Chemistry - Atomic Structure - Quick Guides

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Study GuideChemistry–Atomic Structure1. Structure of the Atom1.1What Is an Atom?Everything around you is made of atoms. Anatomis the smallest unit of an element that still keepsthe properties of that element.For example, a block of iron contains an enormous number of tiny iron atoms packed closely together.1.2Inside an AtomEven though atoms are very small, they are made of even smaller particles:•Protons–positively charged particles•Neutrons–particles with no charge•Electrons–negatively charged particlesProtons and neutrons are tightly packed in the centerof the atom, in a tiny region called thenucleus.Electrons do not sit in one fixed spot. Instead, they move around the nucleus in acloud-like regionthat surrounds it.1.3Atoms and IsotopesAll atoms of the same element have thesame number of protonsin their nuclei. This numberdefines the element.However, atoms of the same element can havedifferent numbers of neutrons. These differentforms are calledisotopes.•Most elements in nature exist as amixture of isotopes.•Each isotope has a slightly differentatomic mass.•The atomic mass listed on the periodic table is aweighted averageof the masses of allnaturally occurring isotopes of that element.

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Study Guide1.4Ions: When Atoms Gain or Lose ElectronsAtoms can also change by gaining or losing electrons:•If an atomloses electrons, it becomes positively charged.•If an atomgains electrons, it becomes negatively charged.Charged atoms are calledions.1.5Radioactive NucleiSometimes, a nucleus becomesunstable. This usually happens when there are too many protonsand neutrons packed together.When this happens, the atom becomesradioactiveand can break apart. During this process:•The atom may change into anew element.•It releases particles calledalphaorbeta radiation.•It also releases energy in the form ofgamma radiation.SummaryAtoms are tiny but complex. They are made of smaller particles, can exist in different forms calledisotopes, can become charged ions, and sometimes even break apart to release energy.Understanding atoms helps explain how matter behaves in everyday life—and in science as a whole.2. Subatomic Particles2.1 Understanding Atomic StructureAs scientists studied the periodic table, they noticed something interesting.As you move from one element to the next,atomic mass usually increases in a steady way.This pattern became clear when scientists discovered that atoms aremade of smaller particles.

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Study Guide2.2Discovery of the NucleusIn1911, the study of atomic structure took a big step forward.A scientist namedErnest Rutherforddiscovered that atoms have asmall, dense center called thenucleus.He found that:•The nucleus containspositively charged particles, calledprotons•Most of the atom isempty space•Tiny negatively chargedelectronsmove around the nucleusTable 1. Parts of an Atom•Themass,charge, andlocationof protons, neutrons, and electrons2.3Protons and Atomic NumberSoon after Rutherford’s discovery, scientists realized something very important:Each element is defined by the number of protons in its nucleus.This number is called theatomic number.For example:•Hydrogen has1 proton•Helium has2 protons•Lithium has3 protonsAs you move through the periodic table, the number of protons increases by one each time.

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Study Guide2.4Neutrons and Atomic MassExcept for the simplest hydrogen atom (which has onlyone proton),all atoms contain neutrons.Neutrons:•Haveno electric charge•Are found in thenucleus•Add to themassof the atomFor mostlight elements, the number ofprotons and neutrons is almost equal.Table 2. Nuclear Structure of the Lightest Elements•Atomic mass (rounded)is thesum of protons and neutrons•This total is called themass numberMass number = protons + neutrons2.5Mass Number Made SimpleAn example:•Carbon has6 protonsand6 neutrons•Its mass number is12This pattern appears clearly in the table and explains why atomic masses increase across theperiodic table.

