2. Atomic Structure & Periodic Trends

Question 1

What is the atomic number (Z)? What is the mass number (A)?

Answer 1

Z = the number of protons in the nucleus of an atom.
A = mass number is the number of protons and neutrons (nucleons) 

we usually refer to atoms by their mass number:
beryllium-9

Question 2

t or f, isotopes have the same atomic number, but differing mass numbers.

Answer 2

true

an isotope is an atom with differing number of neutrons (therefore the mass number changes)

Question 3

what is atomic weight?

Answer 3

elements exist naturally as a collection of their isotopes. Atomic weight is a weighted average of these isotopes based on their prevalence in nature.

Question 4

what holds protons and neutrons together in the nucleus?

Answer 4

the strong nuclear force. It is the strongest force in nature but only acts over very small distances. (it must overcome proton-proton repulsion).

Question 5

Explain Alpha decay.

Answer 5

large nuclei release 2 protons and 2 neutrons (alpha particle/helium) to become more stable. Alpha particles are high energy, but lose this energy fast and therefore stop quickly.
Our skin can stop alpha particles.

Question 6

Explain Beta (-) decay.

Answer 6

an unstable nucleus converts a neutron into a proton and an electron. It then ejects that electron which is called the beta-(-) particle.

here, Z increases by one, A stays the same

Question 7

Explain Beta(+) decay.

Answer 7

Also known as positron emission. An unstable nucleus converts a proton into a neutron and a positron. the positron (B+ particle) is emitted.

here, Z decreases by one, A stays the same

Question 8

Explain Electron capture.

Answer 8

Here an atom captures an electron from its closest shell (n=1 shell) and uses it to convert a proton into a neutron.

Here, the Z decreases by one, A stays the same.

Question 9

What form of beta decay is the most common?

Answer 9

Beta (-) decay is the most common. if not specified, beta (-) decay is default.

Question 10

Explain gamma decay.

Answer 10

When an excited atom (often one that has previously undergone alpha or beta decay) ejects a gamma photon. This photon has no mass or charge but can travel very far with a lot of energy.

unlike alpha and beta decay, gamma decay does not change the atoms composition at all.

Question 11

summarize how an atom’s composition changes for all 5 forms of radioactive decay.

Answer 11

alpha: loses 2 protons and 2 neutrons (A down by 4, Z down by 2)
B- : Z up by one (converts neutron to proton)
B+ : Z down by one (converts proton to neutron)
E capture: Z down by one (converts proton to neutron)
gamma: no change in composition.

Question 12

What is the equation for the half-life of a radioactive substance?

Answer 12

N = No (1/2) ^ T/t

N = substance left
No = starting mass
T = time passed
t = half life

Question 13

Explain nuclear binding energy and the mass defect.

Answer 13

Nuclear binding energy = energy needed to split a nucleus apart.
When nucleons bind together to form a nucleus, some mass is converted to energy. The loss in mass is called the mass defect

Δm = (sum of all nucleons mass) - mass of nucleus

Question 14

Emission spectrum: each element gives off a unique spectrum of light when current is passed through it. The energy of the photons released can be explained by what equation?

Answer 14

E = hf = h(c/λ)

h = planck’s constant = 6.6 x 10e-34 js

Question 15

Bohr model of the atom. Explain absorption and emission of electrons.

Answer 15

Bohrs model explains quantized energy states that surround an atom. Electrons can only occupy these quantized states and nothing in-between.

Absorption: electrons can capture energy and get excited to an outer shell of an atom
Emission: electrons can release energy and fall down to a lower shell (lower energy shell).

due to this quantized nature, emission spectra is not continuous but rather a set of unique lines.

Question 16

t or f, the distance an electron jumps or falls in the Bohr model is directly related to the energy absorbed or emitted.

Answer 16

true. if an electron jumps from ground state to n=4, that is more energy absorption than if it went to n=2

Question 17

write the equation for the energy of the released photon when an electron falls from n=4 to n =2.

Answer 17

E = hf 
E = E2 - E4 

hf = E2 - E4

Question 18

With atoms that only contain one electron (H, He+m etc), we can use what formula to calculate the energy of a discrete energy level?

Answer 18

En = (-2.2 x 10e-18 J) / n^2

Therefore, we can use this formula to find the difference between energy levels

ΔE = E2 - E1 (substitute in above equation)

Question 19

true or false, atoms with one proton are considered Bohr atoms.

Answer 19

false! Atoms with one electron (H Li 2+, etc) are Bohr atoms.

Question 20

The quantum model of an atom. Explain

1. the energy shell
2. the energy subshell

Answer 20

1. the energy shell (n): designates the discrete energy level an electron occupies
2. the energy subshell (s,p,d,f): explains electron orbitals, most likely areas of electron density

Question 21

What is diamagnetic and paramagnetic?

