Unit 1: Matter, Chemical Trends, and Chemical Bonding

atomic radius

one-half the distance between the nuclei of identical atoms that are bonded together (size of the atom)

Democritus (400 BC)

Greek philosopher that said all matter is made of tiny particles called "atomos" (Greek for invisible) or atoms

Aristotle

rejected Democritus's theory and believed that everything was composed of four elements earth, air, fire, water

John Dalton (1804)

Theorized that elements are composed of small, indivisible particles called atoms and that each unique
element is composed of a single type of atom
○ Atoms of one element are different from the atoms
of all other elements
○ During chemical reactions, atoms are combined,
separated, or rearranged into different ratios

J.J. Thomson (1897)

-discovered negative particles in atoms called electrons
-"plum pudding" model electrons (negatively charged subatomic particles) are scattered about the atom like plums in a plum pudding.
- the "pudding" part of the atom was composed of positively charged material

Thomson's Model 1897

An atom is a positively charged mass with negatively
charged particles embedded throughout
○ Negatively charged particles = electrons
○ Model known as the plum pudding model
First model to confirm that atoms are made up of
smaller subatomic particles
● Knew that atoms were neutral, so a positively-charged particle must also exist

Ernest Rutherford (1909)

● Performed the gold foil experiment
○ Shot positively charged alpha particles (He2+)
through a thin sheet of gold
○ Based on Thomson's model, most of the particles
should have gone straight through

What did Rutherford discover?

Nucleus and protons (gold foil experiment)

how did rutherford discover the proton?

Gold foil experiment (while most alpha particles went through the foil, some were deflected at large angles - Rutherford theorized that the alpha particles were hitting something dense and positively-charged)

Planetary Model

electrons move around the nucleus in fixed, circular orbits

issues with Rutherford's model

According to classical physics, electrons orbiting the nucleus should
-lose energy (and collapse into the nucleus)
-emit light

Niels Bohr (1913)

Experimented with applying electricity and thermal energy to hydrogen gas. he observed that the hydrogen atoms emitted light of only certain colors they were "excited"

Bohr's Model

electrons move in fixed in circular orbits around nucleus of an atoms (in definite energy levels)

Three subatomic particles

protons (p+), neutrons (n0), electrons (e-)

atomic number

the number of protons in the nucleus of an atom

mass number

the sum of the number of neutrons and protons in an atomic nucleus

nuclear symbol

the superscript indicates the mass number and the subscript indicates the atomic number

notation of an element

How to find the number of neutrons

mass number - atomic number

How to find the number of protons

same as atomic number

how to find the number of electrons

same as protons

different ways of writing the element

element name - mass # (ex. potassium-40) or element symbol - mass # (ex. K-40)

atomic mass unit (u)

a unit used to express the relative masses of atoms. One u is equal to 1/12 the mass of an atom of carbon-12

Isotopes

Atoms of the same element that have the same number of protons but different numbers of neutrons

difference between a isotope and a regular element?

isotopes have different masses, they have difference physical properties (some are radioactive due to the nuclei breaking down spontaneously)

how are isotopes written?

with their mass # shown (ex. Cl-37)

isotopic abundance

the percentage of a given isotope in a sample of an element

isotopic abundance formula

Atomic mass = (% abundance of isotope 1)(mass of isotope 1) + (% abundance of isotope 2)(mass of isotope 2)+...

radioactive decay

A spontaneous process in which unstable nuclei lose energy by emitting radiation

when do isotopes emit nuclear radiation?

when they are decaying

alpha particle

A cluster of 2 protons and 2 neutrons emitted from a nucleus in one type of radioactivity

beta particle

a negatively charged particle that is emitted from the nucleus of a radioactive atom

gamma ray

a form of high-energy electromagnetic radiation

Radioisotopes

Isotopes that have unstable nuclei and undergo radioactive decay.

valence electrons

Electrons on the outermost energy level of an atom

core electrons

The electrons in the inner shells of an atom; these electrons are not involved in forming bonds.

Cation

A positively charged ion

Anion

A negatively charged ion

Ion

An atom or group of atoms that has a positive or negative charge.

What electrons are involved in bonding?

valence electrons

where can you find valence electrons on the periodic table?

the last digit of the group number

what is the atomic radius of diatomic molecules?

the distance between the two nuclei divided by two

what is atomic radius measured in?

picometers (pm) (1 pm = 1 x 10^-12m)

what are the periodic trends for atomic radius?

atomic radius decreases left to right, it increases going down a group

what increases atomic radius?

more electrons means the electrons from the outermost shells will be further away

shielding or screening

a reduction in nuclear charge felt by an electron due to repulsions from other electrons in the same shell or in a smaller n-shell

effective nuclear charge (Zeff)

the net positive charge experienced by electrons in the valence shell and forms the foundation for all periodic trends

how do we calculate Zeff?

Zeff = Z - S (Zeff = effective nuclear charge, Z = atomic number, S = core electrons or the shielding constant)

where can we find the shielding constant?

by counting the number of core electrons (usually even, electrons without the valence electrons)

what are the smallest atoms in the respective periods?

noble gases

Why does atomic radius increase down a group?

increasing number of energy levels and the core electrons increases

why does atomic radius decrease left to right?

more effective nuclear charge is pulling e-, screening by core electrons is the same within the same energy level

ionic radius

Distance from the center of an ion's nucleus to its outermost electron

what is ionic radius measured in?

picometers

is a cation smaller or larger than their original atom?

smaller (the number of valence electrons decreases, this causes the # of electron orbits to decrease, the remaining electrons have a stronger attractive to the nucleus)

are anions smaller or larger than their original atom?

larger (when anions form, there is a higher electron to electron repulsion, making them spread out more)

ionization energy

The amount of energy required to REMOVE an electron from an atom (in kJ/mol)

the first ionization energy is given by (IE1)

