knowt logo

Chemistry Chapter 3

Section 1

Democritus

  • 400BC Greek thinker

  • atom- nature’s basic particle

Aristotle

  • generation succeeding Democritus

  • didn't believe in atoms

  • all matter is continuous

Foundations of Atomic Theory

element- substance that cannot be further broken down by chemically/ordinary means

elements combine to form compounds

  • different chemical/physical propertied than the elements that make them up

chemical reaction- transformation of a substance to a new substance

law of conservation of mass- mass is neither created nor destroyed during ordinary chemical reaction or physical change

law of definite proportions- chemical compound contains same elements in same proportions by mass regardless of the size of the sample or source of the compound

**law of multiple proportions-**if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always the ratio of small whole numbers

Dalton’s Atomic Theory

  • proposed explanation for law of conservation of mass, definite proportions, and multiple proportions- his 5 postulates

5 Postulates:

  1. all matter is composed of extremely small particles called atoms

  2. atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, other propertied

  3. atoms cannot be subdivided, created, destroyed

  4. atoms of different elements combine in simple whole-number ratios to form chemical compounds

  5. in chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory

  • not all aspects of Dalton’s theory have been proven correct

  • atoms can be divided into smaller parts

    • although law of conservation of mass still holds true

  • a given element can have atoms with different masses

  • Dalton’s theory has been modified

modern atomic theory concepts:

  1. all matter is composed of atoms

  2. atoms of any one element differ in properties from atoms of another element remain unchanged

Section 2- Structure of the Atom

atom- smallest particle of an element that retains the chemical properties of that element

two regions of an atom:

  1. nucleus- small region in the center

    1. at least one proton- positive charge

    2. at least one neutron- neutral charge

  2. area around nucleus occupied by electrons- negatively charge particles

  • subatomic particles- proton, electron, neutron

Discovery of the Electron

Cathode Rays and Electrons

  • current passing through cathode tube causes it to glow

  • observations lead to hypothesis- particles were negatively charged

    • experiments support: Thompson measure ratio of particles to mass

      • always the same

    • cathode rays composed of identical negative charge particles- electrons

Charge and Mass of an Electron

  • electrons in all atoms

  • atoms are divisible

  • large charge to mass ratio

  • atoms are electrically neutral

    • must have a positive

    • electrons have much less mass than atoms, so other particles must have much more mass

  • plum pudding model- electrons evenly spread throughout and the rest is negative

Discovery of Atomic Nucleus

Ernest Rutherford- shoot positive alpha particles at think gold foil

  • some deflect

  • must be hitting something positive and dense in the center- nucleus

Bhor- propose atomic model

  • electrons surround positive nucleus like planets around the sun

Composition of the Atomic Nucleus

atomic nuclei made of protons and neutrons

  • protons= positive charge, electrons= negative charge

    • equal in magnitude

  • atoms are electrically neutral: same number of protons and electrons

  • atoms of different elements have different numbers of protons- affects positive charge

    • protons determine identity of an atom

Forces in the Nucleus

nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

  • usually particles with the same charge repel each other, but the particles are so close that a strong attraction is formed

The Size of Atoms

electron cloud: region outside of the nucleus where electrons are stored

  • radius of an atom- from the center of the nucleus to the outer portion of the electron cloud

    • extremely small

    • measured in picometer (pm)

Section 3- Counting Atoms

Atomic Number

atomic number: the number of protons in each atom of an element

  • identifies the element because different elements have different atomic numbers

  • atoms are neutral, so the atomic number also tell the number of electrons

Isotopes

isotopes: atoms of the same element with different masses

  • same number of protons and electrons but different number of neutrons

Mass Number

mass number: total number of protons and neutrons that make up the nucleus of an isotope

Designating Isotopes

identified by specifying the mass number

2 methods:

