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Química Geral · Atoms, moles and stoichiometry

Átomos, isótopos e a tabela periódica

Uma lição resolvida gratuita e 40 problemas de prática em que você mesmo desenha cada seta curva.

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O que você vai aprender

  • Describe the proton, neutron and electron by charge, mass and location, and connect the gold-foil and oil-drop experiments to the nuclear atom and the elementary charge.
  • Read and write isotope notation, and count protons, neutrons and electrons in atoms, monatomic ions and polyatomic ions.
  • Calculate an average atomic mass from isotope masses and abundances, and work backwards to an unknown abundance or isotope mass.
  • Interpret the mass spectra of elements, including the peak patterns of Cl₂ and Br₂ molecules built from two isotopes.
  • Locate elements by group and period, name the main groups, and classify elements as metals, nonmetals or metalloids.

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Ler a aula gratuita (7 partes)

Inside the atom

An atom has a tiny, dense nucleus of protons and neutrons surrounded by electrons. The proton carries +1 elementary charge and has mass 1.00728 u; the neutron is uncharged with mass 1.00866 u; the electron carries −1 and has mass 0.000549 u, about 1/1836 of a proton. The elementary charge e is 1.602 × 10⁻¹⁹ C. Nearly all the mass of an atom sits in the nucleus, which is only about 10⁻¹⁵ to 10⁻¹⁴ m across while the atom is about 10⁻¹⁰ m across. Two experiments built this picture. In the gold-foil experiment almost all α particles passed straight through a thin gold foil, but a very small fraction bounced back through large angles: the positive charge and mass must be packed into a very small nucleus, and the rest of the atom is mostly empty space. In the oil-drop experiment every measured droplet charge was a whole-number multiple of one smallest charge, 1.602 × 10⁻¹⁹ C, so charge comes in units of e.

Atomic number, mass number and isotopes

The atomic number Z is the number of protons; it defines the element. The mass number A is the number of protons plus neutrons, so neutrons = A − Z. Isotope notation writes A as a left superscript and Z as a left subscript: ⁵⁶₂₆Fe has 26 protons and 30 neutrons, and is also written iron-56 or ⁵⁶Fe. Isotopes are atoms of the same element (same Z) with different numbers of neutrons, such as ¹²C, ¹³C and ¹⁴C. Because isotopes have the same number of protons and electrons, they have the same electron arrangement and essentially the same chemistry; they differ in mass. Atoms of different elements with the same A, such as ⁴⁰Ar and ⁴⁰Ca, are not isotopes. The atomic mass unit is defined as exactly one twelfth of the mass of a ¹²C atom, so ¹²C is exactly 12 u and every other isotope mass, such as ¹⁶O at 15.9949 u, is measured relative to it.

Counting particles in ions

An ion forms when an atom gains or loses electrons; the nucleus does not change. A cation has fewer electrons than protons and an anion has more: electrons = Z − charge. ⁵⁶Fe³⁺ has 26 protons, 30 neutrons and 26 − 3 = 23 electrons, and ³²S²⁻ has 16 protons, 16 neutrons and 16 + 2 = 18 electrons. For a polyatomic ion, add the protons of every atom and then correct for the charge. The ammonium ion (drawn) has 7 + 4 × 1 = 11 protons and 11 − 1 = 10 electrons. Species with the same number of electrons are isoelectronic: Na⁺, Mg²⁺, F⁻ and Ne all have 10.

Average atomic mass

The atomic mass printed in the periodic table is the abundance-weighted average of the isotope masses: average = Σ (fractional abundance × isotope mass). Gallium is 60.11% ⁶⁹Ga (68.9256 u) and 39.89% ⁷¹Ga (70.9247 u): 0.6011 × 68.9256 + 0.3989 × 70.9247 = 41.4312 + 28.2919 = 69.723 u. The average lies closer to the more abundant isotope, and no single gallium atom has a mass of 69.723 u. To work backwards for two isotopes, let x be the fraction of the lighter one: x × 68.9256 + (1 − x) × 70.9247 = 69.723 gives x = (70.9247 − 69.723)/(70.9247 − 68.9256) = 1.2017/1.9991 = 0.6011, or 60.11%. With three isotopes and one abundance unknown, use the fact that the fractions sum to 1. Always convert percentages to fractions, and use isotope masses rather than mass numbers when precise data are given.

