Atomic Structure and the Periodic Table | 原子结构与元素周期表

📚 Atomic Structure and the Periodic Table | 原子结构与元素周期表

Understanding atomic structure is the foundation of all IGCSE science. This article explains the key ideas about atoms, subatomic particles, electronic configuration, and the periodic table — all aligned with the Edexcel IGCSE specification.

理解原子结构是学习IGCSE科学的基础。本文将讲解有关原子、亚原子粒子、电子排布和元素周期表的核心概念，完全对应Edexcel IGCSE考纲。

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1. The Atom | 原子

An atom is the smallest particle of an element that can take part in a chemical reaction. Atoms are extremely small, with a radius of about 0.1 nm (1 × 10⁻¹⁰ m).

原子是元素中能参加化学反应的最小粒子。原子极其微小，半径约为0.1纳米（1 × 10⁻¹⁰ 米）。

Each atom consists of a central nucleus surrounded by electrons. The nucleus is tiny but contains most of the atom’s mass.

每个原子都由位于中心的原子核以及围绕核运动的电子构成。原子核很小，但几乎集中了原子的全部质量。

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2. Subatomic Particles | 亚原子粒子

There are three types of subatomic particles: protons, neutrons, and electrons.

亚原子粒子有三种：质子、中子和电子。

• Protons are positively charged and are found in the nucleus.

  质子带正电荷，位于原子核内。

• Neutrons have no charge and are also found in the nucleus.

  中子不带电荷，也位于原子核内。

• Electrons are negatively charged and move around the nucleus in energy levels.

  电子带负电荷，在原子核周围的能级中运动。

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3. Atomic Number and Mass Number | 原子序数与质量数

The atomic number (proton number) is the number of protons in the nucleus. It defines the element.

原子序数（质子数）是原子核中的质子数目。它决定了元素的种类。

The mass number (nucleon number) is the total number of protons and neutrons in the nucleus.

质量数（核子数）是原子核中质子与中子的总数。

Mass number = Number of protons + Number of neutrons

质量数 = 质子数 + 中子数

For a neutral atom, the number of electrons equals the number of protons. For example, magnesium has atomic number 12, so it has 12 protons and 12 electrons.

对于中性原子，电子数等于质子数。例如，镁的原子序数为12，因此它有12个质子和12个电子。

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4. Isotopes | 同位素

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons.

同位素是同一元素的不同原子，它们的质子数相同，但中子数不同。

For example, chlorine has two common isotopes: chlorine-35 and chlorine-37. Both have 17 protons, but they have 18 and 20 neutrons respectively.

例如，氯有两种常见同位素：氯-35和氯-37。它们都有17个质子，但分别有18个和20个中子。

Isotopes have the same chemical properties because they have the same number of electrons, but they may have different physical properties such as mass or density.

同位素具有相同的化学性质，因为它们具有相同的电子数，但物理性质如质量或密度可能不同。

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5. Electronic Configuration | 电子排布

Electrons are arranged in energy levels (shells) around the nucleus. Each shell can hold a limited number of electrons: the first shell holds up to 2, and the second shell holds up to 8.

电子在原子核周围按能级（电子层）排列。每一层可容纳的电子数有限：第一层最多2个，第二层最多8个。

The electronic configuration of an atom is written by listing the number of electrons in each shell, starting from the innermost shell. For example, sodium (11 electrons) has the configuration 2,8,1.

原子的电子排布从最内层开始，逐层写出每层电子数。例如，钠（11个电子）的排布是2,8,1。

For Edexcel IGCSE, you must be able to draw electron diagrams using dots or crosses. The electrons in the outermost shell are called valence electrons and are responsible for chemical bonding.

在Edexcel IGCSE中，你需要能够用点或叉画出电子示意图。最外层电子叫作价电子，决定原子的成键行为。

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6. The Periodic Table: Groups and Periods | 元素周期表：族与周期

The periodic table arranges elements in order of increasing atomic number. A horizontal row is called a period, and a vertical column is called a group.

