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Introduction to Chemistry: Atoms & Elements

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Termini in questo insieme (28)

  • What is an atom?

    An atom is the smallest identifiable unit of an element.

  • How many elements occur naturally and how many are synthetic?

    There are approximately 91 natural elements and about 20 synthetic elements created by scientists.

  • What was Democritus' contribution to atomic theory?

    Democritus proposed that matter is composed of small indivisible particles called atomos or atoms.

  • State Dalton's atomic theory key points.

    All matter is made of atoms; atoms of an element are identical; atoms cannot be broken down; atoms combine in whole-number ratios to form compounds.

  • What is the law of definite composition?

    The mass ratio of elements in a compound is always the same, e.g., carbon dioxide is always CO\(2\).

  • What did J.J. Thomson discover about the atom?

    Thomson discovered the electron, a negatively charged particle smaller than atoms, and proposed the plum pudding model.

  • Describe Thomson's plum pudding model.

    Electrons are embedded in a sphere of positive charge, making the atom neutral overall.

  • What are the masses of an electron and a proton?

    Electron mass: 9.11 × 10-28 g; Proton mass: 1.67 × 10-24 g.

  • What was Rutherford's gold foil experiment and its conclusion?

    Alpha particles were fired at gold foil; most passed through, but some deflected, proving atoms have a dense, positively charged nucleus.

  • What is the size comparison between an atom and its nucleus?

    An atom is about 1 × 10-8 cm in diameter; its nucleus is about 1 × 10-13 cm, much smaller.

  • What subatomic particles are found in the nucleus?

    The nucleus contains protons (positive charge) and neutrons (neutral charge).

  • Define atomic number (Z) and mass number (A).

    Atomic number (Z) = number of protons; Mass number (A) = protons + neutrons in the nucleus.

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

    Number of neutrons = Mass number (A) - Atomic number (Z).

  • What are isotopes?

    Atoms of the same element with the same number of protons but different numbers of neutrons.

  • Give examples of hydrogen isotopes.

    Protium: 1 proton, 0 neutrons; Deuterium: 1 proton, 1 neutron; Tritium: 1 proton, 2 neutrons (radioactive).

  • What is atomic mass and how is it calculated?

    Atomic mass is the weighted average mass of all isotopes, calculated by summing (fractional abundance × isotope mass).

  • Who developed the modern periodic table and how?

    Mendeleev arranged elements by increasing atomic mass and grouped elements with similar properties together, predicting undiscovered elements.

  • What are the main categories of elements on the periodic table?

    Metals, nonmetals, and metalloids (semimetals) with distinct physical and chemical properties.

  • List key properties of metals.

    Metals are shiny, conduct heat and electricity, malleable, ductile, and tend to lose electrons forming cations.

  • List key properties of nonmetals.

    Nonmetals can be solids, liquids, or gases; poor conductors; brittle solids; tend to gain electrons forming anions.

  • What are metalloids and their properties?

    Metalloids have properties of both metals and nonmetals; they are semiconductors, e.g., silicon is shiny, brittle, and conducts electricity poorly.

  • What is the Periodic Law?

    Element properties repeat periodically when elements are arranged by increasing atomic number.

  • Define groups and periods in the periodic table.

    Groups are vertical columns with similar properties; periods are horizontal rows indicating energy levels.

  • What are the characteristics of Group A elements?

    Group A (representative) elements include alkali metals, alkaline earth metals, pnictogens, chalcogens, halogens, and noble gases with varying reactivities.

  • What are transition elements and inner transition elements?

    Transition elements are in the middle of the table with unpredictable properties; inner transition elements include lanthanides and actinides.

  • How do metals and nonmetals form ions?

    Metals lose electrons to form positive ions (cations); nonmetals gain electrons to form negative ions (anions).

  • Give the ionic charge for Group IA metals and Group VIA nonmetals.

    Group IA metals form +1 ions; Group VIA nonmetals form -2 ions.

  • How are element symbols written?

    Symbols are 1 or 2 letters; first letter capitalized, second letter lowercase if present.