JJC 2008 H2 Chem - Atomic Structure

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JURONG JUNIOR COLLEGE

ATOMIC STRUCTURE Content

I II

The nucleus ofthe atom; neutrons and protons, isotopes, proton and nucleon numbers Electrons; electronic energy levels, ionisation energies, atomic orbitals, extranuclear structure

(a) (b) (c) (d)

Assessment Objectives Candidates should be able to: identify and describe protons, neutrons and electrons in terms oftheir relative charges and relative masses. deduce the behaviour ofbeams ofprotons, neutrons and electrons in both electric and magnetic fields. describe the distribution of mass and charges within an atom. deduce the numbers ofprotons, neutrons and electrons present in both atoms and ions given proton and nucleon numbers (and charge). describe the contribution of protons and neutrons to atomic nuclei in terms of proton number and nucleon number. distinguish between isotopes on the basis of different numbers of neutrons present. describe the number and relative energies of the s, p and d orbitals for the principal quantum numbers 1,2 and, 3 and also the 4s and 4p orbitals. describe the shapes ofs and p orbitals. state the electronic configuration of atoms and ions given the proton number (and charge). explain the trends irr ionisation energies across a period and down a group ofthe Periodic Table. explain the factors influencing the ionisation energies of elements (see Data Booklet\. deduce the electronic configurations ofelements from successive ionisation energy data. interpret successive ionisation energy data of an element in terms of the position of that element within the Periodic Table.

(e) (, (fl

(i,

@ (h) (i) (i) (i, (i) (k)

The Sub-atomic Particles

A. l.

What are the Sub-atomic Particles? Atoms are made up of 3 types ofparticles (known as sub-atomic particles). Sub-atomic Particle

a_

p.ryn

Position in atom

(r) JF*" ;o known

neutron (n)

teons'.

)las'nuc'

electron (e)

Relative charse

Relative mass

nucleus

+1

1

rrlor

0

1

r<

small positively charged nucleus containing protons (+ve) & neutrons (neutral)

Figure

(a)

Atoms are

electricolly neutral

as no.

I

r/t840 )

up most of mass ofatom since mass of e insig. compared to

massofp&n.

Bohr's model of an atom

ofp:

no. of e.

(b) Loss of e give rise to cation (positively charged ion, no. of p > e) while gain of e give rise to anion (negatively charged ion, no. of e > p). Page

I

Nucleus makes

Bohr's model is NOT completely accurate in its representation ofthe atom (e- do not actually orbit around the nucleus) but it is sufficiently useful in explaining many of the phenomena observed in experiments.

-ve electrons in fixed energy levels around nucleus

2.

-t

oround the nucleus (in fixed energy levels)

I

I of

16

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B. 1.

Behaviour of p, n. e in Electric and Magnetic Fields

2.

In electric (,o) field Def lection

occurs. /--a-----r

ln magnetic (B) field

Beoms continue on o

stroight line poth after leaving the field

f

I

tI

p n

x : B field going into the plane Def lection storts only when beams of p, e enter

Direction of deflection predicted by FIe ming Is left-h an d r ul e.

the field

'

motion

Direction of deflection: p to -ve plate e to *ve plate n not deflected (neuhal)

magnetic

field current (similar flow of protons)

. .

Extent of deflection of a charged particle is the angle of deviation caused by ,E or B field. Extent ofdeflection depends on: charge of particle: the greotel the charge, the greoter the extent of deflection

(a) (b)

mass of particle: the .'.

e

are deflected

greater

the mass, the smoller the extent of deflection

to a greater extent than p because of its much smaller mass.

i.e. extent of deflection

oc

C. Atomic Number and Mass Number 1. Atomic Symbol represented as: mass no. (or nucleon no.) ) total no. of p * n Z: atomic no. (or proton no.) ) no. ofp

where A:

)P

Note: The atomic no. defines the element as all atoms of a particular element contain the same no. of z.

2.

dif

Thesctwo numbers give us information on the number of sub-atomic particles.

In an atom, no. of protons

:Z

In an lon,

no.ofneutrons:A-Z no. of electrons:

no. ofprotons

:Z

no. ofneutrons: A

Z

no. of e- in an anion no. of

Page 2

of i6

e

in a cation

Z

to

l

Example

1.

2.

P'

ilp"'.

p, _il,

?ls'-

f,

n _h,

{

Write the symbol for a particle of element

D. 1.

n,

l3s'

which contains 30 p.35 n and 28

Y

e,

Isotopes

Isotopes are otoms of the same element with the some number protons but different number of neutrons. ) same atomic no. but different mass no. E.g.

Exercise

tH,

'H,tHare

1

of

isotopes of hydrogen;3sCl,t7Cl are isotopes of chlorine.

which ofthe following atoms are isotopes of the same element?

wxY 36 i8

Mass number

Number of neutrons

39 20

40

2l

Number of protons

_

=

and

_

are isotopes.

2.

Isotopes of an element have similor chemicol properties. This is because chemical properties of an element are determined by the number and arrangement of electrons, and isotopes have the same no. of e. ttcl atoms accep an electron from Na to give e.g. both "C/ and Cf in the formation of sodium chloride, Na* 3scf and Na* 37Cf respectively.

3.

However, isotopes of an element have different physicol properties (mass, density, melting & boiling pts, rate of diffirsion, etc). This is because theyhave different no. of neutrons and hence, different mosses. e.g.t'cl2 has a higher density thanssclz 1as37cl2has rnore neutrons thus heavier than

Exercise 2

Recoll:

density = moss

/

vo

35Cl2)

When

lH and aHe nuclei are passed between two plates carrying a certain electric

charge, both are deflected as shown below.

(a)

Suggest the polarity (+ or -) of plate because both

lH and aHe nuclei are

(b) Why is rH deflected twice as much

'H:_p,_tr.oHe,_p,_tr Although the

A.

of IH is

as

aHe? (def lection of nuclei is due to protons)

of

aHe,

Using

of rH is thus 0 of

Page 3

of l6

rH

is twice that

that ofaHe.

of

aHe.

II

Electronic Energy Levels and Sublevels

A.

Enerw Levels / Fhells

l.

in shells of fixed energy levels identified by principol quontum numbers (n). numbers known as the Electrons move around

The larger the p.q.n (n)

) )

the greater the energy level of the shell, the further the shell is from the nucleus.

Figure

2

Evidence for different energy levels of electrons wilhin otoms cohes from the emission spectro of otoms.

The Bohr atom with labeling of the p.q.n.

I't.shell (closest to nucleus) 2no shell I n: 2......

)

n=I

a

increosing distonce from nucleus & increosing enerqy level 2.

Each energy level (shell) is made up

subshells),

of subshells

-+ which are in turn made up of orbitols.

(known as s. p. d.

f

principal quantum shells

I

V

protons

are made up

o7

[',t,''*l

neutrons

L----r-----J

!

are made un

of

lr,ffil --f v

,or-,o*,r*

electrons

B. l.

Enersy Sublevels / Subshells and Orbitals

Typ.! of subshells:

s subshell: contains

one s orbitol

three

p

orbitols d subshell: contains f ive d orbitals f subshell: contains seven f orbitols p subshell: contains

2.

An orbital is a rcgion of space in which there is a high probobility of finding an electron (although the electron is not confined to this region). Each orbitalcan accommodate 2 electrons. Page 4

of l6

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C.

Relative Energies of Energv Levels and Sublevels

4f

Increasing energy

3d

n:3

a^ L5

Figure n

*

:

I

3

Energy levels of subshells up to

--------------------- ls

n:4

a

Within each energy level (same p.q.n.), the subshells are associated with different energies:

s