元素周期表按照原子序数递增排列元素。横行为周期，纵列为族。

Elements in the same group have the same number of outer electrons, so they have similar chemical properties. For example, Group I elements all have one valence electron.

同一族元素具有相同的最外层电子数，因此化学性质相似。例如，第I族元素都只有1个价电子。

The group number tells you how many electrons are in the outer shell (for Groups I to VII). The period number tells you how many occupied shells there are.

对于第I族到第VII族，族号表示最外层电子数；周期号则表示有核外电子层的数目。

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7. Trends in the Periodic Table | 元素周期表中的递变规律

Going down a group, the atomic radius increases because extra electron shells are added. This affects other properties such as reactivity.

在同一族中，从上到下原子半径增大，因为增加了新的电子层。这会影响金属性、非金属性等性质。

In Group I (alkali metals), reactivity increases down the group because the outer electron is further from the nucleus and is lost more easily. In Group VII (halogens), reactivity decreases down the group because it becomes harder to gain an electron.

在第I族（碱金属）中，从上到下反应性增强，因为外层电子离核更远，更容易失去。在第VII族（卤素）中，从上到下反应性减弱，因为获得电子变得更困难。

Across a period, elements change from metals to non-metals. For example, in Period 3, sodium and magnesium are metals, silicon is a metalloid, and chlorine and argon are non-metals.

同一周期中，元素从金属逐渐变为非金属。例如，第三周期中钠、镁是金属，硅是准金属，氯和氩是非金属。

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8. Formation of Ions | 离子的形成

Atoms are electrically neutral because they have equal numbers of protons and electrons. To become stable, atoms often lose or gain electrons to achieve a full outer shell.

原子因质子和电子数目相等而呈电中性。为了达到稳定，原子经常通过失去或获得电子来使最外层填满。

When an atom loses electrons, it forms a positive ion (cation). When an atom gains electrons, it forms a negative ion (anion).

当原子失去电子时，形成带正电的离子（阳离子）；当原子获得电子时，形成带负电的离子（阴离子）。

Metal atoms tend to lose electrons, while non-metal atoms tend to gain electrons. For example, sodium (2,8,1) loses one electron to become Na⁺, with configuration 2,8.

金属原子倾向于失去电子，而非金属原子倾向于获得电子。例如，钠（2,8,1）失去1个电子变为Na⁺，其电子排布为2,8。

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9. Ionic Bonding | 离子键

Ionic bonding occurs when electrons are transferred from a metal atom to a non-metal atom, forming oppositely charged ions held together by strong electrostatic forces of attraction.

离子键发生在电子从金属原子转移到非金属原子时，形成的带相反电荷的离子之间通过强烈的静电吸引力结合在一起。

In sodium chloride, sodium gives one electron to chlorine. The sodium atom becomes Na⁺ and the chlorine atom becomes Cl⁻.

在氯化钠中，钠给氯一个电子。钠原子变为Na⁺，氯原子变为Cl⁻。

Ionic compounds have high melting points and conduct electricity when molten or dissolved in water. They do not conduct electricity when solid because their ions are held in a fixed lattice.

离子化合物熔点高，在熔融或溶于水时能够导电。它们在固态时不导电，因为离子被固定在晶格中不能自由移动。

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10. Covalent Bonding | 共价键

Covalent bonding occurs when two non-metal atoms share pairs of electrons so that each atom achieves a full outer shell.

共价键发生在两个非金属原子之间，通过共用电子对使每个原子达到最外层全满的状态。

For example, in a water molecule (H₂O), each hydrogen atom shares one electron pair with the oxygen atom. The shared pair counts as belonging to both atoms.

例如，在水分子（H₂O）中，每个氢原子与氧原子共用一对电子。这对电子同时属于两个原子。

Covalent substances can exist as simple molecules, such as CO₂ and CH₄, or as giant structures, such as diamond and silicon dioxide. Simple molecular substances have low melting points and usually do not conduct electricity.

共价物质可以形成小分子，例如CO₂和CH₄，也可以形成巨型结构，例如金刚石和二氧化硅。小分子物质熔点低，通常不导电。

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