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ArOMS AND RADPOACT!VITY

Inside atoms

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Charge essentials

There are two types of eieciric charge: positive {+) and negatri/e (-). Like charges repet; unlike charges attract.

A aimplt; model of the atom

Everything iii madę of atoms. Atoms are far Loo smali tu be scen wilii any ordinary microscope - there ure morę than a billion billion of them on the surface oftliis tuli stop. Howerer, by shooting tiny aminie partieles through atoms, scientists have been able to develop models (descriptions) of their struć turę. [n advanced work, scientists use a maLhcmatieul model of the atom. However, the simple modei below is often used to explain the basie idcas.

! element	Chemical symbol	atomie number (proton number)
hydrogen	H	1
helium	He	2
lithium	Li	3
beryllium	Be	4
borort	B	5
carbon	C	6
n itrogen	N	7
oxygen	O	8
radium	Ra	88
thorium	Th	90
u rani u m	U	92
piutonium	Pu	94

An atom is madę up of smailer partieles:

•    There is a central nucleus madę up of protons and neutrons. Around rhis, electrons orbit at high speed. The numbert of partieles depends on

the iype of atom.

•    Protons have a positive (+) charge. Electrons haee an equal negative {-) eharge. Normally, an atom has the same number of electrons as protons, so lis to tal charge is zero.

•    Protons and neutrons are called nucleons. Each is about 1800 times morę massive than an electron, so virrually all of an aiom's mass is in its nucleus.

•    Electrons are held in orbit by the furce of attracrion between opposite charges, Protons and neutrons are bound tightly togerher in rlie nucleus by a different kind of force, called the strong nuelear force,

Elements and atomie number

AJl materials are madę from about 1.00 basie substances called elements. An atom is rbe smallest ‘piece’ of an element you can have. Each element has a different number of protons in its atoms: k has a different atomie number (sometim.es called the proton number), There are some esamples on the left. The atomie number also tells you tłie number of electrons in the atom.

Isotopes and mass uurnber

The atoms oi any one element are not all esuetly alike. Some may have morę neutrons than others. Thcse different cersions of the element are called isotopes. They havc identical Chemical properties, although their atoms have different masses. Most elements are a mixture of two or morę isotopes. You can see some exampies in the chart on drc opposite page.

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The co tal number of protons and neutrons in the nucleus is calkd the mass mass number (nudeon number) number (or nu cl eon number). Isotopes hcwc the same atomie number but \

different mass numbers. For exumple, the metal lithium (atomie number 3) is    ' U — symbol for element

a mimire of two isotopes with mass numbers 6 and 7. Lithium-7 is Lhe morę j

cornmon: over 93% of lichium atoms are of his type. On the right, you can    _at'_{m|c number} (proton number)

see how to represent an atom uf lithium-7 using a symbol and numbers,

, j    u.:X .'.7,, f-f-f    ' _e= eiedrori {-) i

■ . . element • f -    . ^: _i • ' • fi, _i; • '    . 'ć • " .'    .    , .V.ć.    • ;    . Isotopes .0.,:. yy;¹;. 7'    '■ l; '.'.fp = proton _fm) ¹

fiydrogen

helium.

. [ithium

The periodic table groups ai) the elements according to their Chemical properties, For exampie_r all the elements in Group I are highly reactiye metals - and aII have a single electron :n their outer shefl.

Electron shclls

Electrons orbit the nucleus at certain lixcd levels only. These levcls are called shells. Ją^trons normally occupy shdls as near the nucleus as possible, but there is a limit to how many electrons each Shell can hołd - for example, no morę than 2 electrons tn dte first shell and 8 in the second. It is an atom’s outermost electrons whieh form the Chemical bonds with otber atoms, so elements with similar electron arrangements have similar Chemical properlies.

For ąuestions 4 and 3, you will need data frum che table of elements on the opposite page.

3    Chlorine is a mixture of two isotopes, with mass numbers 33 and 37. Whar is the difference between the two cypes of atom?

4    In symbol form, nitrogen-14 can be written ^{1 2}iN How can each of the folio wing be written?

a) carbon-12 b)oxygen-l6 c) radium-226 3 Atom X has 6 electrons and a mass number of 12. Atom Y has 6 elecLrons and a mass number of 14. Atom Z has 7 neutrons and a mass number of 14. Idcntify the elements X, Y, and Z.

Related topics: electńc charge 8,01-8,02; experimerstal ev^;,dence for nucleus 11.08

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   An atoms eoncains electrons, prosura, and nemrons. Whieh of these partieles

a) are outside the nucleus b) are uucharged

c) have a negative charge d) are nucleons e) are much lighter than the othersr

2

   An aluminium atom has an atomie number of 13 and a mass number of 27. How many

a) protons b) electrons c) neutrons does it have?