CHEM 132 - Ch 2 Atoms, Molecules and Ions (Exam 1)

What are the four postulates of Dalton's atomic theory?

1) Matter is made of indivisible atoms. 2) All atoms of an element are identical. 3) Compounds form when atoms combine in fixed whole-number ratios. 4) Chemical reactions rearrange atoms; they do not create or destroy them.

What is the law of definite proportions?

A given compound always contains the same elements in the same fixed proportion by mass, no matter the sample or its source.

What is the law of multiple proportions?

When two elements form more than one compound, the masses of one element combining with a fixed mass of the other are in small whole-number ratios. CO and CO2 have a 1:2 oxygen ratio.

What did J.J. Thomson's cathode ray experiment discover?

The electron — a negatively charged particle far lighter than any atom. It showed atoms are divisible, and gave the charge-to-mass ratio.

What was Thomson's plum pudding model?

Electrons scattered through a diffuse sphere of positive charge, like plums in a pudding. Rutherford's experiment disproved it.

What did Millikan's oil drop experiment determine?

The charge on a single electron (about -1.602 x 10^-19 C), which combined with Thomson's ratio gave the electron's mass.

What did Rutherford's gold foil experiment show?

Most alpha particles passed straight through, but a few deflected sharply back — so the positive charge and nearly all the mass sit in a tiny, dense NUCLEUS, and the atom is mostly empty space.

What are the three subatomic particles and their charges?

Proton +1 (in nucleus), neutron 0 (in nucleus), electron -1 (outside nucleus).

How do the masses of protons, neutrons and electrons compare?

Protons and neutrons are each about 1 amu. An electron is roughly 1/1836 of a proton — negligible for mass calculations.

What is the atomic number (Z)?

The number of PROTONS in the nucleus. It defines which element an atom is.

What is the mass number (A)?

The total number of protons PLUS neutrons in the nucleus.

How do you find the number of neutrons in an atom?

Neutrons = mass number (A) minus atomic number (Z).

What are isotopes?

Atoms of the SAME element (same protons) with DIFFERENT numbers of neutrons, and therefore different mass numbers. Carbon-12 and carbon-14.

What are isobars?

Atoms of DIFFERENT elements that happen to share the same mass number. Argon-40 and calcium-40.

What is an ion?

An atom or group of atoms carrying a net electrical charge because it has gained or lost electrons.

What is a cation and how does it form?

A POSITIVELY charged ion, formed when an atom LOSES one or more electrons. Metals typically form cations.

What is an anion and how does it form?

A NEGATIVELY charged ion, formed when an atom GAINS one or more electrons. Nonmetals typically form anions.

What is atomic mass on the periodic table actually reporting?

The weighted AVERAGE mass of all naturally occurring isotopes of that element, weighted by their natural abundance.

How do you calculate average atomic mass from isotopes?

Multiply each isotope's mass by its fractional abundance, then add the results.

What is an atomic mass unit (amu)?

Exactly 1/12 the mass of one carbon-12 atom, about 1.66 x 10^-24 g.

What is a molecule?

Two or more atoms held together by covalent bonds, acting as a discrete unit.

What is the difference between an empirical and a molecular formula?

The EMPIRICAL formula gives the simplest whole-number ratio of atoms (CH2O). The MOLECULAR formula gives the actual number in one molecule (C6H12O6 for glucose).

What is a structural formula?

A representation showing how the atoms are actually connected and, often, the geometry — not just how many of each there are.

What is an ionic compound?

A compound held together by electrostatic attraction between cations and anions, typically a metal with a nonmetal. It forms a lattice, not discrete molecules.

What is a covalent compound?

A compound in which atoms share electrons, typically nonmetal with nonmetal. It forms discrete molecules.

What is a polyatomic ion?

A group of covalently bonded atoms carrying an overall charge, acting as a single unit. Sulfate SO4 2-, nitrate NO3 -, ammonium NH4 +.

What is Avogadro's number?

6.022 x 10^23 particles per mole. It is a counting unit, like a dozen, chosen so one mole of a substance weighs its atomic mass in grams.

What is a mole?

The amount of substance containing exactly 6.022 x 10^23 elementary entities — atoms, molecules, ions, whatever you specify.

What is molar mass and what are its units?

The mass of one mole of a substance, in grams per mole (g/mol). Numerically equal to the atomic or formula mass in amu.

How do you convert grams to moles?

DIVIDE by the molar mass. moles = grams / (g per mol).

How do you convert moles to grams?

MULTIPLY by the molar mass. grams = moles x (g per mol).

How do you convert moles to number of particles?

Multiply by Avogadro's number, 6.022 x 10^23.

Calculate the molar mass of CO2 given C = 12.01 and O = 16.00.

12.01 + 2(16.00) = 44.01 g/mol.

How many moles are in 36.0 g of water (molar mass 18.02 g/mol)?

36.0 / 18.02 = 2.00 mol.

What is the difference between 44.01 amu and 44.01 g/mol for CO2?

44.01 amu is the mass of ONE molecule. 44.01 g/mol is the mass of a MOLE of them. Numerically identical, physically very different.

How do you calculate percent composition by mass?

(mass of that element in one mole of compound / molar mass of the compound) x 100.