Mass spectra of elements

A mass spectrometer ionizes a sample, separates the ions by mass-to-charge ratio (m/z) and records how many arrive at each m/z. For singly charged atomic ions, each peak is one isotope and the peak heights give the relative abundances; together with the isotope masses they give the average atomic mass. Chlorine atoms give peaks at m/z 35 and 37 in a ratio of about 3 : 1, and bromine atoms give m/z 79 and 81 in a ratio of about 1 : 1. Diatomic molecules combine two atoms at random. Br₂ (drawn) gives molecular ions at m/z 158 (⁷⁹Br⁷⁹Br), 160 (⁷⁹Br⁸¹Br) and 162 (⁸¹Br⁸¹Br). The middle peak can form two ways, ⁷⁹Br first or ⁸¹Br first, so with equal abundances the heights are ½ × ½ : 2 × ½ × ½ : ½ × ½ = 1 : 2 : 1. The middle peak is not a third isotope. Cl₂ likewise gives three molecular-ion peaks, at m/z 70, 72 and 74, whose heights follow from multiplying the chlorine abundances in the same way.

The periodic table

The modern periodic table lists elements in order of atomic number, not atomic mass. That is why tellurium (Z = 52, 127.60 u) comes before iodine (Z = 53, 126.90 u): iodine belongs with the halogens it resembles. Rows are periods (1 to 7) and columns are groups, numbered 1 to 18. Groups 1, 2 and 13 to 18 are the main-group elements, groups 3 to 12 are the transition metals, and the lanthanides and actinides are the inner transition metals shown below the table. Named groups: group 1 alkali metals (hydrogen sits there but is a nonmetal), group 2 alkaline earth metals, group 16 chalcogens, group 17 halogens, group 18 noble gases. Metals lie to the left of a staircase line running from boron to astatine, nonmetals to the right; the metalloids B, Si, Ge, As, Sb and Te sit along it. Periods 1, 2 and 3 hold 2, 8 and 8 elements; periods 4 and 5 hold 18 each; periods 6 and 7 hold 32 each because they include the f block. Main-group metals tend to lose electrons to reach the electron count of the previous noble gas (Na⁺, Ca²⁺, Al³⁺), and nonmetals tend to gain electrons to reach the next one (N³⁻, O²⁻, Cl⁻).

What to carry forward

Z counts protons and fixes the element; A − Z counts neutrons; electrons = Z − charge. Isotopes share Z and chemistry but differ in mass, and the atomic mass is their abundance-weighted average, which you can also solve backwards for an unknown abundance or mass. In a mass spectrum, atomic peaks are isotopes, while diatomic molecules give combined peaks whose heights are products of the isotope abundances. The periodic table is ordered by Z, and an element's group and period locate it among the metals, nonmetals and metalloids.

Problemas de prática (40)

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Mass spectra of elements

  • Molecular-ion peaks of Cl₂Predict the positions and relative heights of the molecular-ion peaks of a diatomic element with two isotopes.

The periodic table

  • Predict the ion from the element's positionPredict the common monatomic ion of a main-group element from its group and period, and count its electrons.

Classificar 6

The periodic table

  • Find the alkaline earth metalRecognize a member of a named group of the periodic table.
  • Find the metalloidClassify period 3 elements as metals, metalloids or nonmetals.
  • Locate seleniumGive the group and period of a main-group element.

Particles, isotopes and ions

  • Spot the isotopesDistinguish a pair of isotopes from pairs of different elements, allotropes, or an atom and its ion.
  • Write the isotope symbolWrite the isotope symbol of a neutral atom from its proton and neutron counts.
  • Which particle is it?Identify a subatomic particle from its charge, mass and location.

Comparar 3

Particles, isotopes and ions

  • Rank by number of electronsRank atoms and monatomic ions by their electron counts.
  • Rank by number of neutronsRank nuclides, including several with the same mass number, by neutron count.

