Atoms and Elements

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Dalton’s Atomic Theory

atoms are tiny particles of matter

atoms of an element are similar and different from other elements

atoms of two or more different elements combine to form compounds

atoms are rearranged to form new combinations in a chemical reaction

Law of Multiple Proportions

Subatomic Particles

Protons: +

Electrons:-

Neutrons: neutral

Like charges repel and unlike charges attract

Rutherford’s Gold Foil Experiment

Positively charged particles were aimed at atoms of gold

most of the particles went straight through the atoms and some were deflected occasionally

This concludes that there must be a positively charged nucleus in the atom that deflects positive particles that come close.

The Nuclear Atom

J.J. Thomson discovered the electron

Electrons are negatively charged

they are much smaller and lighter than atoms

uniformly present in many substances

he proposed that atoms must contain postive charge that balanced the negative charge of electrons

He thought of the Plum Pudding model; neg charged electrons were held in a sphere of positive charge

Structure of the Atom

consists of a nucleus that have protons and electrons

electrons are contained in the electron cloud around nucleus

Atomic Mass (amu)

one-twelfth the mass of a carbon 12 atoms

1u = 1.66054 x 10^-24 grams

Atomic Mass: weighted average of the masses of the naturally occurring isotopes of that element

a proton has a mass of about 1 (1.007) amu

a neutron has a mass of about 1 (1.008) amu

an electron has a very small mass (0.000549) amu

the atomic mass listed in the periodic table represents the average mass of atoms that compose the element

listed below the symbol of each element on periodic table

NOT the same as mass number

Atomic number

specific for each element

the same for all atoms of an element

it is equal to the number of protons in an atom

appears above the symbol of an element

EX: Hydrogen has an atomic number of 1; every H atom has one proton

Electrons in an Atom

an atom of an element is electrically neutral; the net charge of an atom iz zero

an atom has an equal number of protons and electrons

# of p = # of e

Mass Number

Mass Number: represents the number of particles in the nucleus

# of p + # of n

Isotopes

Isotopes: atoms of the same element that have different mass numbers; has the same # of protons but different # of neutrons

all elements have their own unique percent or natural abundance of isotopes

to represent an isotope the chemical name/symbol is listed first, followed by the hyphen and then the mass number

EX: Ne-20 or neon-20

Some nuclei of isotopes are unstable

nuclear radiation: occurs when atoms emit subatomic particles from their nuclei and change into different isotopes of different elements

Radioactive: isotope that emit energetic subatomic particles

Harmful to humans and other living organisms because the energy from the particles messes with the biological molecules

Some isotopes have a short or long radiative life

Not all radioactive isotopes are harmful; like the dye that is given for x-rays

Nuclear Symbols

represents a particular atom of an element

gives the mass number in upper left corner and the atomic number in the lower left corner

mass number ------->23

Na

atomic number------->11

using the nuclear symbol can help us determine the # of P E and N

Groups and Periods

Groups: contain elements with similar properties in vertical columns

Periods: horizontal rows

Letter A elements are representative

Letter B elements are transition

Metals, Nonmetals, and Metalloids

Metals: shiny and ductile; good conductors of heat and electricity

Nonmetals: dull, brittle and poor conductors; good insulators

Metalloids: better conductors than nonmetals; used as semiconductors and insulators

Atoms & Elements