  • hyphen notation

    • mass number written with a hyphen after the element name

    • Hydrogen-3

  • nuclear symbol

    • shows composition of the nucleus

    • superscript= mass number

    • subscript= atomic number

    • 32 He

Relative Atomic Masses

Chemistry Chapter 3

Section 1

Democritus

  • 400BC Greek thinker

  • atom- nature’s basic particle

Aristotle

  • generation succeeding Democritus

  • didn't believe in atoms

  • all matter is continuous

Foundations of Atomic Theory

element- substance that cannot be further broken down by chemically/ordinary means

elements combine to form compounds

  • different chemical/physical propertied than the elements that make them up

chemical reaction- transformation of a substance to a new substance

law of conservation of mass- mass is neither created nor destroyed during ordinary chemical reaction or physical change

law of definite proportions- chemical compound contains same elements in same proportions by mass regardless of the size of the sample or source of the compound

**law of multiple proportions-**if two or more different compounds are composed of the same two elements, then the ratio of the masses of the second element combined with a certain mass of the first element is always the ratio of small whole numbers

Dalton’s Atomic Theory

  • proposed explanation for law of conservation of mass, definite proportions, and multiple proportions- his 5 postulates

5 Postulates:

  1. all matter is composed of extremely small particles called atoms

  2. atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, other propertied

  3. atoms cannot be subdivided, created, destroyed

  4. atoms of different elements combine in simple whole-number ratios to form chemical compounds

  5. in chemical reactions, atoms are combined, separated, or rearranged

Modern Atomic Theory

  • not all aspects of Dalton’s theory have been proven correct

  • atoms can be divided into smaller parts

    • although law of conservation of mass still holds true

  • a given element can have atoms with different masses

  • Dalton’s theory has been modified

modern atomic theory concepts:

  1. all matter is composed of atoms

  2. atoms of any one element differ in properties from atoms of another element remain unchanged

Section 2- Structure of the Atom

atom- smallest particle of an element that retains the chemical properties of that element

two regions of an atom:

  1. nucleus- small region in the center

    1. at least one proton- positive charge

    2. at least one neutron- neutral charge

  2. area around nucleus occupied by electrons- negatively charge particles

  • subatomic particles- proton, electron, neutron

Discovery of the Electron

Cathode Rays and Electrons

  • current passing through cathode tube causes it to glow

  • observations lead to hypothesis- particles were negatively charged

    • experiments support: Thompson measure ratio of particles to mass

      • always the same

    • cathode rays composed of identical negative charge particles- electrons

Charge and Mass of an Electron

  • electrons in all atoms

  • atoms are divisible

  • large charge to mass ratio

  • atoms are electrically neutral

    • must have a positive

    • electrons have much less mass than atoms, so other particles must have much more mass

  • plum pudding model- electrons evenly spread throughout and the rest is negative

Discovery of Atomic Nucleus

Ernest Rutherford- shoot positive alpha particles at think gold foil

  • some deflect

  • must be hitting something positive and dense in the center- nucleus

Bhor- propose atomic model

  • electrons surround positive nucleus like planets around the sun

Composition of the Atomic Nucleus

atomic nuclei made of protons and neutrons

  • protons= positive charge, electrons= negative charge

    • equal in magnitude

  • atoms are electrically neutral: same number of protons and electrons

  • atoms of different elements have different numbers of protons- affects positive charge

    • protons determine identity of an atom

Forces in the Nucleus

nuclear forces: the interaction that binds protons and neutrons, protons and protons, and neutrons and neutrons together in a nucleus

  • usually particles with the same charge repel each other, but the particles are so close that a strong attraction is formed

The Size of Atoms

electron cloud: region outside of the nucleus where electrons are stored

  • radius of an atom- from the center of the nucleus to the outer portion of the electron cloud

    • extremely small

    • measured in picometer (pm)

Section 3- Counting Atoms

Atomic Number

atomic number: the number of protons in each atom of an element

  • identifies the element because different elements have different atomic numbers

  • atoms are neutral, so the atomic number also tell the number of electrons

Isotopes

isotopes: atoms of the same element with different masses

  • same number of protons and electrons but different number of neutrons

Mass Number

mass number: total number of protons and neutrons that make up the nucleus of an isotope

Designating Isotopes

identified by specifying the mass number

2 methods:

  • hyphen notation

    • mass number written with a hyphen after the element name

    • Hydrogen-3

  • nuclear symbol

    • shows composition of the nucleus

    • superscript= mass number

    • subscript= atomic number

    • 32 He

Relative Atomic Masses

robot