Average atomic mass

  • Which iridium isotope is more common?Decide which of two isotopes is more abundant from where the average atomic mass falls.

Calcular 11

Particles, isotopes and ions

  • Particles in an aluminium ionGive the proton, neutron and electron counts of a monatomic cation.
  • Neutrons in heavy waterCount the neutrons in a molecule built from specified isotopes.
  • Neutrons in a lead(II) ionCount the neutrons in a monatomic cation from its mass number and atomic number.
  • How much heavier is a proton?Compare the masses of the proton and the electron as a ratio.
  • Total electrons in sulfateCount all the electrons, core and valence, in a polyatomic anion.

Mass spectra of elements

  • Height of the m/z 72 peak of Cl₂Calculate the relative height of the mixed-isotope molecular-ion peak of Cl₂ from precise abundances.

Average atomic mass

  • Average atomic mass of chlorineCalculate an average atomic mass from two isotope masses and their percent abundances.
  • Atomic mass of enriched uraniumCalculate the average atomic mass of an isotopically enriched sample.
  • Average atomic mass of magnesiumCalculate an average atomic mass for an element with three isotopes.
  • Mass of an oxygen-16 atom in uUse the definition of the atomic mass unit to convert a mass ratio relative to carbon-12 into an isotope mass.

The periodic table

  • How many elements are in period 6?Count the elements in a period from the atomic numbers of its first and last members.

Encontre o erro 4

Mass spectra of elements

  • Three peaks, three isotopes?Find the error in reading the molecular-ion peaks of Br₂ as separate isotopes.

Particles, isotopes and ions

  • Does Mg²⁺ lose protons?Find the error in a description of how a cation forms from its atom.

The periodic table

  • Is hydrogen an alkali metal?Identify the error in classifying hydrogen by its position at the top of group 1.

Average atomic mass

  • A shortcut for bromine's atomic massFind the flaw in an average-atomic-mass calculation that ignores isotope abundances.

Interprete as evidências 4

Mass spectra of elements

  • Abundance from a chlorine atom spectrumConvert relative peak heights in an atomic mass spectrum into a percent abundance.
  • Atomic mass of copper from its spectrumCalculate an average atomic mass directly from relative peak heights and isotope masses.
  • Which element gives this spectrum?Identify an element from the positions and heights of its isotope peaks.

Particles, isotopes and ions

  • Find the unit of charge from oil dropsExtract the elementary charge from a set of droplet charges that are whole-number multiples of it.

Trabalhe de trás para frente 5

Average atomic mass

  • Percent boron-10 from the atomic massWork backwards from an average atomic mass to the abundance of one of two isotopes.
  • Abundance of magnesium-26Find an unknown abundance in a three-isotope element using the average atomic mass and the sum of the fractions.
  • Mass of the silver-109 isotopeWork backwards from an average atomic mass to the mass of one isotope.

Particles, isotopes and ions

  • Name the ion from its particlesWork back from proton, neutron and electron counts to the full symbol of an ion.

The periodic table

  • Which element forms this ion?Identify an element and its position in the table from the electron count and charge of its ion.

Planeje uma síntese 1

  • What to measure for an atomic massChoose the measurements needed to determine an element's average atomic mass.

Explique por quê 4

The periodic table

  • Why argon comes before potassiumExplain why the periodic table is ordered by atomic number rather than atomic mass.

Average atomic mass

  • Why 35.45 u?Explain why a periodic-table atomic mass need not match the mass of any single atom.

Particles, isotopes and ions

  • What the gold-foil experiment showedExplain how the scattering of α particles by gold foil leads to a small, dense, positive nucleus.
  • Why both chlorine isotopes react alikeExplain why isotopes of an element have essentially the same chemical properties.

Travou em átomos, isótopos e a tabela periódica? Escaneie o problema.

Tire uma foto da reação da sua lista ou do livro e desenhe o mecanismo com cada seta corrigida. 1 scan grátis agora, sem cadastro; mais 1 com uma conta gratuita.

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