chem quest #2

How to calculate Atomic Mass

Number of protons + Number of neutrons = atomic mass amu

Why decimal places

The atomic mass of a single atom is a whole number. Element masses are based on Carbon-12 = 12 amu. Hydrogen (1 amu) is 1/12th the mass of carbon

Whats an Isotope

A version of an element with a unique mass and number of neutrons

Isotopes

Nearly every element on the periodic table has at least two isotopes

Relative Abundance

The differing amounts isotopes are found in nature

Trace

the isotope was found in very small (trace amounts)

Calculating weighted average

A_r = abundance of isotope x mass of isotope / 100%

Steps for calculating weighted average

1. Set equation with subscripts

2. sub in values

3. final answer with units

Radioisotopes

a radioactive isotope

Properties of radioisotopes

very unstable nucleus

can occur naturally but many are created artificially

when the nucleus decays, energy is released in the form of radiation

Atomic radius

The distance from the centre of an atom (nucleus) to the valence shell

nuclear charge

since the number of protons determines the positive charge, more protons results a stronger pull on electrons

electron shells

electrons only exist where electron shells or energy levels allow them to

a change in the number of energy levels affects the distance electrons can be from the nucleus

shielding

inner electrons block, or shield, the attractive force between outer electrons and the nucleus

electron-electron repulsion

electrons in the same shell repel each other since they have the same charge. an increase of protons outweighs this affect

Trends in the periodic table across a period

An increase in the number of protons in the nucleus, the number of shells and shielding remain constant, atomic radius decreases (because there is a greater nuclear charge/ pull on electrons, making them closer to the nucleus, therefore smaller)

Trends in the periodic table down a group

despite the increasing number of protons in the nucleus, there is an increasing number of electron shells and shielding between the nucleus and the valance electrons, making the atomic radius bigger since there is less of an attraction to the nucleus. this also makes it easier to remove electrons (electronegativity and ionization)

What is a trend

a pattern with exceptions, making it different from a rule

Ionic radius

ideally, the distance from the centre of an ion (nucleus) to its valence shell. Though, it is difficult to measure consistently

Metals

Metals tend to lose all of their outer valance electrons to form cations

Non-metals

non-metals tend to gain electrons to fill their valance shells to form anions. they have the same number of protons and the same amount of shells and shielding, but they have a different amount of electrons, causing more electron repulsion.

Cation and anion size

an anion is larger than its parent atom

cations are smaller than their parent atom (due to fewer shells and less shielding)

cations are smaller than most anions

Ionic radius across a period

Across a period, the ionic radius decreases because there is an increase in nuclear charge. The shielding between the nucleus and the valence electrons remains constant. Despite the repulsion between electrons in the same shell, the stronger pull from the nuclear charge causes the overall ionic radius to become smaller. Metallic cations and non-metallic anions are isoelectronic.

Ionic radius down a group

Down a group, the ionic radius increases as the number of electron shells increases. The additional shells place the valence electrons farther from the nucleus and create more shielding between the nucleus and the valence electrons. Despite the increasing number of protons in the nucleus drawing electrons inward, the effect of the added shells and greater shielding is stronger, so the nucleus has less pull on the valence electrons. As a result, the ionic radius becomes larger down a group.

Electron affinity

ideally, the measure of the attractive force an atom has for adding an electron to its valance shell

nuclear charge electron affinity

more protons in the nucleus create a greater attraction to an additional electron

electron shells / shielding

if there is an increased distance and shielding, the attraction will be decreased between the nucleus and incoming electrons

electron affinity across a period

an increasing number of protons in the nucleus increases attraction between the nucleus an incoming electrons. the number of shells and shielding between the nucleus and incoming electrons is constant. there is an increase in electron affinity.

electron affinity down a group

despite the increasing number of protons in the nucleus increasing attraction between the nucleus and incoming electrons there is an increasing number of shells and shielding between the nucleus and the incoming electron, therefore a less attractive force. electron affinity decreases.

first ionization energy

a measure of the energy required to remove one electron from the valance shell of a neutral atom

nuclear charge fie

more protons in the nucleus will have a greater attraction to the valance electrons increasing the amount of energy needed to remove them

electrons/shielding

more electron shells results in a greater distance between outgoing electrons and the positively charged nucleus

greater distance = decreased attraction to valence electrons, making them easier to move

across a period fie

increasing number of protons in the nucleus, increasing attraction between the nucleus and outgoing electrons. the number of shells and shielding remains constant, so thereʼs no blocking the attraction between the nucleus and the outgoing electron

down a group fie

despite the increasing number of protons in the nucleus, there is an increase in attraction between the nucleus and the outgoing electron. there is an increasing number of shells and shielding, which adds more distance and decreases the attraction between the nucleus and the outgoing electron

Electronegativity

the ability of an atom to hold on to valance electrons in a covalent bond

it is a relative measurement and helps determine the type of bond between two elements (pure covalent, polar covalent, ionic bond)

greater the attractive force between a nucleus and shared electrons = greater the electronegativity of an element

across a period electronegativity

there is an increasing number of protons in the nucleus, increasing nuclear charge and attraction between the nucleus and the shared pair of valance electrons

number of shells and shielding remain constant, causing no change or disruption to the attraction between the nucleus and the shared pair of valance electrons

down a group electronegativity

despite the increasing number of protons in the nucleus there is an increased number of shells and shielding, blocking the attraction between the nucleus and the shared pair of valance electrons

key word summary

• electron affinity - incoming electron

• electronegativity - shared pair of valance electrons

• fie - outgoing electron

• if protons do not make a difference (typically down a group) - despite

  • Down a group (top → bottom)

    • Protons are increasing, but their effect is outweighed by the addition of more shells and more shielding.  This is where you use “despiteˮ (because the trend goes in the opposite direction of what youʼd expect from just proton increase).