The Atom

Question 1

Dalton’s atomic theory

Answer 1

1. atoms are tiny particles of matter
2. atoms of an element are similar to each other and different from those of other elements
3. atoms of 2 or more different elements combine to form compounds
4. atoms are rearranged to form new combinations in a chemical reaction: they cannot change into atoms of another element

Question 2

JJ Thomson’s Cathode Ray experiment

Answer 2

cathode ray tube
1. applied a high electrical voltage at either end of the tube
2. found that the beam of particles, cathode rays, traveled from the negatively charged electrode to the positively charged electrode
3. measured the charge-to-mass ratio of the cathode ray particles by deflecting them using electrical and magnetic fields

he measured a value of -1.76 x 10^8 coulombs per grams imlying the cathode ray particles were about 2000x lighter than hydrogen

Question 3

what are the properties of the particles that compose the cathode ray?

Answer 3

they travel in straight lines

independent of the composition of the material from which they originate

carry a negative electrical charge

Question 4

what did thomson realise from his experiment?

Answer 4

cathode rays contain negatively charged particles

particles have a much smaller mass than the atom

Question 5

rutherfords fold foil experiment

Answer 5

rutherford employed alpha particles at an ultrathin sheet of gold foil. the particles that came close to the atomic nuclei are deflected from their straight path. rutherford realized that to account for the deflections he observed, the mass and positive charge of an atom must be concentrated in a space much smaller than the size of the atom itself.

Question 6

rutherfords proposal of the nuclear theory of the atom

Answer 6

• most of the atom’s mass and all of its positive charge are contained in a small core called the nucleus
• most of the volume of the atom is space throughout which negatively charged electrons are dispersed
• there are many negatively charged electrons outside the nucleus as there are positively charged particles within the nucleus, so the atom is electrically neutral

Question 7

who proposed that the previously accounted mass was due to neutrons

Answer 7

James Chadwick

Question 8

atomic diameter

Answer 8

10^-10m

Question 9

nuclear diameter

Answer 9

10^-15m

Question 10

atomic mass unit

Answer 10

1amu has a mass equal to 1/12 of the mass of the 12C atom

1amu= 1 dalton

Question 11

mass of proton

Answer 11

1.672 x 10^-24

Question 12

mass of a neutron

Answer 12

1.675 x 10^-24

Question 13

mass of an electron

Answer 13

9.109 x 10^-28

Question 14

atomic number Z

Answer 14

the number of protons in an atom

Question 15

mass number A

Answer 15

the number of particles in the nucleus

Question 16

what holds protons and neutrons together?

Answer 16

neutrons are necessary to make protons stick together: they interdigitate between the protons to stabilize them with a force that goes beyond coulomb force

Question 17

how to calculate atomic mas

Answer 17

the weighted average of all naturally occurring isotopes of that element

Question 18

hydrogen isotopes

Answer 18

deuterium and tritium

Question 19

how to calculate the atomic mass?

Answer 19

• use the experimentally % abundance of each isotope of the element
• multiply the %abundance by the atomic mass of that isotope
• sums the abundance-weighted masses of all isotopes

Question 20

Brownian motion

Answer 20

Robert brown looked through his microscope at water-suspended particles that come from pollen grains. he noticed that the pollen particles were in continuous motion. einstein’s model explained the motion brown had observed as a result of the molecular bombardment of the particles due to the thermal energy of the surrounding water

Question 21

which microscope can form images of atoms themselves?

Answer 21

scanning-tunnel microscope STM

Question 22

law of conservation of mass

Answer 22

matter is neither created nor destroyed

Question 23

law of definite proportions

Answer 23

all samples of a given compound have the same proportions of their constituent elements

Question 24

law of multiple proportions

Answer 24

when 2 elements form 2 different compounds, the masses of element B that combine with 1g of element A can be expressed as a ratio of whole numbers

Question 25

cations

Answer 25

positively charged ions

Question 26

anions

Answer 26

negatively charged ions

Question 27

mass spectrometry

Answer 27

measured the masses of atoms and percent abundances of isotopes of elements

Question 28

mass spectrometry procedure

Answer 28

1. sample is injected into the instrument and vaporised
2. the vaporised are ionized by an electron beam, thus removing electrons creating positively charged ions
3. the ions are accelerated into a magnetic field
4. when ions drift through a mf they experience a force that bends their trajectory.
5. ions strike a detector and produce an electrical signal that is recorded

the amount of bending depends on the mass of the ions: the trajectories of lighter ions are bent more than those of heavier ones

Question 29

avogadro’s number

Answer 29

1 mol: 6.022 x 10^23

Question 30

molar mass

Answer 30

the mass of one mole of atoms of an element

Question 31

value of one mole

Answer 31

equal to the number of atoms in exactly 12g of pure carbon-12