of element Z in its

compounds? (1991 P1 Q4)

A Z2-

B Z-

C Z+

D Z2+

E Z3+

( )

This question is very straightforward if you know the trick.

Since number of protons NEVER change, look for the answer with 11 protons.

Ans: (B)


If X is a noble gas, then Y will belong to Group I.

Z will belong to group II since they are consecutive. Group II elements need to lose 2 electrons in order to form Ion, so the answer is Z2+.

Ans: (D)



5. An element X has two isotopes, which may be represented as 238

X and 235

X.

How does 238

X differ from 235

X? (2000 P1 Q6)

A It has 3 more protons and 3 more electrons.

B It has 3 more protons, but no more electrons.

C It has 3 more neutrons and 3 more electrons.

D It has 3 more neutrons, but no more electrons. ( )

If X is a noble gas, then Y will belong to Group I.

Z will belong to group II since they are consecutive. Group II elements need to lose 2 electrons in order to form Ion, so the answer is Z2+.

Ans: (D)



Practice (MCQs)

1. The atomic number of magnesium is 12. When magnesium reacts to form an ionic compound,

the electronic configuration of the magnesium ion formed is (1980 P1 Q10)

A 2, 8

B 2, 8, 1

C 2, 8, 2

D 2, 8, 3

E 2, 8, 4 ( )

2. The mass number of an atom is equal to (1980 P1 Q12)

A the number of neutrons in the atom.

B the number of protons in the atom.

C the number of electrons in the atom.

D the sum of the numbers of protons and electrons in the atom.

E the sum of the numbers of protons and neutrons in the atom. ( )

3. The atomic number of calcium is 20. The electronic structure of the calcium ion can be

represented as (1981 P1 Q2)

A 2, 8

B 2, 8, 4

C 2, 8, 8

D 2, 8, 8, 2

E 2, 8, 8, 4 ( )

4. A sodium atom has mass number 23 and atomic number 11. It follows that the atom contains

(1981 P1 Q4)

A 11 electrons.

B 11 neutrons.

C 12 protons.

D 23 neutrons.

E 23 protons. ( )

5. Which one of the following sets consists only of the electronic structures of metals?

(1982 P1 Q4)

A 2, 8, 3 2, 8, 4 2, 8, 5

B 2, 7 2, 8, 7 2, 8, 18, 7

C 2, 1 2, 8, 1 2, 8, 8, 1

D 2, 5 2, 6 2, 7

E 2 2, 8 2, 8, 8 ( )


6. The symbol for an atom of sodium can be written as 2311Na. The nucleus of this sodium atom

contains (1981 P1 Q4)

A 11 protons and 12 neutrons.

B 11 neutrons and 12 protons.

C 12 neutrons and 11 electrons.

D 11 protons and 12 electrons.

E 12 protons and 11 electrons. ( )

7. Which one of the following correctly gives the numbers of the different particles to be found in

an aluminium ion? (1984 P1 Q4)


A 10 27 13

B 13 14 27

C 13 26 13

D 13 14 10

E 27 13 27 ( )

8. What is the atomic structure of the Z2-

ion in which Z has atomic number 8 and mass number

18? (1985 P1 Q2)

electrons protons neutrons A 8 8 8

B 8 8 10

C 8 8 18

D 10 8 8

E 10 8 10 ( )

9. What is the atomic structure of a phosphorus atom? (1985 P1 Q6)


A 15 16 15

B 15 16 16

C 16 15 15

D 16 15 16

E 16 16 15 ( )


10. The atoms of five elements in the Periodic Table have atomic numbers and mass numbers as

follows:

element V W X Y Z

atomic number 4 5 6 7 8

mass number 9 11 12 14 16

Which of the following, A, B, C, D or E represents the number of neutrons in the nucleus of

each of the elements? (1985 P1 Q6)

V W X Y Z

A 4 5 6 7 8

B 5 6 6 7 8

C 8 10 12 14 16

D 9 11 12 14 16

E 13 16 18 21 24 ( )

11.

An element Z exists in three isotopic forms as shown below. (FHSS Prelim 2016)

isotope 150

Z 155

Z 157

Z

isotopic abundance 50% 25% 25%

What is the relative atomic mass of element Z?

A 150

B 152

C 153

D 154 ( )


Practice (Structured Questions) 1. Complete the following table

Element Chemical

Symbol

No of

Protons

No of

Neutrons

No of

Electrons

Electronic

Configuration

Group

No

Period

No

Metal/

Non-

Metal

Ions

Formed

Helium

Fluorine

Aluminium

Phosphorus

Potassium

Carbon

Argon

Magnesium

Sodium

Sulfur

Calcium ---

Nitride Ion

Magnesium

Ion

Oxide Ion

Chlorine Ion

Boron Ion

Potassium

Ion

Carbon Ion

Fluoride Ion

Sulfide Ion

2. Use the Periodic Table to help you to complete the following table.

Element Atomic

Number

Relative

Atomic

Mass

Particles in one atom Electronic

Structure Protons Neutrons Electrons

Magnesium 2, 8, 2

6

2, 8, 5


3. (a) Complete the following table.

(b) What name is used to describe atoms which have the same atomic number but

different mass numbers?

(c) How many electrons, protons and neutrons are present in the hydrogen ion H+?

(June 83)

4. Use the following information to answer the questions below about the elements A, B, C, D

and E.

atomic mass electronic

element number number structure

A 3 7 2, 1

B 18 40 2, 8, 8

C 8 16 2, 6

D 12 24 2, 8, 2

E 19 39 2, 8, 8, 1

(a) Which element has 22 neutrons in each atom?

(b) Is the bonding between C and E ionic or covalent?

(c) Which element is a noble gas?

(d) Which two elements form ions with the same electronic structure as neon?

(e) Which two elements are in the same Group of the Periodic Table?

(June 84)

5. Selecting from the following ions

Ca2+

Cl- Li

+ Na

+ O

2-

choose the ion that

(i) has the same arrangement of electrons as neon,

(ii) has only two electrons

(iii) has the same arrangement of electrons as Al3+

(June 87)

Mass

number

Atomic number of elements

Number of

electrons protons neutrons

sodium atom 23 11

calcium ion 40 18

chloride ion 17 20


6. Complete the following table concerning the three types of particles present in atoms.

relative mass relative charge position in the atom neutron

proton

electron

[3]

(Nov. 88)

7. An atom of an element contains twelve electrons.

(a) Name this element.

(b) In which Group of the Periodic Table is this element?

(c) Give the formula for the ion formed by this element. [3]

(Nov. 88)

Secondary 3 Chemistry Atomic Structure - Week 1.pdf · Atom • An atom is the smallest building...

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