Chapter 4

Atomic Structure

___________• means indivisible, from Democritus (Greek,

400BC)• smallest particle of an element that retains the

chemical properties of that element

____________: transformation of substances into one or more new substances

Up to late 1700’s, info about reactions were qualitative

Technology improved scales/balances so that quantitative analysis would be more accurate.

Dalton’s Atomic Theory

In ______, John Dalton transformed Democritus' ideas into scientific theory

1) All ________ is composed of tiny indivisible particles called atoms

2) Atoms of the same __________ are identical – size, mass, etc…Atoms of different elements are different.

3) Atoms of different elements combine in simple whole-number ratios to form _________________

4) In chemical reactions, _______________________, separated or rearranged. (but not created, destroyed or transformed)

Dalton’s Atomic Theory

Modern Atomic Theory

Dalton thought atom was solid & indivisibleThere are 3 major differences between

Dalton’s and the Modern theory1) Atoms are made up of ___________________ –

protons, neutrons & electrons2) Atoms can be changed from one element into

another – by __________ reaction3) Atoms of the same element are not all exactly

the same - ___________

Structure of the Atom

Atom: smallest particle of an element that retains the _____________________ of that element

Although teeny tiny, the atom can be seen using a scanning tunneling microscope.

Once we can see it, we can manipulate itThere is an emerging branch of chemistry…

___________________ that is concerned with those things that are very small… like microchips etc

Structure of an AtomAtom consists of ___________________ – that

occupy the small nucleus and the larger surrounding area

_________ at the center of the atom contains ________ (positively charged) and ________

_____________ (negatively charged) occupy a region surrounding the nucleus

… but how did we discover these things?

Discovering the Electron

Late 1800’s, experiments were carried out on Cathode Ray tubes – basic research to see what would happen if electric current passed through a gas at low Pressure producing ______________________

J.J. Thomson (English guy) used this device to calculate the ratio of charge of these cathode ray particles to their mass, which he found to be negatively charged and constant

These cathode ray particles are now known as ________________________

1909: Robert Millikan (American physicist) showed that mass of electron ~ 1/2000 mass of Hydrogen atom (its actually 1/1837th)

Millikan’s experiments confirmed that ____________________________________________________________________________________________

More Conclusions from Millikan

1) _______________, therefore, there must be a positive charge to balance the negative electrons

2) Electrons are a small % of the atom’s mass, therefore there must be _______ ______that make up most of the atom’s mass

Protons

Since atoms are neutral, atoms must contain positive charge as well.

Thompson believed that the atom was a cloud of positive charge with negative electrons embedded into it.

This is known as the ________________________.

The Nuclues

In 1911, Ernest Rutherford did an experiment where he bombarded a thin Gold foil with ______ _______– small, positively charged particles

They believed the _______________________ evenly distributed

Rutherford, along with Geiger & Marsden, expected the alpha particles to simply go straight through, like a small ball bearing through a thin veil, but some were actually deflected back

Rutherford concluded that the atom consisted of a very small, dense centre that is positively charged, which he called the _____________

Also, that the nucleus was surrounded by a very large empty region in which the electrons occupied, like planets around the sun

Later, it was calculated that mass of a proton is ___________________________of an electron, even though they have equal but opposite charge

The Neutron

The __________ is just slightly heavier that a proton but is neutral

All atoms, except ____________, contain neutronsAn atom is neutral when it contains equal numbers

of protons and electronsi.e. # protons = # electrons

It is solely the _____________that determine what type of element the atom is

Protons, which are positively charged, can stay close to each other in the nucleus because of _______________

Some Facts…

Particle Symbol Charge Mass(kg) Relative Mass

__________ e- -1 9.109 x 10-31 1__________ p+ +1 1.673 x 10-27 1836__________ no 0 1.675 x 10-27 1837

Size of atom is measured in picometres (pm) = 10-12m

Counting Atoms

_____________(Z): of an element is the number of protons in the nucleus of each atome.g. every Carbon atom has 6 protons (and also 6 e-)

The Periodic Table is arranged in order of ____________________________

The Atomic Number identifies the element(look at Periodic Table at back of book)

Isotopes: are atoms of the _____________________________different masses.

Because all atoms of the same element must have the same number of protons (and electrons), they can only differ in the number of neutrons, which do not change the atoms identity

EXAMPLE: • Hydrogen consists of just 1 proton & 1 electron and

is also known as __________ • There is another kind of Hydrogen that consists of 1

proton, 1 electron & 1 neutron, called _________which is only 0.015% of all Hydrogen

• There is still another type of Hydrogen which has 1 proton, 1 electron & 2 neutrons, called _________

_________________(A): is the total number of protons & neutrons in the nucleus of an atom

So, for Hydrogen:Name Atomic #(Z) Mass #(A) # protons # neutrons protons+neutronsProtium 1 0 1Deuterium 1 1 2Tritium 1 2 3

So, # Neutrons = Mass #(A) – Atomic #(Z)

__________: general term for any isotope of any element

Isotope/Nuclide Symbol p+ e- no

Protium 11H 1 1 0

Deuterium 21H 1 1 1

Tritium 31H 1 1 2

Atomic MassesMass (not weight) of atoms is very, very small, i.e. around

____________Better to use a relative scale to compare mass of atoms to

each other, but you need a standard.Choose ______, which has 6 protons & 6 neutrons, 12

6C, also known as ______________

It has been assigned a mass of exactly _________________units (amu)

So:_________= 1/12 mass of Carbon-12 = 1.660540 x 10-27kg

All other atoms are determined using this relative scale.

______________________: is the weighted average of the atomic masses of the naturally occurring isotopes of an element

Example from book: Copper-________ of all Copper is Copper-63 (63

29Cu) which has a mass of 62.929599 amu

-________ of all Copper is Copper-65 (6529Cu) which has a

mass of 64.927793 amu

Calculate Average Atomic Mass

(_______ x 62.929599amu) + (______ x 64.927793amu)= 63.545642 amu = 63.55 amu (2DP)

(Revise Z#, A#, Atomic Mass & Average Atomic Mass)

Suppose you have:10 pens (20%) “weighing” ½ oz each, and40 pens (80%) “weighing” ¾ oz each.

What is the average mass of each pen?

Or, another way:(20% x ½ oz) + (80% x ¾ oz)

= (0.2 x ½ oz) + (0.8 x ¾ oz) = 0.7 oz