Chem 2 Exam

How to find the number of neutrons

mass number - atomic number

How to find the number of electrons

same as the number of protons

How to find the number of protons

same as atomic number

What is the atomic weight of a hypothetical element consisting of two isotopes, one with mass = 64.23 amu (26.00%), and one with mass = 65.32 amu?

65.04 amu

The most convincing evidence for the existence of electrons came from ____.

Crookes tubes

Ernest Rutherford's model of the atom did not specifically include the ____.

neutron

Which of the following statements is false?

A. In nature some elements exist in only one isotopic form.

B. Mass spectrometers can be used to determine isotopic abundance.

C. Mass spectrometers can be used to measure the masses of isotopes.

D. Mass spectrometers detect the negative ions formed by bombarding a gas sample with low energy electrons.

E. Mass spectrometers measure the charge-to-mass ratio of charged particles.

D. Mass spectrometers detect the negative ions formed by bombarding a gas sample with low-energy electrons.

The mass spectrum of a compound shows a peak at m/z 64 with intensity 100, and a second peak at m/z 66 with an intensity of about 35. (There are also 2 peaks of very low intensity at m/z 65 and 67.)What is the probable formula of the compound, given that it could possibly contain one or more of the following elements: C, H, O, Cl, Br, S?Remember: C forms 4 bonds, N 3 bonds, O and S 2 bonds, and H,F,Cl,Br form 1 bond each.

C2H5CL

H H

I I

H-C-C-Cl

I I

H H

All of the following are true statements about electromagnetic radiation (light) except?

A. As energy increases frequency decreases.

B. The product of wavelength and frequency is constant.

C. As wavelength increases frequency decreases.

D. As wavelength increases energy decreases.

A. As energy increases frequency decreases

Who developed an explanation for the photoelectric effect?

A. Einstein

B. Rutherford

C. Millikan

D. Bohr

E. Planck

A. Einstein

One of the spectral lines in the emission spectrum of mercury has a wavelength of 623.4 nm. What is the frequency of the line?

A. 4.81 x 10^14 s-1

B. 1.87 x 10^14 s-1

C. 1.87 x 10^6 s-1

D. 4.81 x 10^6 s-1

E. 6.45 x 10^11 s-1

A. 4.81x10^14 S-1

The emission spectrum of mercury shows a line of wavelength 579 nm. How much energy is emitted as the excited electron falls to a lower energy level?

A. 3.43 x 10^-19 J/atom

B. 1.07 x10^-20 J/atom

C. 3.60 x 10^-20 J/atom

D. 5.16 x10^-20 J/atom

E. 6.05 x 10^-19 J/atom

A. 3.43x10^-19

Calculate the energy for the transition of an electron from the n = 4 level to the n = 1 level of a hydrogen atom.

A. +2.042 x 10-18 J

B. -3.165x10-19 J

C. +3.165x10-19 J

D.-2.042x10-18 J

E. -1.361x10-19 J

D. -2.042x10^-18

A(n) ____ is a region of space in which there is a high probability of finding an electron in an atom.

A. major energy level

B. core

C. shell

D. atomic orbital

E. nucleus

D. atomic orbital

What is the de Broglie wavelength of a 16.0 lb shotput moving at a velocity of 7.26 m/s? (1 kg = 2.20 lbs) [

A. 1.85 x 10^-30 m

B. 6.63 x 10^-31 m

C. 1.26 x 10^-35 m

D. 1.30 x 10^-38 m

E. 2.60 x 10^-36 m

C. 1.26x10^-35

Atomos

greek word for atom- Indivisible (means not able to be divided)

Law of Conservation of Mass

Matter can neither be created nor destroyed

Law of Constant Proportion

Compounds always contain the same elements in the same proportions by mass

Law of Multiple Proportions

If two elements form more than one compound between them, then the ratios of the masses of the second elements that combine with a fixed mass of the first element will be ratios of small whole numbers

Dalton's Theory

He proposed that all elements are composed of atoms. Atoms are indivisible and indestructible particles. Atoms of the same element are exactly alike. Atoms of different elements are different. Atoms of one element cannot be converted to atoms of another element. Compounds are formed by the joining of atoms of two or more elements.

Who proposed the current nomenclature system of using one and two letters based on the Latin names of the elements?

Berzelius

What was JJ Thomson's experiment?

Cathode Ray Tube- as the current passed through the gas, it gave off rays of negatively charged particles. This experiment gave the first hint that an atom is made an smaller particles.

