Atomic theory

LecturePLUS Timberlake 1

Atomic Theory

Atoms are building blocks of elements

Similar atoms in each element

Different from atoms of other elements

Two or more different atoms bond in simple ratios to

form compounds


Subatomic Particles

Particle Symbol Charge Mass

Electron e- 1- 1/1840

Proton p+ + 1

Neutron n 0 1

Charge measure in terms of electronic charge: e=1.6x10^-19

Mass is measure in unified atomic mass: units(u)1 u is 1/12 of the mass of a carbon-12 atom.


Location of Subatomic Particles

10-13 cm electrons

protons

neutrons

10-8 cm

nucleus


Atomic Number

Counts the number of

protonsin an atom


Atomic Symbols

Show the mass number and atomic number

Give the symbol of the element

mass number

23 Na sodium-23

atomic number 11


Number of Electrons

An atom is neutral The net charge is zeroNumber of protons = Number of electronsAtomic number = Number of electrons


Subatomic Particles in Some Atoms

16 31 65 O P Zn

8 15 30

8 p+ 15 p+ 30 p+

8 n 16 n 35 n8 e- 15 e- 30 e-


Isotopes

Atoms with the same number of protons, but

different numbers of neutrons. Atoms of the same element (same atomic

number) with different mass numbers Isotopes of chlorine

35Cl 37Cl17 17

chlorine - 35 chlorine - 37


Learning Check

Naturally occurring carbon consists of three isotopes, 12C, 13C, and 14C. State the number of protons, neutrons, and electrons in each of these carbon atoms.

12C 13C 14C 6 6 6

#p _______ _______ _______

#n _______ _______ _______

#e _______ _______ _______

Nuclear Density

• Mass of proton m(p)=1.67 x 10^-27 kg• Radius of proton r = 0.80 x 10^-15 m

• Find the density=

IsotopesAtoms made of protons, neutrons and electrons.

Neutrons+ Protons: Nucleus

• Electron + Proton # influences-> Chemical Properties• Protons+ Neutron # influences-> Physical Properties

IsotopesAtoms with same number of protons but different

number of protons are Isotopes.

Radioactivity

• A stable nuclei emits radiations. Radiation was discovered by Henri Becquerel in 1896. These radiations are not influenced by any outside conditions such as temperature or pressure.

• It is a spontaneous process, but it occurs at fixed intervals. Hence a proportion of sample decay will occur in that time interval.

Three particles present in these Radiations:

• Alpha, Beta And Gamma

AlphaBeta

Gamma

AlphaBeta

Gamma

• We know that nuclei consists of protons and neutrons, if the balance between these two types of particles is too far to one side, the nucleus may emit alpha or B radiations.

• Gamma emitted after alpha or B radiation to release excess energy.

A third conserved quantity

• In radioactive decay, both nucleon number A and proton number Z are conserved.

• On comparison b/w mass before and after decay, we witness a decrease in mass.

• This is because energy lost caries mass- recall E=mc^2, so we witness a net decrease in energy.

Properties of Ionizing radiation

• On moving past atoms, alpha or B collide/rub with them and this as a result causes to knock or drag away electrons from an atom.

• This process is called Ionization.• Now, Energy is lost during ionization.

Size: Motion: Ionization Ability:• Alpha: more mass &charge | slow | Most• Beta:lighter | faster | Moderate• Gamma: no charge | Least

Lesson ContentsLesson Contents1. Physical properties of a, b

and g2. Penetrating power of a, b

and g3. N v Z graphs4. Decay laws

Alpha RadiationAlpha RadiationAlpha particles contain two protons and two neutrons

Alpha RadiationAlpha Radiationa has the same constitution as a helium nucleusAlpha particles may be written as

They have a double positive charge and a mass of 4 u

Alpha• Since most effective ionization therefore,

most energy is lost during travel, hence cover least distance and in a cloud chamber, most prominent tracks are formed.

Detection of all three:

Beta-minus RadiationBeta-minus RadiationBeta-minus particles are electrons

Beta-minus RadiationBeta-minus Radiationb- is produced when a neutron decaysBeta-minus particles may be written as

They have a negative charge and a mass of 1/1800 u

Beta-minus RadiationBeta-minus Radiation

b- is produced when a neutron decays

The surplus mass is released as kinetic energy in the b- and as an antineutrino

+ Energy

Gamma RadiationGamma RadiationGamma rays are a form of electro-magnetic radiation

Gamma RadiationGamma Radiationg release is often associated with a or b decayGamma rays remove energy from an unstable nucleus

Type of radiation emitted &

symbol

Nature of the radiation

(higher only)

Nuclear Symbol

(higher only)

Penetrating power, and what will block it (more dense material, more radiation is absorbed BUT smaller mass or charge of particle, more

penetrating)

Ionising power - the ability to remove electrons from atoms to form positive ions

Alpha

a helium nucleus of 2 protons and 2

neutrons, mass = 4, charge = +2

Low penetration, biggest mass and charge, stopped by a few cm of air

or thin sheet of paper

Very high ionising power, the biggest mass and charge of the three radiation's, the

biggest 'punch'!

Beta

high kinetic energy electrons, mass = 1/1850, charge = -

1

Moderate penetration, 'middle'

values of charge and mass, most stopped by a few mm of metals like

aluminium

Moderate ionising power, with a smaller mass and charge than the alpha particle

Gamma

very high frequency

electromagnetic radiation, mass =

0, charge = 0

Very highly penetrating, smallest

mass and charge, most stopped by a thick layer of steel or concrete, but

even a few cm of dense lead doesn't stop all of it!

The lowest ionising power of the three, gamma radiation carries no electric

charge and has virtually no mass, so not much of a 'punch' when colliding with an

atom

Penetrating powerPenetrating power

Effect of Magnetic Fields

Decay laws - alphaDecay laws - alphaWhen an isotope emits an a particle Its nucleon number decreases by 4 Its proton number decreases by 2For example:

Decay laws – beta-minusDecay laws – beta-minus

When an isotope emits a b- particle Its nucleon number is unchanged Its proton number increases by 1For example:

Decay laws – beta-plusDecay laws – beta-plusWhen an isotope emits a b+ particle Its nucleon number is unchanged Its proton number decreases by 1For example:

Decay lawsDecay lawsTry writing the nuclear equations for the decay of these isotopes

Decay lawsDecay lawsAnswers

Decays

Randomness and Decay:

Spontaneous:• Decays not effected by the presence of other

nuclei.• No chemical reaction, external factors like

pressure and temperature effect the decay.• Random:• Impossible to predict the time• Each nucleus has an equal probability to decay

Atomic theory

Education

Transcript of Atomic theory