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Número atômico e número de massa
O número de prótons no núcleo de um átomo é seu número atômico (Z). Esta é a característica definidora de um elemento. Seu valor determina a identidad…
O número atômico, denotado como Z, é o número total de prótons no núcleo de um átomo.
Na tabela periódica, cada quadrado representa um elemento com um número atômico único. Esses elementos são organizados em ordem crescente de número atômico.
Um átomo com um próton é hidrogênio, um átomo com dois prótons é hélio e assim por diante.
O número de massa de um átomo é a soma dos prótons e nêutrons no núcleo atômico, representado pela letra A.
Assim, um átomo de hidrogênio com um próton e nenhum nêutron tem um número de massa de um, enquanto um átomo de hélio com dois prótons e dois nêutrons tem um número de massa de quatro.
Os elementos podem ter vários isótopos com o mesmo número de prótons, mas diferentes números de nêutrons.
Por exemplo, os átomos de carbono existem como carbono-12 com seis prótons e seis nêutrons, carbono-13 com seis prótons e sete nêutrons ou carbono-14 com seis prótons e oito nêutrons.
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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.