Plum Pudding Model

J.J Thomsons model of an atom, atoms are made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding

Eugen Goldstein

Bored holes in the cathode, and found opposite-charged radiation behind it in 1886, for which he proposed the name canal rays

Wilhelm Wien

measured the mass of the positively charged canal rays, finding they have the mass of ions

Millikan's Oil Drop Experiment

Robert Millikan and Harvey Fletcher balanced the gravitational and electric forces on tiny charged droplets of oil suspended between two metal electrodes. They discovered charge of electron

Rutherford's Gold Foil Experiment

Ernest Rutherford fired a stream of tiny positively charged alpha particles at a thin sheet of gold foil. He concluded that an atom had a small, dense, positively charged center that repelled his positively charged "bullets"

Rutherford's Atomic Model

All of an atom's positively charged particles were contained in the nucleus. The negatively charged particles were scattered outside the nucleus around the atom's edge. According to the model, electrons move around the nucleus like planets around the sun. The flaw is that a moving charged particle would emit radiation, therefore slowing down and eventually allowing it to be captured by the nucleus.

Atomic Number(Z)

Refers to the number of protons in the nucleus of an atom. It is also the number of electrons in a neutral atom. The atomic number is a characteristic property of a particular element.

Mass number(A)

Refers to the number of protons and neutrons in an atom.

Atomic Number and Mass Number Calculations

Z=#p=#e in a neutral atom

A=#p+#n

#n=A-Z

Isotopes

Atoms with the same atomic number but different mass numbers. Same number of protons and electrons but a different number of neutrons

Natural abundance of isotopes

The relative amount of each different isotope in a naturally occurring sample of a given element is roughly constant.

Mass spectrometry

Provides a means for identifying the isotopes of an element, their relative abundances, and their exact masses

Average Atomic Mass

The weighted average of the masses of its isotopes

Mass spectrometers consist of three basic parts

An ion source, a mass analyzer, and a detector system

The stages within the mass spectrometer are

1. Producing ions from the sample

2. Separating ions of differing masses

3. Detecting the number of ions of each mass produced

4. Collecting the data and generating the mass spectrum

Light

A type of electromagnetic radiation that exists at a wide range of wavelengths

The electromagnetic spectrum

A continuum of all electromagnetic waves arranged according to frequency and wavelength.

Frequency

The number of waves that pass through a point in 1 second

c=vλ

c= speed of light (3x10^8)

v= frequency

λ= wavelength

As λ increases, v decreases, to keep c constant

E=hv=hc/λ

E- Energy of photon in joules

h- planks constant- 6.626e-34 J x S

c= speed of light

λ= wavelength

As v increases, λ decreases, E increases

Photoelectric Effect- Albert Einstein

Photons falling on a metal can knock off electrons and transfer energy to electrons released from the metal. More photons- more electrons can be knocked off

Kirchhoff's Three Laws of Spectroscopy

1. A hot solid object produces light with a continuous spectrum

2. A hot gas produces light with spectral lines at discrete wavelengths (specific colors) which depend on the energy levels of the atoms in the gas

3. Hot solid object surrounded by a cool tenuous gas produces light with an almost continuous spectrum which has gaps at discrete wavelengths depending on the energy levels of the atoms in the gas

Rydberg Equation

1/λ= R (1/nf^2 - 1/ni^2)

R= Rydberg constant= 1.097x10^7m-1

nf and ni= positive integers, such that nf

Bohr Model of the Hydrogen Atom

Electrons can jump to higher orbits(excited states) by absorbing certain quanta of energy. When electrons jump down from the excited state to the ground state, they emit energy in the form of light. The Bohr Model explained well the spectra of hydrogen atoms and those of other one-electron systems like He+, Li2+, but when it came to atoms with 2 or more electrons it proved not quite satisfactory.

Louis de Broglie

Speculated that nature did not single out light as being the only entity that exhibits a wave-particle duality. He proposed that ordinary particles such as electrons, protons, or bowling balls could also exhibit wave characteristics in certain circumstances.

de Broglie's equation

λ=h/mv

λ=wave property

h= Planck's constant=6.626x10^-34JS

mv= momentum=particle property

Consequences of Wave Nature

-Electrons do not obey classical mechanical laws like heavier objects

-Electrons do obey Quantum Mechanical Laws, and this Heisenberg Uncertainty Principle

Quantum Numbers

Describe the size, shape, and orientations of the orbitals of the electrons

Heisenberg's Uncertainty principle states

It is impossible to simultaneously know both the exact position (x) and the exact momentum (p) of a particle with infinite precision

