Atomic Structure and the Periodic Table

Chapter 2-1

Chemical symbols

Abbreviated way to write the name of an element

Always starts with a capital letter If double letters – first is capitalized second is lower case Cl Ar Ne

atom

Smallest part of matter Basic building block of matter Parts of an atom

– Nucleus – center– – protons - + charge– neutrons - neutral charge

size: mass p = mass n

Electron cloud

– Surrounds the nucleus– Contains electrons– Electrons have a negative charge– Size: mass e = 1/2000 of m p– mass so tiny that it isn’t even considered

in the mass of an atom– Moves around the nucleus at such speeds that they

cannot be seen. – Cloud like appearance

– Neutral atom – the number of positive charges equal the number of negative charges

– All elements on periodic table are stable atoms.

Atomic number

Number of protons in an atom Every atom of the same element has the same

atomic number Identity number of an element

Danish scientist Neils Bohr 1913 Developed the first

model of what the atom looks like

Bohr diagram Central nucleus Electrons moving around

it in well defined paths or orbits.

Electron clouds Electrons moving so fast

that they cannot be seen Electrons move within a

certain region NOT on orbits

Energy Levels and Electrons

Electrons are at varying distances from nucleus

– e near nucleus – low energy

– e farther from nucleus – high energy

Energy levels

– Each holds a different amount of electrons

– There is a maximum number of electrons each can hold

1st 2 e

2nd 8 e

3rd 18 e

4th 32 e

Mass number

– Number of protons + neutrons in nucleus

Number of Neutrons

Mass number - # p = #n

Isotopes

Atoms of the same element that have a different number of neutrons

Average atomic mass – average mass of an element and its isotopes

Written H-1 H-2 H-3

element – mass #

Isotopes of hydrogen

H-1 protium

H-2 deuterium

H-3 tritium

Importance of Isotopes

Used in: Medical field –

radioactive to detect cancers, thyroid disorders

Dating – to date age of rocks or artifacts

Radioactive power plants