chapter 2 - part 2 -the structure of the atom

8/7/2019 Chapter 2 - Part 2 -The Structure of the Atom

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THETHE ATOMICATOMIC

STRUCTURESTRUCTURE


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The existence of atoms was proposed as

early as in the 5th century by the Greek

philosophers Leucippus and his pupilDemocritus, for which they were called

atomists.


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Democritus, develop the idea of atoms. He

asked this question:

If you break a piece of matter in half, andthen break it in half again, how many

breaks will you have to make before you

can break it no further?


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Democritus thought that it ended at some

point, a smallest possible bit of matter. He

called these basic matter particles, atoms.


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The word "atom" is derived from the Greek

word "atomos", which means "indivisible".

The history of the development of atomic

models begins with John Daltons

discovery.


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J.J. Thompson (1856J.J. Thompson (1856--1940)1940)

An Englishman discovered electron. He

described the atom as a sphere of positive

charge which contains a few negatively-charged particles called electron


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ERNEST RUTHERFORDERNEST RUTHERFORD

(1871(1871--1937)1937)

Discovered proton, apositive charge in anatom. The positive charge

and most of the mass ofthe atom areconcentrated in a small,central region called thenucleus.

The electron move in aspace that is larger thanthe space occupied bythe nucleus.


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Neils BohrNeils Bohr

Proposed that the

electrons in an atom

move in shells aroundthe nucleus.


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James ChadwickJames Chadwick

James Chadwick proved the existence of

neutrons, the neutral particles in the

nucleus. Neutrons contribute

approximately to half the mass of an atom.


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Modern Atomic ModelModern Atomic Model

The atomic model in the present day is

based on the contributions of the abovescientists.


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According to the modern atomicAccording to the modern atomic

model :model :

1.The central nucleus consists of protons

and neutrons. It containing almost all themass of the atom.

2.The nucleus of an atom is very small

compared to the size of the atom


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3. the electrons are orbiting outside the

nucleus in the electron shells

4. the electrons are moving in electronshells at a very high speed and we

cannot determine the position of the

electrons at a particular time


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The subatomic particles of an atomThe subatomic particles of an atom

An atom contains three types of subatomic

particles:

proton,

neutron and

electron,


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The proton and neutron form the nucleus

at the centre of an atom.

The electron moves around the nucleus ata very high speed.


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The nucleus is positively charged because

of the presence of protons, which are

positively charged. The neutrons are

neutral.


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Proton number and nucleon numberProton number and nucleon number

Proton Number

The proton number represent the number

of protons found in the nucleus of an atom. Proton number = the number of protons

The proton number is also known as the

atomic number.


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In an atom of neutral charge, the number

of electrons also equals the atomic

number.

Hence, the proton number of an atom can

also represent the number of electrons.


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Nucleon Number

The nucleon number, also called atomicmass number or mass number, is the

number of protons plus the number of

neutrons in an atomic nucleus.

Nucleon number = Number of protons +

Number of neutrons


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The nucleon number of an atom is about

the same as the mass of the atom

because the mass of an electron is very

small and can be ignored.

Example


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Nucleon number = proton

number + number of neutrons


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Atoms of various elements differ from one

another because of the different number of

protons.

Each element has a specific proton

number.


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For example, sodium (Na) has a proton

number of 11 and oxygen has a proton

number of 8


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Symbol of ElementsSymbol of Elements

Each element is given

a name and a symbol.

Examples :

Element Symbol

Hydrogen H

Beryllium Be

Lithium Li

Argon Ar


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- Each symbol consists of one or two

letters ( H, Li)

- For elements with two-letter symbols, the

first letter is always a capital letter whilethe second letter is always a small lettter.


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The standard representation for an atom

of any element shows the proton number

and the nucleon number of the element.

A( nucleon number )

X( symbol of element )Z ( proton number )


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Sometimes an element is represented byusing only the nucleon number.

For example : 23

Na11

is reperesented by sodium - 23


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Isotopes and theirImportanceIsotopes and theirImportance

We have learnt that the number of protons

in atoms of the same element is always

the same.

However, chemists discovered that some

elements could have atoms with different

masses.


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Therefore, these atoms must have

different number of neutrons.

Such atoms are called isotopes.

Isotopes : are atoms of the same elements

with different numbers of neutrons


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Three important points to define isotopes.

1. Isotopes are different atoms of thesame element.

2. Isotopes have the same number ofprotons or same proton numbers.

3. Isotopes have different numbers ofneutrons or nucleon numbers.


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Uses of isotopes in ourdaily lives

1. There are two types of isotopes, namely

- the stable isotopes (non-radioactive)

- the non-stable isotopes (radioactive).


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2. Unstable isotopes go through

radioactive decay and emit radiation and

they are known as radioisotopes.

3. Radioisotopes have many applications

in daily life.

4. Several uses of radioisotopes in daily

life are shown as below :


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cobalt-60 is used to kill cancer cells

without surgery in patients. This treatment

is known as radiotherapy.

Gamma rays of cobalt-60 are used to

destroy bacteria in food without changing

the quality of food.


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Electron Configuration in Atom

- We have learnt that electrons occupy

orbits with definite energy level of an atom.

- These orbits with definite energy level areknown as the shell.

- Every single shell is capable of holding up

to certain amount of electrons.


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- The first shell can hold up to two

electrons. This is called a duplet.

- The second shell can hold up to eight

electrons. This is called an octet.

- The third shell can hold up to eighteen

electrons.


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- The electrons in the outermost occupied

shell are known as the VALENCE

ELECTRON.

chapter 2 - part 2 -the structure of the atom

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