Principal Quantum NUmber(n)

Main energy level describes the size and energy of the orbital n=1,2,3...any positive integer

Angular Momentum Quantum Number(l)

Describes the shape of the orbital. l=0,1,2,3 (n-1), corresponding to s,p,d,f... orbitals

n=1, l=0 only s subshell

n=2, l=0, 1 s and p subshell

n=3, l=0,1,2 s, p, and d subshell

n=4, l=0,1,2,3 s,p,d, and f subshells

Magnetic Quantum Numbers( m1)

Describes the orientation of the orbital in space. m1= 0, ±1, ±2, ±3...., ±l

l=0, m1=0, s subshell- spherical

l=1, m1=0, ±1, p subshell- 3 orientations

Spin Quantum Number(ms)

Describe the spin of the electron

ms= ±1/2

Each atomic orbital can contain no more than 2 electrons, one with ms= ±1/2 and the other with ms= -1/2

Diamagnetic

Atoms that contain only paired electrons are slightly repelled by a strong magnetic field- appear to weigh less

Paramagnetic

Atoms that contain unpaired electrons are attracted by a strong magnetic field- appear to weigh more

Pauli Exclusion Principle

No two electrons in an atom can have the same set of four quantum numbers, so no orbital can contain more than two electrons. As a result of Pauli exclusion principle, no more than 2 electrons wth opposite spins can be held in an orbital

Why is the energy of the 2s orbital at a value lower than that of the three 2p orbitals

The 2s electrons spend more time near the nucleus and are less effectively shielded by the core electrons than a 2p electron. So, a 2s electron experiences a higher effective nuclear charge and is held closer to the nucleus than a 2p electron. This gives the 2s orbital a lower energy than the 2p orbitals

Box diagrams

Orbitals are represented by boxes and electrons are represented by arrows

spdf Notation

Orbitals are represented by their n, l values and the number of electrons written as a superscript

Hund's Rule

The most stable arrangement of electrons is that with the maximum number of unpaired electrons, all with the same spin direction

What are the exceptions to the box diagrams

Cr and Cu have special cases; a half-filled or fully filled d shell has extra stability. This is true for all d4/d9 systems and f6/f13 systems

Electron Configuration of Ions

When atoms form ions, electrons are lost from the valence shells or added to the valence shell.

Groups

Vertical columns of elements that have similar chemical and physical characteristics. There are 18 groups

Periods

Horizontal rows in the periodic table of elements. There are 7 periods

Noble gas configuration

An outer main energy level is fully occupied, in most cases, by eight electrons

Isoelectronic

Describes atoms, ions, or molecules that share the same number of electrons, leading to the same total electron configuration and often similar chemical properties

N-3, O-2, F-, Ne, Mg+2, and Al+3 form an isoelectronic series(a group of atoms or ions that all contain the same number of electrons)

Why do atoms form ion?

-stability of noble gas configuration(ns2np6 configuration- octet of electron)

-loss or gain of electrons to attain the closest noble gas configuration

- metal lose electron(s) to form cations

- nonmetal gains an electron(s) to form anions

- Ions held by electrostatic forces to form ionic compounds

Atomic Radius

The size of an atom, measured as the typical distance from the nucleus to the outermost electron. It decreases across periods and increases down groups

Atomic Size

Lower "lefter" larger

Ion size

For an isoelectronic series of ions, the size decreases with increasing atomic number

Metallic Character trend

Metallic character decreases across a period

Period 3 for example

Na, Mg, Al are metals

Si is a metalloid

P,S are solid nonmetals

Cl, Ar are gaseous nonmetals

Metallic character increases down group

Group 4 for example

C is a nonmetal

Si, Ge are metalloids

Sn, Pb are metals

Ionization Energy

Energy required to move a ground-state electron from an atom or ion. Electrons further from the nucleus are always lost first NOTE: Second ionization always requires more energy

Ionization Energy Trend

Increase across a period (radius decreases, so electrons are held more tightly) and decrease down a group( radius increases, so electrons are held less tightly)

The stability of completely-filled and half-filled subshells modifies the trend

Electron Affinity

Energy changes are observed when an atom gains an electron NOTE: Most atoms release energy when gaining an electron, but some atoms will not take in an electron

-Most atoms release energy when they gain an electron. This is defined to be a negative electron affinity

-Some atoms require energy; this is defined to be a positive electron affinity

Electronegativity

The power of an atom, when in a molecule to attract electrons to itself

Pauling Electronegativity

measures how strongly one atom can pull on other atoms