X(g) -> X+(g) + e- (the g and the first plus sign is smaller)

the second ionization energy is given by (IE2)

X+(g) → X2+(g) + e-

the (number) ionization energy is given by (IEn)

X(n-1)+(g) → Xn+(g) + e- (every other number from 3)

do metals have a low ionization energy? how about non-metals?

metals have a low ionization energy, while non-metals have high ionization energies

do successive ionization energies increase or decrease, why?

increases, since you are removing electrons from already positively charged ions

trends in ionization energies

increases from left to right a group, increases going down a group (opposite of atomic radius)

why does ionization energy increase from left to right?

the effective nuclear charge increases so atomic radius decreases, valence electrons are held more closely to the nucleus (these factors make it harder to remove a valence electron from the atom/ion)

why does ionization energy decrease going down a group?

the atomic radius increases due to increased shielding, the attraction between valence electrons and the nucleus decreases (these factors make it easier to remove a valence electron from the atom/ion)

electron affinity (Eea)

the energy change that occurs when an electron is ADDED to a neutral atom or ion in the gaseous state

general electron affinity equation

X(g) + e- → X- (g) + energy

what does it mean if the electron affinity value is negative?

this indicates that energy is released when the electron is added to the atom/ion (if attraction > repulsion -> energy < 0, if attraction < repulsion -> energy > 0)

Trends in Electron Affinity

increases from left to right (number becomes more negative), decreases going down (number becomes more positive)

electron affinity increases from left to right because

effective nuclear charge increases so atomic radius decreases, the force of attraction between the nucleus and valence electrons increases (these factors mean that more energy is released when a new electron is acquired)

electron affinity decreases down a group because

atomic radius increases due to increased shielding, attraction between valence electrons and the nucleus decreases

Electronegativity

the ability of an atom to attract electrons in a covalent bond (A chemical bond formed when two atoms share electrons)

electronegativity increases left to right because

atomic radius decreases and effective nuclear charge increases so there is a stronger attraction for electrons

electronegativity decreases down a group because

atomic radius increases so atoms have a weaker attraction for electrons

periodic trends of electronegativity

it increases from left to right across a period and decreases from top to bottom in a group

summary of all periodic trends

atomic radius (increases, left and down) ionization energy (increases, right and up), electron affinity (increases, right and up), electronegativity (increases right and down)

ionic compounds

composed of a metallic element combined with one or more non-metallic elements

ionic bond

an electrostatic force of attraction between a positive ion and a negative ion, DONATES its electrons (electrons are transferred from the metal atom(s) to non-metal atom(s), NON-METALS + METALS)

When do cations form?

When a neutral atom LOSES one or more electrons to form a positively charged ion (brackets around the element with a plus on the side + e-)

when do anions form?

when a neutral atom GAINS one or more electrons to form a negatively charged ion (brackets around the element with a minus sign on the side)

are anions smaller than original atoms or larger?

larger, there is more electrostatic repulsion between the negatively charged electrons

are cations smaller or larger than their original atoms?

smaller, the removal of electrons causes the electrons to come closer together since there is less electrons for electrostatic repulsion

Lewis symbols (dot structures)

the chemical symbol for an atom surrounded by one or more dots representing the valence electrons

structure of ionic compounds

consists of alternating units of positive and negative ions in a rigid arrangement called a crystal lattice structure (no single unit of an ionic compound, they exist as a giant, endlessly repeating lattice of ions)

formula unit

the lowest whole-number ratio of ions in an ionic compound (tiny section of the repeating pattern)

properties of ionic compounds

- high melting and boiling points due to many strong bonds between the ions, since the attraction between ions of opposite charges
- many (but not all) are soluble in water (water molecules are able to pull apart the ions and separate them)
- conduct electricity as molten liquids (but not as solids) or when dissolved in water
- hard, but brittle

Why do ionic compounds conduct electricity only when molten or in solution?

in liquid form, ions are free-flowing and are able to carry electric charges

Electrolyte

An ionic compound whose aqueous solution conducts an electric current

Why are ionic compounds brittle?

They are brittle because when the ions are forced out of position, like charges line up and cause repulsion and cracking of the ionic compound.

molecular elements

elements whose particles are multi-atom molecules

molecular compound

a pure substance composed of molecules made up of two or more non-metallic elements

properties of molecular substances

- can exist as gases, liquids, and solids at room temp. (solids can be soft, waxy, flexible, or hard)
- generally have low boiling and melting points
- poor conductors of electricity
- molecular elements are typically gases at room temp. due to their low boiling points, typically do no dissolve readily in water

covalent bond

a covalent bond forms when a pair(s) of electrons are SHARED by two atoms (usually occur between non-metals)
- The shared electrons are attracted to both nuclei, but simultaneously repelled by the other electron
- Each atom achieves a full valence shell and has a stable electron arrangement (like a noble gas)

each atom in a covalent bond has a

bonding capacity (the number of covalent bonds that it can form)

bonding electron

an electron, in the valence shell of an atom, that is available to form a covalent bond with another atom

Three types of covalent bonds

single - 2e- (share one PAIR of electrons, double 4e-(share two PAIRS of electrons, triple 6e-(share three pairs of electrons)

lone pairs

pairs of valence electrons remaining that are not involved in covalent bond formation

octet rule

States that atoms lose, gain or share electrons in order to acquire a full set of 8 valence electrons

how can we represent molecules?

- lewis structures, lines represent a pair of bonding electrons, pairs of individual dots are used to represent lone pairs

exceptions to the octet rule

atoms in period 3 and higher can have an expanded octet (valence shell with more than 8e-), beryllium and boron can have an incomplete octet (valence shell with less than 8e-)