3.5
Atomic Number and Mass Number
The number of protons in the nucleus of an atom is its atomic number (Z). This is the defining trait of an element. Its value determines the identity…
The atomic number, denoted as Z, is the total number of protons in the nucleus of an atom.
On the periodic table, each square represents an element having a unique atomic number. These elements are arranged in order of increasing atomic number.
An atom with one proton is hydrogen, an atom with two protons is helium, and so on.
The mass number of an atom is the sum of the protons and neutrons in the atomic nucleus, represented by the letter A.
So, a hydrogen atom with one proton and no neutrons has a mass number of one, while a helium atom with two protons and two neutrons has a mass number of four.
Elements can have multiple isotopes with the same number of protons but different numbers of neutrons.
For example, carbon atoms exist as carbon-12 with six protons and six neutrons, carbon-13 with six protons and seven neutrons, or carbon-14 with six protons and eight neutrons.
View the full transcript and gain access to JoVE Core videos
Q1: What is atomic number and how does it identify an element?
Atomic number, denoted as Z, is the total number of protons in an atom's nucleus. It is the defining trait of an element and determines the atom's identity. Any atom with six protons is carbon with atomic number 6, regardless of neutron or electron count. On the periodic table, elements are arranged in order of increasing atomic number.
Q2: How does atomic number relate to the number of electrons in a neutral atom?
In a neutral atom, the number of protons equals the number of electrons because atoms must contain equal positive and negative charges. Since atomic number indicates the number of protons, it also indicates the number of electrons in a neutral atom. This relationship ensures electrical neutrality.
Q3: What is mass number and how is it calculated?
Mass number, represented by the letter A, is the sum of protons and neutrons in an atom's nucleus. For example, a helium atom with two protons and two neutrons has a mass number of four. The number of neutrons can be determined by subtracting atomic number from mass number: A minus Z.
Q4: What are isotopes and why do they have different masses?
Isotopes are atoms of the same element that differ in mass but are chemically identical. They have the same number of protons but different numbers of neutrons. For example, carbon-12 has six protons and six neutrons, while carbon-14 has six protons and eight neutrons. Understanding isotopes and radioisotopes is essential for advanced chemistry.
Q5: Why are isotopes chemically indistinguishable despite having different masses?
Isotopes are chemically identical because chemical properties depend on the number of protons and electrons, not neutrons. Since all isotopes of an element have the same atomic number, they possess the same number of protons and electrons. This identical electron configuration makes them chemically indistinguishable in reactions.
Q6: How are elements arranged on the periodic table?
Elements on the periodic table are arranged in order of increasing atomic number. Each square represents an element with a unique atomic number. Hydrogen, with one proton, appears first, followed by helium with two protons, and continuing in this systematic order throughout the table.
Q7: What is the relationship between atomic number and element identity?
Atomic number is the sole determinant of element identity. The number of protons in an atom's nucleus defines which element it is. An atom cannot change its element identity without changing its proton count. This fundamental principle means atomic number uniquely identifies every element on the periodic table.