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The identity of
an element is controlled by the number of protons in the nucleus. The
number of protons equals the atomic number. Every element has its own
unique atomic number. The atomic number represented by the symbol, Z.
The periodic table is arranged in sequence of increasing atomic number
or proton count. Hydrogen has one proton and has atomic number one,
Z=1. The atomic number increases by one unit for every additional proton.
helium has two protons and is therefore atomic number Z= 2.
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- What
is the atomic number for nitrogen, N?
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- Answer:
Nitrogen is in the seventh position in the periodic table. This means
nitrogen atoms have 7 protons in the nucleus, and they have an atomic
number of 7.
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- The count of
neutrons and protons equals the mass number for an atom. This has
its origin in the fact that the "massive" particles in the atom are
protons and neutrons. The symbol for mass number is A. The entries
in the periodic table do not show the mass numbers for atoms. The
periodic table is useless when we want to figure mass numbers. The
mass number for an atom is commonly indicated with a number after
the element name. For example lead-210 indicates a type of lead atom
with a mass number A = 210. For people who like formulas the mass
number and atomic number are related as shown below.
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number
of neutrons = A - Z
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number
of neutrons = mass number - atomic number
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- How
many neutrons and protons are in an atom of sodium-23?
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- Answer:
Sodium, Na, has atomic number 11. This means sodium atoms have 11
protons in the nucleus. The atom has a mass number equal to the number
of protons and neutrons, so there must be 12 neutrons in the nucleus.
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number
of neutrons = A - Z
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number
of neutrons = 23 - 11 = 12
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- Atomic
number and electron count:
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- Atoms are neutral
particles. The number of positively charged protons are canceled out
by an equal number of negatively charged electrons. Hydrogen has one
proton and one electron.
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- The illustration
shows a beryllium-8 atom, Be-8, with four electrons and four protons.
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- How
many electrons and protons are in a neutral sodium atom?
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- Answer:
Sodium, Na, has atomic number 11. This means sodium atoms have 11
protons in the nucleus. The neutral atom has to have equal numbers
of protons and electrons, so there must be 11 electrons moving outside
the nucleus.
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- How many neutrons
are in an atom of sulfur, S, with mass number 33?
- Answer:
The atomic number for sulfur is 16. The number of neutrons = A - Z
= 33 - 16 = 17
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- An atom contains
24 neutrons and 25 protons, what is the mass number of the atom?
- Answer:
Mass number = A = number protons + number of neutrons = 24 + 25 =
49
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- An atom with
a mass number of 39 contains 20 neutrons. What is the atomic number
and identity of the element?
- Answer:The
atomic number is Z = 39- 20 =19 . The identity is potassium because
K is element 19.
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