Atoms

Question 1

Isotopes

Answer 1

• Atom of the same element with a different number of neutrons.
• therefore it has a different mass number ~ different physical properties
• has the same number of protons (atomic number) and electrons ~ react chemically in the same way.

Question 2

Relative atomic mass ( Ar)

Answer 2

The weighted mean mass of an atom of an element, relative to one twelfth of the weight of an atom of carbon-12.

Ar = (mass of each isotope x % abundance of each isotope) / 100

Question 3

Relative isotopic mass

Answer 3

The isotopic mass of an isotope, relative to one twelfth of the mass of carbon-12.

Question 4

Relative molecular mas (Mr)

Answer 4

The weighted mean mass of a molecule of a compound, relative to one twelfth of the mass of an atom of carbon-12.

Question 5

Relative formula mass

Answer 5

The weighted mean mass of a formula unit compared with one twelfth of the mass of an atom of carbon-12.

Question 6

Daltons theory- 1800s

Answer 6

• atoms are tiny particles that make up elements
• atoms cannot be divided
• all atoms of a given element are the same.
• atoms of one element are different from those of a different element.

Question 7

Thompson- 1897

Answer 7

Discovered that cathode rays were a stream of particles and is now known as electrons.

electron properties:
- they have a negative charge
- deflected by a magnet and electric field
- very very small mass

Plum pudding model:
Atoms are made up of negative electrons moving around in a sea of positive charge.

Question 8

Rutherford-1909

Answer 8

Fired alpha particles at a thin sheet of gold foil.

Results:
- most did not deflect at all
- small % were deflected at various angles
- very few deflected back to the source

Conclusions:
- The positive charge and most of the atoms mass is located in the centre in the nucleus.
- negative electrons orbit the nucleus
-most of the atoms volume is located in the space between the nucleus and the orbiting electrons.
- The overall positive and negative charge must balance.

Question 9

Modern Day atom

Answer 9

• protons and neutrons are found in the nucleus
• electrons orbit the nucleus in shells
• most of the atom is empty space.

Question 10

positive ions with 1+ charge

Answer 10

Hydrogen ~ H+
lithium ~ Li+
sodium ~ Na+
potassium ~ k+
silver ~ Ag+
ammonium ~ NH4+
copper(I) ~ cu+

Question 11

positive ions with 2+ charge

Answer 11

Magnesium ~ mg 2+
Calcium ~ ca 2+
Barium ~ Ba 2+
Zinc ~ Zn 2+
Lead (II) ~ Pb 2+
Copper (II) ~ Cu 2+
Iron (II) ~ Fe 2+

Question 12

positive ions with 3+ charge

Answer 12

Iron(III) ~ Fe 3+
Aluminium ~ Al 3+

Question 13

Negative ions with 1- charge

Answer 13

Fluoride ~ f-
chloride ~ Cl-
Bromide ~ Br -
Iodide ~ I-
Hydroxide ~ OH-
Nitrate ~ NO3 -
Nitrite ~ NO2 -
Hydrogen carbonate ~HCO3 -
Manganate ~ MnO4 -

Question 14

Negative ions with 2- charge

Answer 14

oxide ~ O2-
sulfide ~ S2-
sulfate ~ SO4 2-
Sulfite ~ SO3 2-
Carbonate ~ CO3 2-
Dichromate ~ Cr2O7 2-
Chromate ~CrO4 2-

Question 15

Negative ions with 3- charge

Answer 15

Phosphate ~ PO3 3-
Nitride ~ N 3-

Question 16

Writing formulae for compounds

Answer 16

• in an ionic compound the charges of the metal and non metal have to balance.

Examples:
Calcium Oxide ~ CaO
( The positive 2 charge and negative 2 charge cancel out)

Sodium Carbonate ~ Na2CO3
(Na has a +1 charge and carbonate has a 2- charge therefore 2 sodium are needed to balance the charges)

Chromium (III) oxide ~ Cr2O3
(Cr has a 3+ charge and oxide has a 2- charge, so the charges are switched)

Question 17

Formation of ions in the periodic table

Answer 17

• metals lose electrons to form cations
• non metals gain electrons to form anions.
• Be ,B,C and Si do not form ions as the loss of their outer electrons requires too much energy.
• Transition metals except zinc and silver can form ions of different charges.

Question 18

Representing elements in equations

Answer 18

• elements are shown as their symbol because they exist as giant structures.
• some exist as small molecules so are shown as follows:

Hydrogen ~ H2
nitrogen ~ N2
oxygen ~ O2
fluorine ~ F2
Chlorine ~ Cl2
Bromine ~ Br2
Iodine ~ I2
Phosphorus ~ P4

Question 19

Mass spectrometry

Answer 19

• used to identify the abundance of isotopes of elements and the structure of unknown compounds.

Uses:
- monitoring the breath of anaesthetised patients during surgery
- detecting banned substances in athletes
- analysing compounds in space

How it works:

• inject the sample
• the sample is ionised as a gas
• electrons are fired at the sample to turn the atoms into positively charged ions.
• the sample is accelerated into magnetic field using positively charge plates
• the magnetic field deflects and separates ions based on their m/z value(all the relative isotopic masses of the element)
• the lighter the isotope, the more it is deflected

Question 20

interpreting a mass spectrum

Answer 20

x axis ~m/z ratio
(the ratio of the mass of each ion to its charge)

y axis ~ relative abundance (%)

Question 21

why is the term RELATIVE used?

Answer 21

We want to know how much HEAVIER one subatomic particle is COMPARED with another .

                  OR

What the CHARGE of one subatomic particle is COMPARED to another.

We are not concerned with ACTUAL masses or charges.

Question 22

The atom : 1913-1932

Answer 22

1913 ~ Neils Bohr
- Electrons orbit the nucleus in shells.
- Helped to explain the energy of electrons at different distances from the nucleus

1918 ~ Rutherford
Discovered the proton

1932 ~ James Chadwick
Discovered the neutron.

Question 23

Subatomic particle masses and charges

Answer 23

Proton:
charge ~ 1+
mass~ 1

Electron :
charge ~ 1-
mass ~ 1/2000

Neutron :
Charge ~ 0
Mass ~ 1