Notes
Atomic structure
- The nucleus holds positive protons and neutral neutrons. Negative electrons surround it. Neutral atoms have equal protons and electrons.
- Atomic number = protons. Mass number = protons + neutrons. Neutrons = mass number - atomic number.
Isotopes
- Isotopes have the same number of protons but different numbers of neutrons. Atomic mass on the table is a weighted average of isotopes.
The periodic table
- Elements are ordered by atomic number. Rows are periods; columns are groups. Metals, nonmetals and metalloids occupy different regions.
- Group 1 alkali metals are very reactive, Group 17 halogens are reactive nonmetals, and Group 18 noble gases are unreactive.
- Elements in a group share the same number of valence electrons. Energy levels hold 2, then 8 electrons.
Topics in this unit
Each topic has its own short notes and practice.
Practice
Atomic number = protons = 15. Mass number = $15 + 16 = 31$.
Isotopes differ in neutron number.
Atomic number, the proton count, identifies the element.
$0.80(20) + 0.20(22) = 16 + 4.4 = 20.4$.
They are halogens in Group 17.
Valence electrons are those in the outermost level.
Free response
Write your answer first, then open the scoring guide.
Free-response question
An atom has 19 protons, 20 neutrons and 19 electrons.
(a) Identify its atomic number and mass number.
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Atomic number is 19 (number of protons). Mass number is $19 + 20 = 39$.
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(b) Explain how you know the atom is neutral.
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The number of protons (positive charges) equals the number of electrons (negative charges), 19 each, so the charges cancel.
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(c) An isotope of this element has 22 neutrons. Give its mass number and explain why it is the same element.
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Mass number is $19 + 22 = 41$. It is the same element because it still has 19 protons; only the neutron number differs.
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