Topic 2- Atomic Structure

IB Chemistry SL Study PowerPoint

2.1 The structure of atoms

• Most atoms contain protons, electrons, and neutrons.• A neutron has a mass that is slightly greater

than, but almost the same as, that of a proton (mass of a proton- 1.66x10-27 kg).• A proton has a positive charge, while a neutron

has a neutral (no) charge.• Protons and neutrons are called nucleons, and

are both found in the nucleus.

• Electrons are very tiny in comparison to protons and neutrons (its mass is 1/1830 [around 1/2000] that of a proton.)• Electrons have negative charges, and orbit the nucleus

of an atom.• The charges of protons and electrons are equal in

strength, but are opposite in polarization (+, -)• Therefore, an atom with equal numbers of electrons

and protons has no overall charge.

Relative Mass Relative ChargeProton 1 + 1

Neutron 1 0

Electron 5 x 10-4 - 1

Isotopes of Elements

• All elements can be characterized by two numbers: the atomic number and the mass number• Atomic number (Z)- equal to the number of

protons in the nucleus of an atom; also the number of electrons (since charge neutrality must be maintained); atoms of different elements have different atomic numbers

• Mass number (A)- number of protons and neutrons in an atom; if two atoms of the same element have different mass numbers, then one is an isotope.• Isotopes are atoms of the same element with different

numbers of neutrons from the standard.• Atoms of the same element will always have the same

number of protons, but the number of neutrons may vary.

• Isotopes of atoms are written in the following format:• A

ZX , where A is the mass number, Z is the atomic number, and X is the elemental symbol• Examples: 35

17Cl- 17 protons, 18 neutrons

• 3717Cl- 17 protons, 20 neutrons

• Ions- charge is written to top right of element symbol- Na+, K+, Cl-, O-2, Br-, Ca+2

Properties of Isotopes

• Isotopes have identical chemical properties as the standard atoms (chemical properties are based on electron arrangement.)• However, isotopes will have different physical

properties, because the isotopes will have different masses than the standard atoms.• Density, rate of diffusion, melting point, and

boiling points will differ

Uses of Isotopes

• Some isotopes are radioactive (radioisotopes.)• Uses for radioisotopes include- nuclear power

generation, sterilization of surgical instruments, crime detection, metal stress detection, food preservation, artifact dating, and medical diagnosis and treatment (radiology.)

2.2 The Mass Spectrometer

• Machine that determines the mass of individual elements.• Principle of operation- positively charged

particles pass through a magnetic field that deflects the ions along a circular path on a radius that is proportional to the mass to charge ratio, m/e

Steps

• Vaporization- atoms are vaporized either before/immediately after injection.• Ionization- vapor is ionized by bombarding vapor

with a stream of high energy electrons to make positive ions- M(g) + e- M+

(g) + 2e-.• Acceleration- positive ions pass through holes in

parallel plates, where they are accelerated.

• Deflection- ions are deflected by an external magnetic field- heavier/less charged ions are deflected less; lighter/more charged ions are deflected more.• Detection- detector measures mass to charge

ratio and the relative amounts of ions present.• From this, the mass spectrum for an element can

be determined.

Diagram

Calculation of Relative Atomic Mass

• Relative atomic mass can be calculated from mass spectrum; equal to weighted mean mass of all naturally occurring isotopes of that element relative to 1/12 of Carbon-12• Draw spectrum on board

• Total detector current (sum of values)- 6.83+9.13+2.60+12.17+2.60= 33.33• Abundance of germanium-70= charge for

germanium-70/total charge 6.83/33.33=20.5%• All relative abundances are calculated in the

same way.• Ar= (70x20.5)+(72x27.4)+(73x7.8)+(74x36.5)+(76x7.8)/100

• Ar= 72.7

2.3 Electron arrangement

• When metal ions are heated, have electricity added to them, etc., they release energy in the form of colored light, which is a form of electromagnetic radiation.• Each color has a specific wavelength and

frequency, and all of the colors released are in the visible light spectrum.• Each color also has a particular amount of

energy.

The electromagnetic spectrum

• EM waves can travel through space and matter• There are two relationships between the speed

of light [m/s], wavelength (λ) [m], and frequency (f) [1/sec].• C = λ x f• EM radiation is also a form of energy, and its

energy [J] is related to its frequency [1/sec] and Plank’s Constant (h) [6.63x10-23 J s]• E = h x f

• Smaller wavelengths have higher frequency and more energy• Larger wavelengths have lower frequency and

less energy• The EM spectrum has a range of wavelengths

from high-energy gamma (γ) radiation to low energy radio waves.

Emission Spectra

• When observed through a spectrometer, each of the elements gives its own colors and lines at fixed wavelengths- this is the emission spectrum of the element.• Emission spectra are not continuous, but are

separate lines that converge on the higher end of the spectrum.• The EM spectrum is a continuous spectrum,

while the emission spectrum is a line spectrum.

• Each series (ultraviolet, infrared, radio, microwave, gamma wave, etc.) has its own set of lines that converge at the higher end of the spectrum (convergence point.)• Note: the letter n is used to describe which

energy level an electron is on. This is the known as the principal quantum number.

Explanation of emission spectrum

• Electrons travel in orbits around the nucleus of an atom, and each orbit corresponds to a fixed energy level or shell.• Addition of energy to an electron will cause it to rise

to a higher energy level (ground state excited state.)• When the electron falls back down, light (a quantum)

with a specific energy is emitted (this energy corresponds to a particular wavelength of light; one of the separate lines on the emission spectrum.)

• From this, the line spectrum is formed- why?- electrons can only exist at fixed energy levels, not in between. • When an electron falls just 1 energy level, light

with a large wavelength (relative to the series) is released. When the electron falls 2 energy levels, more energy is released. As the electron falls more energy levels, more energy is released, until the electron falls from infinity energy levels (which is the point of convergence on the emission spectrum.)

• Electrons dropping to the lowest energy level (n=1) emit the most energy, so the ultraviolet spectrum for hydrogen is produced.• The visible spectrum occurs when electrons fall

to the second energy level (n=2.)• The first infrared series of the hydrogen

spectrum occurs when electrons fall to the third energy level (n=3.)

Electron arrangement

• Each energy level as described by the principal quantum number can only contain a certain number of electrons.• Generally, when an energy level is full, the electrons

will begin to fill on the next energy level.• Exception- third energy level- after 8 electrons are on

n=3, 2 electrons are added to n=4, then 10 more electrons complete the third energy level.

Energy Level Number of Electrons

1 2

2 8

3 8/18

• Atomic numbers can be used to determine the atomic electron arrangements, as the atomic number determined the number of electrons in a neutral atom.

Element Electron Arrangement

Element Electron Arrangement

H 1 Na 2.8.1

He 2 (first level full) Mg 2.8.2

Li 2.1 Al 2.8.3

Be 2.2 Si 2.8.4

B 2.3 P 2.8.5

C 2.4 S 2.8.6

N 2.5 Cl 2.8.7

O 2.6 Ar 2.8.8 (third full)

F 2.7 K 2.8.8.1

Ne 2.8 (second level full)

Ca 2.8.8.2

• 2.8.8.2• First level- 2 electrons• Second level- 8 electrons• Third level- 8 electrons• Fourth level- 2 electrons

Picture citations

• http://www.kollewin.com/EX/09-15-03/electromagnetic-spectrum.jpg• http://www.mhhe.com/physsci/chemistry/carey/

student/olc/graphics/carey04oc/ch13/figures/1334.gif

• General- IB Chemistry Course Companion, Geoffrey Neuss, Second Edition