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Study Guide2.6Dalton’s Early Idea and Modern UnderstandingEarlier,John Daltonsuggested that atomic masses were simple multiples of hydrogen’s mass.While this idea wasnot completely correct, it wasclose to the truth.Today we know that:•Atomic mass increases because atoms gainnucleons•Nucleonsare the particles found in the nucleus:oProtonsoNeutronsBy adding these nucleons, nature creates the series of elements with increasing atomic masses.Summary•Atoms have anucleuswith protons and neutrons•Atomic number= number of protons•Mass number= protons + neutrons•Atomic masses increase by addingnucleons3. Quiz: Subatomic Particles1.QuestionIons areAnswer Choices•atoms of an element that have different numbers of protons•atoms of an element that have different numbers of neutrons•atoms that have a charge

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Study GuideCorrect Answeratoms that have a chargeWhy This Is CorrectIons are formed when atomsgain or lose electrons, which gives them anet electrical charge.•If an atomloses electrons, it becomes apositively charged ion (cation).•If an atomgains electrons, it becomes anegatively charged ion (anion).Changing the number ofprotonswould change the element itself, and changing the number ofneutronscreates isotopes—not ions.2.QuestionWhat particles are found in the nucleus of an atom?Answer Choices•protons and neutrons•protons and electrons•neutrons and electronsCorrect Answerprotons and neutronsWhy This Is CorrectThenucleusis the dense center of an atom and contains:•Protons→ positively charged particles•Neutrons→ neutral (no charge) particlesThese particles make up nearly all of an atom’s mass.

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Study GuideElectronsarenotfound in the nucleus. Instead, they move around the nucleus in regions calledelectron clouds.Example:In a carbon atom, the nucleus contains6 protons and 6 neutrons, while6 electronsare foundoutside the nucleus.3. QuestionWhich subatomic particle has the smallest mass?Answer Choices•proton•neutron•electronCorrect AnswerelectronWhy This Is CorrectAmong the three main subatomic particles, theelectronhas the smallest mass.•Electronshave a mass about1/1836that of a proton.•Protonsandneutronsare much heavier and have nearly the same mass.Because of its very small mass, the electron contributes almost nothing to an atom’s total mass, eventhough it plays a major role in chemical behavior.For example:Even though every hydrogen atom containsone electron, almost all of hydrogen’s mass comes fromitsproton.

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Study Guide4. QuestionWhich subatomic particle has a charge of zero?Answer Choices•proton•neutron•electronCorrect AnswerneutronWhy This Is CorrectA neutron hasno electric charge, which is why it is considered neutral.•Protonshave a positive (+) charge•Electronshave a negative (–) charge•Neutronshavezero chargeNeutrons are found in the nucleus of an atom and help stabilize it by reducing repulsion betweenpositively charged protons.5.QuestionWhich subatomic particle has a charge of +1?Answer Choices•proton•neutron•electron

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Study GuideCorrect AnswerprotonWhy This Is CorrectAprotoncarries a positive electric charge of+1. Protons are found in the nucleus of an atom anddetermine the atom’s identity (atomic number).•Neutronshaveno charge (0)•Electronshave anegative charge (–1)For example, hydrogen hasone proton, giving it a +1 nuclear charge, which defines it as hydrogen.6. QuestionWhich section of an atom has the highest density?Answer Choices•electron cloud•nucleus•all sections have the same densityCorrect AnswernucleusWhy This Is CorrectThe nucleus has the highest density because it contains almost all of an atom’s mass packed into avery small volume. Protons and neutrons are located in the nucleus, while electrons in the electroncloud have very little mass and occupy most of the atom’sspace.For example:Even though the nucleus is tiny compared to the whole atom, it contains nearly99.9% of the atom’smass, making it extremely dense.

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Study Guide7.QuestionWhich section of an atom has the lowest density?Answer Choices•electron cloud•nucleus•all sections have the same densityCorrect Answerelectron cloudWhy This Is CorrectTheelectron cloudhas the lowest density because it occupies a very large volume of space aroundthe nucleus, but contains extremely little mass (only electrons).In contrast, thenucleusis extremely dense because it contains protons and neutrons packed into avery small space, holding almost all of the atom’s mass.So, even though the electron cloud makes up most of the atom’s size, it has thelowest density.8.QuestionThe atomic number tells theAnswer Choices•number of electrons in the atom•number of protons in the atom•number of neutrons in the atomCorrect Answernumber of protons in the atom

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