Answer 21

Diamagnetic: paired electrons in an electron orbital Paramagnetic: lone electron in an electron orbital. Paramagnetic atoms are attracted to magnetic fields (the lone electron wants a partner)

Question 22

Electron configurations. Explain

1. Aufbau principle
2. Hunds rule
3. Pauli exclusion principle

Answer 22

1. Aufbau principle: Electrons occupy the lowest energy states first.
2. Hunds rule: Electrons of the same subshell must occupy all subshell orbitals before pairing up. (e.g. p oribital has 3 orientations, one electron in each before pairing)
3. Pauli exclusion principle: a maximum of 2 electrons can occupy one orbital at a time.

Question 23

Why are noble gases non-reactive?

Answer 23

because they have a full valence shell (i.e. they complete the octet rule with their electron configuration).

Question 24

t o f, diamagnetic atoms are attracted to magnetic fields

Answer 24

false, they are repelled (paramagnetic are attracted)

Question 25

periodic table: what are periods what are families

Answer 25

period = row = horizontal 
family = group = column = vertical

Question 26

electron configurations: what occurs when we get to the d-block?

Answer 26

the electron energy shell (n) that were are in is the period number - 1.

e.g. bromine
[Ar]-4s2-3d10-4p5

Note: for the f block we subtract 2 (won’t have to deal with this likely)

Question 27

What major exception applies to filling out electron configurations?

Answer 27

d-block electrons prefer to be half fulled or entirely fulled (5 or 10 electrons). s orbital electrons can be promoted into the d-shell leaving 1 electron in the s shell.

Question 28

t or f, atoms that gain an electron (anions) accommodate that electron in the lowest energy orbital possible.

Answer 28

true

Question 29

What is isoelectronic?

Answer 29

isoelectronic indicates two atoms with the same electron configuration.

Neon and F- are isoelectronic

Question 30

What happens if a transition metal loses an electron? How does the electron configuration change?

Answer 30

When electrons are ionized, they leave the highest energy shell (n) available. This means that transition metals lose their s shell electrons before their d shell electrons.

Ti+ –> [Ar]-4s1-3d2

Question 31

What is the electron configuration of Cu+

Answer 31

transition metal promotes s electron in d shell
copper is [Ar]-4s1-3d10
Cu+ is [Ar]-3d10
since the largest energy shell loses its electrons first (n=4)

Question 32

What is the difference between an excited state atom and an ion?

Answer 32

an ion has lost or gained electrons. An excited state atom is existing in a non-ideal electron configuration. For example oxygen 1s2-2s2-3p4 may exist as
1s2-2s2-3p3-4s1. an electron has become excited into the 4th energy shell

Question 33

Explain why alkali and alkali earth metals are strong reducing agents while halogens are strong oxidizing agents?

Answer 33

alkali and alkali earth metals may lose 1 or 2 electrons, respectively, to obtain a full outer ring (which is stable). They really want to be oxidized making them strong reducing agents

halogens really want to be reduced to have 8 valence electrons. Therefore they are strong oxidizing agents. This is also why halogens naturally exist in a diatomic state.

Question 34

What is electron shielding and effective nuclear charge?

Answer 34

the more electrons exist between the nucleus and the valence electrons, the more negative repulsion the valence electrons feel. This reduces the positive nuclear charge they feel giving an effective nuclear charge (Z-eff < Z)

Question 35

What is ionization energy?

Answer 35

the amount of energy required to remove an electron from an atom. Note that first ionization energy is always the easiest. (IE1 < IE2)

Question 36

What is electron affinity?

Answer 36

EA is how much an atom wants an electron. If an electron is added and energy is released (E is negative), than the addition was favorable.

Question 37

What is electronegativity?

Answer 37

EN is a measure of an atoms ability to pull electrons towards itself in a covalent bond. FONClBrISCH

Question 38

Explain acidity of atoms.

Answer 38

Acidity explains how likely an atom would give up a hydrogen (donates a proton) or accept electrons.

Question 39

periodic trends: explain the trends of 
atomic radius 
ionization energy 
electron affinity 
electronegativity 
acidity

Answer 39

in increasing value

atomic radius –> bottom left (electron shielding and number of shells)
ionization energy –> top right (EN)
electron affinity –> top right
electronegativity –> top right
acidity –> bottom right (accepting electrons or donating protons: larger nuclei stabilize the negative charge better)

Question 40

t or f, noble gases have very negative electron affinities.

Answer 40

False - despite the periodic trend (increasing top and to the right), noble gases do not want more electrons as they already satisfy the octet rule.

Note that exceptions can exist in periodic trends.